JPS62256913A - Manufacture of high strength black heat malleable cast iron - Google Patents

Abstract

PURPOSE:To manufacture high strength black heat malleable cast iron by chilling the surface of white pig iron and subjecting the pig iron to austenitizing, rapid cooling and isothermal transformation under specified conditions so as to make the structure fine. CONSTITUTION:A part or the whole of the surface layer of white pig iron is melted by rapid heating and is rapidly cooled to form a chilled structure. The pig iron is then held at 870-1,050 deg.C for >=30min to carry out graphitizing as well as austenitizing and it is rapidly cooled to 200-400 deg.C and is held at the temp. for >=10min to convert the structure of the matrix into bainite by isothermal transformation. By this method, high strength black heart malleable cast iron having a fine structure can be manufactured in a short time.

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for manufacturing high-strength black-core malleable cast iron. [Prior Art] Conventionally, forged steel has been used as a high-strength material, but forging has higher manufacturing costs than casting, and it is difficult to manufacture products with complex shapes. Therefore, efforts are being made to improve the strength of cast iron, which can be manufactured easily and inexpensively even in complex shapes. In order to increase the strength of cast iron, for example, white pig iron is subjected to first and second stages of graphite dead annealing to obtain core malleable cast iron. Recently, in order to increase the strength, white pig iron is austenitized by holding it in a salt bath at 950°C for 4 to 8 hours, and austempering by holding it in a salt bath at 250 to 450°C for 1 hour. Austempering treatment of malleable cast iron to make the base into a bainite structure has been proposed (Uchiyama et al., Japan-Foundry Association 106th National Lecture Conference Abstracts, p. 16). [Problems to be Solved by the Invention] However, the surface of the above-mentioned core malleable cast iron is chilled even when it is as cast, but because the chill structure is rough, a large amount of graphite precipitates during graphitization treatment, and the strength is not sufficient. . Also,
As mentioned above, in malleable cast iron that has been subjected to austempering treatment, the chill structure of ordinary white pig iron is rough, so the graphite that precipitates in the subsequent graphitization treatment is large, and the bainite structure formed by austempering treatment is also rough. , the strength cannot be said to be sufficiently high. The present invention is intended to solve the above-mentioned problems, and an object of the present invention is to provide a method for manufacturing high-strength focused malleable cast iron. [Means for Solving the Problems] The high-strength focused malleable cast iron of the present invention rapidly heats part or all of the surface layer of white pig iron to melt it, chills it by rapid cooling, and then melts the surface layer of the white pig iron. It is characterized by holding the pig iron at 870 to 1050°C for 30 minutes or more to austenitize it, then rapidly cooling it, and holding it at 200 to 400°C for 10 minutes or more to perform isothermal transformation treatment. A method of rapidly heating and melting only the surface layer is as follows:
Tungsten gas arc can provide energy at high density.
), plasma jet torch, laser, etc. for a short period of time. Magnesium (Mg) may be added to the white pig iron used in the present invention in order to make the precipitated graphite spherical. [Function] Rapidly heats and melts part or all of the surface layer of white pig iron,
Since dithylation occurs by rapid cooling, part or all of the surface layer becomes a fine chilled structure. This chilled white pig iron is heated at 870 to 1050°C for 30 minutes.
By holding the temperature for more than a minute, the cementite in the fine chilled structure becomes fine graphite and the base becomes austenite. Next, it is rapidly cooled and held at 20G to 400°C for 10 minutes or more to perform isothermal transformation treatment, so that the υ base becomes a fine bainite structure. The reasons for this limitation are explained below. The austenitization process is to austenite the base and graphitize the cementite at the same time.
At temperatures below 0°C, graphitization is significantly slowed down and 1050
If the temperature exceeds ℃, the austenite grains and graphite will become coarse, so the treatment temperature was set to 870 to 1050'' (l).The treatment time was set to 50 minutes or more, since graphitization would not be sufficient if it was less than 30 minutes. Transformation treatment is to convert austenite in the matrix into bainite. If the treatment temperature is less than 200°C, the matrix becomes brittle, and if it exceeds 400°C, the bainite structure becomes rough and the strength decreases, so the treatment temperature is 200 to 400°C. If the treatment time is less than 10 minutes, sufficient bainite formation cannot be achieved and the material becomes brittle, so the treatment time was set to 10 minutes or more. [Example] The present invention will be explained by an example. Example 1 Carbon (C) 2%, silicon (Si) t3, manganese (M
n) α3, phosphorus (P) IllL01%, sulfur (S) α
The surface of white pig iron consisting of 0.8% iron (Fe) and the balance was melted to a depth of about 2n from the surface by scanning with a TIG. Since the TIG scans, the melted part radiates heat to other unheated parts and is rapidly cooled, becoming chilled, and the surface layer becomes a fine chilled structure. Next, 900”
(l) for 2 hours to austenitize and graphitize. Next, it was rapidly cooled to 575℃, and kept at that temperature for 1 hour to perform isothermal transformation treatment to convert the base to bainite. Using white pig iron with the same composition as that used in Example 1, an austempering treatment consisting of an austenitization treatment and a constant temperature transformation treatment was performed under the same conditions as in Example 1 without TIG treatment.Example 1 and Micrographs showing the metal structure of the surface layer of the focused malleable cast iron of Comparative Example 1 are shown in FIGS. 1 and 2, respectively. In Example 1, the graphite and bainite structures were finer than in Comparative Example 1. This is because the chilled structure became finer due to chilling.Also, because the chilled structure was finer, it was possible to graphitize in a relatively short time.Table 1 shows the tensile strength and fatigue limit of both. The fatigue limit was determined by the Ono rotary bending fatigue test using a smooth test piece. Table 1 Tensile strength and fatigue limit were 824 for Comparative Example 1, respectively.
f/in, 35 Qf/d, whereas in Example 1
are as large as 102 f/ld and 43 f/xi, respectively, and have excellent strength. This is because the metal structure of Example 1 is fine. Example 2 Using white pig iron having the same composition as that used in Example 1, the surface of the white pig pig was subjected to TIG treatment under the same conditions as in Example 1 to chill it. Next

[Austenitization was performed under the same conditions as in Example 1, rapidly cooled to 230° C., and maintained at that temperature for 2 hours to carry out a constant temperature transformation treatment to perform austempering]. As Comparative Example 2, austempering treatment was performed under the same conditions as Example 2 using white pig iron having the same composition as in Example 2 without TIG treatment. Microscopic photographs showing the metallographic structure of the surface layer of the black-core malleable cast iron of Example 2 and Comparative Example 20 are shown in FIGS. 3 and 4, respectively. In Example 2, the graphite and bainite structures were finer than in Comparative Example 2. This is because the chilled structure became finer due to chilling. In addition, since the chill structure was fine, graphitization could be achieved in a relatively short time. Table 2 shows the tensile strength and fatigue limit of Example 2 and Comparative Example 2. The fatigue limit was determined by the Ono rotary bending fatigue test using a smooth specimen. Table 2 The tensile strength and fatigue limit of Comparative Example 2 are 98 instant f/Id and 32 instant f/d, respectively, whereas Example 2 has a large and excellent tensile strength of 152 instant f/Id and 40 QiAII, respectively. There is. This is because the metal structure of Example 2 is fine. [Effects of the Invention] As described above, in the manufacturing method of the present invention, the white pig iron is re-chilled to make the chill structure fine and then subjected to auschipper treatment, so that the graphite and bainite structures become fine υ, resulting in high strength. Nausea malleable cast iron can be manufactured. Furthermore, since the chill structure is fine, it can be graphitized in a relatively short time. Therefore, manufacturing costs can be reduced. Furthermore, since the core malleable cast iron manufactured by the method of the present invention has high strength, it can be used as a substitute for forged steel to reduce costs and weight, and it is also possible to easily manufacture high-strength products even with complex shapes. can be manufactured.

[Brief explanation of drawings]

Figure 1 is a micrograph of the metal structure of the surface layer of the core malleable cast iron produced in Example 1 of the present invention, and Figure 2 is a micrograph of the metal structure of the surface layer of the core malleable cast iron produced in Comparative Example 1. , Figure 5 is a microscopic photograph of the metal structure of the surface layer of the core malleable cast iron manufactured in Example 2 of the present invention, and Figure 4 is a microscope photo of the metal structure of the surface layer of the core malleable cast iron manufactured in Comparative Example 2. Represents a photo. Patent applicant Toyota Motor Corporation No. 1114 3rd! ㅅ (100 cases) ・ρ, 2 :', (1001 sounds) 凉 4' (100 cases)

Claims (1)

[Claims] Part or all of the surface layer of the white piglet is melted by rapid heating and chilled by rapid cooling, and then the white pigtail is held at 870 to 1050°C for 30 minutes or more to undergo austenitization treatment, and then rapidly cooled. , a method for producing high-strength black-core malleable cast iron, characterized by holding it at 200 to 400° C. for 10 minutes or more to undergo isothermal transformation treatment.