Blackheart malleable cast iron and its manufacturing method

Abstract

A black heart malleable cast iron according to one embodiment of the present invention comprises a ferrite matrix and graphite aggregates contained in the matrix, while containing, in terms of the mass ratio, from 50 ppm to 100 ppm of boron and from 65 ppm to 200 ppm of nitrogen. With respect to this black heart malleable cast iron, the crystal grain size number of the matrix is from 8.0 to 10.0, said crystal grain size number being obtained by quantifying the grain size of the matrix by comparing the metal structure photograph thereof with the standard diagram of crystal grain sizes.

Description

Technical Field

[0001] The present disclosure relates to malleable cast iron and a method for manufacturing the same.

Background Art

[0002] Cast iron, which is a general term for iron-based materials, can be classified into flake graphite cast iron, nodular graphite cast iron, malleable cast iron, etc. according to the form of carbon present. Malleable cast iron can be further classified into white-heart malleable cast iron, black-heart malleable cast iron, pearlite malleable cast iron, etc.

[0003] The black-heart malleable cast iron according to the present disclosure, also called malleable cast iron, has a form in which graphite is dispersed in a matrix made of ferrite. Black-heart malleable cast iron is superior in mechanical strength compared to flake graphite cast iron and is also superior in toughness because the matrix is ferrite. Therefore, black-heart malleable cast iron is widely used as a material for constructing automotive parts and pipe fittings that require mechanical strength.

[0004] In flake graphite cast iron and nodular graphite cast iron, flake or nodular graphite (graphite) precipitates during the cooling process after casting. In contrast, in black-heart malleable cast iron, carbon in the cast iron obtained by cooling after casting exists in the form of cementite (Fe3C), which is a compound with iron. Thereafter, by heating and holding the cast iron at a temperature of 720°C or higher, cementite is decomposed and graphite precipitates. In this specification, the process of precipitating graphite by heat treatment is hereinafter referred to as "graphitization". Also in this specification, a product (intermediate product) after casting and before graphitization is referred to as "cast iron" or "cast iron before graphitization", and a material (including the final product) after graphitization is referred to as "black-heart malleable cast iron". The cementite contained in black-heart malleable cast iron is preferably as little as possible, for example, from the viewpoint of increasing mechanical strength.

[0005] The graphitization of cast iron in the manufacturing process of blackheart malleable cast iron takes an extremely long time. Graphitization consists of two stages: a first-stage graphitization that decomposes free cementite in austenite at temperatures above 900°C, and a second-stage graphitization that follows the first stage and decomposes cementite in pearlite at temperatures around 720°C. Both the first and second-stage graphitization processes involve the diffusion of carbon in the matrix and the precipitation of graphite, and therefore generally take several hours to tens of hours. This long graphitization process contributes to the increased manufacturing cost of blackheart malleable cast iron.

[0006] Various methods have been considered to shorten the time required for graphitization. For example, Patent Document 1 describes that the time required for graphitization can be shortened by adjusting the content of silicon, an element that promotes graphitization, to a higher amount than usual. Patent Document 2 describes that the time required for graphitization can be shortened compared to conventional methods by performing heat treatment at a low temperature range of 100°C to 400°C for at least 10 hours.

[0007] Patent Document 3, previously filed by the present applicant, describes a black-heart malleable cast iron containing at least one of bismuth and manganese, and aluminum and nitrogen, with a matrix grain size of 8.0 or higher and 10.0 or lower in grain size number, and a method for producing black-heart malleable cast iron, comprising a step of preheating the cast iron containing the above-mentioned additive elements at 275°C or higher and 425°C or lower before graphitizing. The black-heart malleable cast iron described in Patent Document 3 has fine matrix grains, so the time required for graphitization can be significantly reduced compared to the conventional technique. Furthermore, it is stated that if the cast iron is preheated before graphitization, a metallic structure with fine grains can be easily formed compared to when it is not preheated. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Special Publication No. 46-17421 [Patent Document 2] U.S. Patent No. 2,227,217 [Patent Document 3] International Publication No. 2018 / 180424 [Overview of the project] [Problems that the invention aims to solve]

[0009] In the method described in Patent Document 1 above, the silicon content that promotes graphitization is increased, so depending on the shape of the mold, the cooling rate immediately after casting, and other cooling conditions, a metallic structure called "mottle" with crystallized graphite (crystallized graphite) is likely to be formed during casting and the subsequent cooling process. The crystallized graphite formed during casting does not disappear with subsequent heat treatment and causes a decrease in the mechanical strength of blackheart malleable cast iron. In the method described in Patent Document 2 above, the time required for heat treatment, which is performed at a temperature lower than the temperature required for graphitization, is long, about 8 to 10 hours. For this reason, the total heat treatment time, including the newly performed heat treatment and the conventional graphitization, is not necessarily shorter.

[0010] In the method for producing blackheart malleable cast iron described in Patent Document 3 above, the preheating step is performed at a temperature of 275°C or higher and 425°C or lower. On the other hand, as mentioned above, graphitization is performed at a temperature of 720°C or higher. This presents a challenge, for example, when using a continuous heat treatment furnace for a large number of cast iron pieces, as it is difficult to perform graphitization, which has significantly different temperature conditions, at the same processing rate following preheating.

[0011] Furthermore, when processing a large number of cast iron pieces at once, or when processing large-sized cast iron, it is difficult to maintain a uniform temperature during preheating and process under the same temperature conditions. This presents a challenge in that the quality of blackheart malleable cast iron may not be stable.

[0012] This disclosure has been made in view of the above-mentioned issues, and aims to provide black-heart malleable cast iron and a method for producing the same, which can be graphitized in a short time without preheating, eliminate the risk of mottling during casting, and enable stable operation at low cost. [Means for solving the problem]

[0013] One aspect of the present invention (first embodiment) is: A black-heart malleable cast iron having a ferrite matrix and massive graphite contained in the matrix, It contains, by mass ratio, boron at 50 ppm or more and 100 ppm or less, and nitrogen at 65 ppm or more and 200 ppm or less. The grain size of the aforementioned matrix is ​​between 8.0 and 10.0, as determined by comparing a metallographic photograph with a standard grain size chart. It is black-heart malleable cast iron.

[0014] According to the first embodiment described above, when boron and nitrogen are included in predetermined amounts, the crystal grains of the matrix become finer compared to when they are not included, and a metal structure is more likely to be formed in which the crystal grain size of the matrix is ​​8.0 or higher and 10.0 or lower in grain size number.

[0015] Aspect 2 of the present invention is, When the boron content is B (ppm) and the nitrogen content is N (ppm) in mass ratio, the values ​​of B and N satisfy the following equation (1): This is the blackheart malleable cast iron described in Embodiment 1. In this preferred embodiment, the blackheart malleable cast iron according to this embodiment satisfies the following formula (1) when the boron content is B (ppm) and the nitrogen content is N (ppm). In this case, it is possible to effectively prevent a decrease in mechanical strength, especially elongation, due to the addition of excessive boron.

[0016] N≧1.3B-10 (1)

[0017] A third aspect of the present invention is: Further containing titanium, when the content of titanium is Ti (ppm), the content of boron is B (ppm), and the content of nitrogen is N (ppm) in terms of mass ratio, the values of Ti, B, and N satisfy the following formula (2): It is the black heart malleable cast iron according to Embodiment 1 or 2. N≧1.3B + 0.3Ti - 10 (2)

[0018] Embodiment 4 of the present invention is The massive graphite is dispersed and present at the grain boundaries of the matrix. It is the black heart malleable cast iron according to any one of Embodiments 1 to 3.

[0019] Embodiment 5 of the present invention is The average particle diameter of the massive graphite is 10 micrometers or more and 40 micrometers or less. It is the black heart malleable cast iron according to any one of Embodiments 1 to 4.

[0020] Embodiment 6 of the present invention is The number of particles of the massive graphite per square millimeter of cross-sectional area is 200 or more and 1200 or less. It is the black heart malleable cast iron according to any one of Embodiments 1 to 5.

[0021] Embodiment 7 of the present invention is Further containing carbon in a mass ratio of 2.0% or more and 3.4% or less, and silicon in a mass ratio of 0.5% or more and 2.0% or less, with the balance being iron and inevitable impurities. It is the black heart malleable cast iron according to any one of Embodiments 1 to 6.

[0022] Embodiment 8 of the present invention is Further containing manganese in a mass ratio of more than 0% and 1.0% or less. It is the black heart malleable cast iron according to any one of Embodiments 1 to 7.

[0023] Embodiment 9 (the second embodiment) of the present invention is A step of adjusting the component composition of the molten metal obtained by melting the raw materials, A process of obtaining cast iron by casting using molten metal with adjusted component composition, wherein the mass ratio of carbon is 2.0% or more and 3.4% or less, silicon is 0.5% or more and 2.0% or less, boron is 50 ppm or more and 100 ppm or less, nitrogen is 65 ppm or more and 200 ppm or less, and the remainder is iron and unavoidable impurities. The process includes a step of graphitizing the cast iron at a temperature exceeding 680°C. This is a method for manufacturing black-heart malleable cast iron.

[0024] Aspect 10 of the present invention is In the process of graphitizing the cast iron, the total time spent graphitizing the cast iron at a temperature exceeding 680°C is between 1 hour and 6 hours. This is a method for producing blackheart malleable cast iron as described in Embodiment 9.

[0025] Aspect 11 of the present invention is The step of adjusting the component composition of the molten metal is as follows: The process includes adding a nitrogen-containing compound to the first molten metal, followed by adding the second molten metal. This is a method for producing blackheart malleable cast iron as described in embodiment 9 or 10.

[0026] Aspect 12 of the present invention is: The step of adjusting the component composition of the molten metal is as follows: The process includes adding at least one of ferroboron and manganese nitride to the molten metal before casting to adjust the composition of the metal. A method for producing blackheart malleable cast iron according to any one of embodiments 9 to 11.

[0027] Aspect 13 of the present invention is The raw materials include at least one of cast iron or black-heart malleable cast iron produced using the method for producing black-heart malleable cast iron described in any of embodiments 9 to 12. A method for producing blackheart malleable cast iron according to any one of embodiments 9 to 12. [Effects of the Invention]

[0028] According to the blackheart malleable cast iron and its manufacturing method according to this embodiment, the crystal grains of the matrix can be refined without preheating. As a result, the distance traveled by carbon diffusion during the graphitization process is shortened, significantly reducing the time required for post-casting heat treatment. Furthermore, the quality of the blackheart malleable cast iron is stabilized, and the mechanical strength is improved due to the refinement of the crystal grains of the matrix. [Brief explanation of the drawing]

[0029] [Figure 1] This is an example of a metallographic photograph of blackheart malleable cast iron according to this embodiment. [Figure 2] This is an example of a metallographic photograph of blackheart malleable cast iron according to conventional technology. [Figure 3] This is an example of a metallographic photograph of cast iron before graphitization according to this embodiment. [Figure 4] This is an example of a metallographic photograph of cast iron before graphitization using conventional technology. [Modes for carrying out the invention]

[0030] Embodiments for carrying out the present invention will be described in detail below with reference to the figures and tables. However, the embodiments described herein are merely illustrative, and the embodiments for carrying out the present invention are not limited to those described herein.

[0031] <Metal structure> The microstructure of the blackheart malleable cast iron according to this embodiment will be described.

[0032] In the first embodiment of the present invention, the blackheart malleable cast iron has a ferrite matrix. In this specification, "ferrite" refers to the α phase in the iron-carbon equilibrium phase diagram. In this specification, "matrix" refers to the residual structure excluding graphite, which is the main phase or matrix that occupies most of the volume (area in cross-sectional observation) of the alloy. Specifically, for example, when the cross-section of a sample is polished and the metallic structure is observed under a microscope, if ferrite accounts for 80% or more of the area of ​​the total structure, then ferrite can be said to be the main phase or matrix that occupies most of the alloy, and corresponds to the matrix in this embodiment. The matrix after graphitization is complete is composed of ferrite that hardly dissolves carbon. Therefore, the blackheart malleable cast iron according to this embodiment has excellent toughness, similar to conventional blackheart malleable cast iron.

[0033] The black-heart malleable cast iron according to this embodiment has massive graphite contained in the matrix. In this specification, "massive graphite" refers to a precipitated phase consisting of graphite, in which a plurality of granular graphite particles aggregate to form massive aggregates. In this embodiment, "contained in the matrix" means that the massive graphite is contained in a form surrounded by the ferrite matrix, and does not mean that the graphite is solid-dissolved in the ferrite matrix.

[0034] The blackheart malleable cast iron according to this embodiment has a matrix grain size of 8.0 or greater and 10.0 or less, which is quantified by comparing a metallographic photograph with a standard grain size diagram. In this specification, "standard grain size diagram" refers to a set of drawings that represent the grain boundaries of a metallographic structure having various grain sizes. As the standard grain size diagram, we use "Annex B (Normative) Measurement of grain size - Standard grain size diagram" from Japanese Industrial Standard JIS G 0551 ("Steel - Microscopic test method for grain size", Japan Standards Association, revised January 21, 2013). The microscopic test method for grain size specified in this Japanese Industrial Standard is substantially the same as the test method specified in the ISO standard (ISO 643:2019). These standards specify a microscopic test method for measuring the grain size of ferrite or austenite in steel, but in this disclosure, this method is applied mutatis mutandis to the measurement of the grain size of the ferrite matrix in blackheart malleable cast iron.

[0035] In this specification, "grain size number" refers to the value of G calculated using the following formula (3), where m is the average number of crystal grains per square millimeter of cross-sectional area. In formula (3) below, G is a base-2 power exponent. For example, when m is 16, the grain size number G is 1. A smaller grain size number indicates a coarser crystal grain, while a larger grain size number indicates a finer crystal grain.

[0036] m = 8 × 2 G (3)

[0037] The comparison between a metallographic photograph and a standard grain size chart is performed by comparing a micrograph showing the metallographic structure of blackheart malleable cast iron with a standard grain size chart displayed at the same magnification, and visually identifying the grain size number in the standard grain size chart that has the closest grain size to the grain size shown in the micrograph. Unlike the method specified in the above standard, in this comparison, the bulk graphite portions included in the micrograph are ignored, and only the size of the grain boundaries of the ferrite matrix is ​​considered when comparing it with the standard grain size chart. In this specification, "metallographic photograph" is not limited to a micrograph of the metallographic structure printed on paper, but may also refer to image data obtained using a CCD camera installed in a metallographic microscope, etc.

[0038] In conventional black-heart malleable cast iron, lump graphite was not always located at the grain boundaries of the matrix, but was often located near the center, away from the grain boundaries, or even spanning multiple grain boundaries. Furthermore, the grain size of the matrix was often 7.5 or less. In such a microstructure, carbon atoms had to travel long distances by diffusion through the matrix before precipitating as lump graphite during the graphitization process, and in some cases, had to travel across multiple grains of the matrix. Consequently, the graphitization process took a long time, ranging from several hours to tens of hours, to complete.

[0039] On the other hand, in the black-heart malleable cast iron according to this embodiment, the grain size of the matrix in the final product, i.e., after graphitization is complete, is 8.0 or higher in grain size number, and the crystal grains of the matrix are finer than those of conventional black-heart malleable cast iron. In the production of black-heart malleable cast iron having such a metallic structure, carbon atoms can reach the position of the crystal grain boundary by diffusion, at most moving a distance from the center of the crystal grain of the refined matrix to the position of the crystal grain boundary, where they can precipitate as graphite.

[0040] Furthermore, the diffusion rate of carbon atoms at the grain boundaries of the matrix is ​​faster than the diffusion rate of carbon atoms within the crystal grains. In the manufacturing process of the black-heart malleable cast iron according to this embodiment, the supply of carbon atoms necessary for the precipitation and growth of massive graphite present at the grain boundaries of the matrix can be carried out at high speed via the grain boundaries of the matrix. In this way, by shortening the distance traveled by the diffusion of carbon atoms and making the grain boundaries available as diffusion pathways, the black-heart malleable cast iron according to this embodiment can significantly reduce the time required for graphitization compared to conventional techniques.

[0041] When the matrix grain size is 8.0 or higher in grain size number, the distance carbon atoms travel due to diffusion until graphite precipitates is short, thus shortening the graphitization time. The finer the matrix grain size, the better, and there is no upper limit to the grain size number. However, generally, the grain size number of the matrix grain size in blackheart malleable cast iron will never exceed 10.0, no matter how large it is. Therefore, in this embodiment, the matrix grain size of the blackheart malleable cast iron is set to a grain size number of 8.0 or higher and 10.0 or lower. The grain size number is preferably 8.5 or higher.

[0042] Using examples of metallographic images, the metallographic structure of the black-heart malleable cast iron according to this embodiment will be explained in comparison with the prior art. Figure 1 is an example of a metallographic image of the black-heart malleable cast iron according to this embodiment. Figure 2 is an example of a metallographic image of the black-heart malleable cast iron according to the prior art. The length of the scale bar in the lower right of the figures is 200 micrometers in both cases. In Figures 1 and 2, the light gray phase is the ferrite matrix. The black phase is the lumpy graphite contained in the matrix. The lines seen in the matrix are the grain boundaries of the matrix visualized by etching. The grain size of the matrix in Figure 1 is 9.0, which falls within the range of grain size numbers in this embodiment. The grain size of the matrix in Figure 2 is 7.0, which is coarser than that in Figure 1. In Figures 1 and 2, there appears to be no significant difference in the amount of lumpy graphite present. However, in the prior art, as shown in Figure 2, the lumpy graphite accumulates to form large lumps. In contrast, in this embodiment, as shown in Figure 1, the lumpy graphite is finely dispersed.

[0043] In a preferred embodiment, the black-heart malleable cast iron according to this embodiment has blob graphite dispersed at the grain boundaries of the matrix. In this specification, "blob graphite present at the grain boundaries of the matrix" means that, in the metal structure of the black-heart malleable cast iron as a final product, the blob graphite is present at the grain boundary between two ferrite grains of the matrix, and / or at the triple point of the grain boundary between three ferrite grains. It is rare for blob graphite to span multiple grain boundaries of the matrix. It is sufficient that the majority of the blob graphite is present at the grain boundaries of the matrix. For example, in a metallographic photograph illustrated in Figure 1, it is preferable that 70% or more of the total area of ​​blob graphite is present at the grain boundaries of the matrix as described above. The proportion of blob graphite present at the grain boundaries is more preferably 80% or more, even more preferably 90% or more, and most preferably 100%. In the preferred embodiments of the present invention described above, it is permissible for a small number of bulk graphite particles to be located near the center of the matrix grains, away from the grain boundaries of the matrix, or for a small number of bulk graphite particles to be located across four or more grain boundaries of the matrix.

[0044] Furthermore, in this specification, "bulk graphite is dispersed" means that the bulk graphite is not concentrated in the positions of some of the crystal grains in the matrix, but is evenly distributed across many of the crystal grains in the matrix. In other words, it means that in many of the crystal grains in the matrix, bulk graphite is present at the grain boundaries between that crystal grain and the surrounding crystal grains. Only a small number of crystal grains do not have bulk graphite at the grain boundaries. It is sufficient that bulk graphite is present in many of the crystal grains in the matrix. In the preferred embodiments of the present invention described above, it is acceptable for bulk graphite to be absent in a small number of crystal grains, or, if present, to be located near the center of the crystal grain rather than at the grain boundaries.

[0045] When precipitates are present at the grain boundaries of a matrix, interphase grain boundaries are formed between the matrix and the precipitates. Generally, the grain boundary energy of interphase grain boundaries is smaller than that of grain boundaries between the same phase. When small crystal grains in a matrix merge with larger crystal grains to undergo grain growth, movement of the grain boundaries is necessary. However, for grain boundaries to move away from the precipitates, new grain boundaries must be formed to replace the interphase grain boundaries, requiring more energy for grain boundary movement than in the case where precipitates are absent. As a result, the grain boundaries remain fixed at the positions of the precipitates, hindering grain growth. This effect is sometimes called the "pinning effect" of grain boundaries by precipitates.

[0046] In the blackheart malleable cast iron according to this embodiment, when lumpy graphite is dispersed at the grain boundaries of the matrix, grain growth of the matrix during the graphitization process is hindered by a pinning effect. This pinning effect occurs for almost all crystal grains. As a result, a metallic structure with a matrix grain size unique to the blackheart malleable cast iron according to this embodiment tends to be formed more easily.

[0047] The black-heart malleable cast iron according to this embodiment tends to have improved mechanical strength compared to black-heart malleable cast iron according to the prior art. Specifically, it tends to have improved tensile strength and elongation. The reason for this is not clear, but it is thought that this is related to the fact that the black-heart malleable cast iron according to this embodiment has a finer grain size in the ferrite matrix compared to black-heart malleable cast iron according to the prior art, and that the lump graphite is located at the grain boundaries of the matrix.

[0048] In a preferred embodiment, the black-core malleable cast iron according to this embodiment has an average particle diameter of lumpy graphite of 10 micrometers or more and 40 micrometers or less. When the average particle diameter of lumpy graphite is 10 micrometers or more, the number of lumpy graphite particles does not become too large and tends to be dispersed at the grain boundaries of the matrix. When the average particle diameter of lumpy graphite is 40 micrometers or less, the number of lumpy graphite particles does not become too small and the diffusion distance of carbon required for the growth of lumpy graphite does not become too long, so it tends to shorten the time required for graphitization. Therefore, it is preferable that the black-core malleable cast iron according to this embodiment has an average particle diameter of lumpy graphite of 10 micrometers or more and 40 micrometers or less. The average particle diameter of lumpy graphite is more preferably 12.0 micrometers or more, even more preferably 15.0 micrometers or more, more preferably 19.0 micrometers or less, even more preferably 18.5 micrometers or less, and even more preferably 18.0 micrometers or less.

[0049] In a preferred embodiment, the black-heart malleable cast iron according to this embodiment has 200 to 1200 lump graphite particles per square millimeter of cross-sectional area. Since the final volume of graphite contained in the black-heart malleable cast iron according to this embodiment is approximately constant, the larger the average particle diameter of the lump graphite, the fewer the number of particles, and the smaller the average particle diameter, the more particles there are. When the number of lump graphite particles is 200 or more, the diffusion distance of carbon required for the growth of lump graphite becomes shorter, and the time required for graphitization tends to be shortened. The more lump graphite particles there are, the better, and there is no upper limit to the number of particles. However, in a preferred embodiment of the present invention, the number of lump graphite particles per square millimeter of cross-sectional area that can be formed will not exceed 1200 at most. Therefore, it is preferable that the number of lump graphite particles per square millimeter of cross-sectional area be 200 to 1200. The number of lump graphite particles per square millimeter of cross-sectional area is more preferably 300 or more, even more preferably 500 or more, and may also be 1000 or less.

[0050] In this embodiment, the "average particle size of the lump graphite" and the "number of particles per square millimeter of cross-sectional area" are both determined based on the microstructure observed in any cross-section of the blackheart malleable cast iron. More specifically, as described in the examples below, the same micrograph showing the microstructure of the blackheart malleable cast iron used to identify the particle size number is used to digitize the image of the micrograph using a scanner or CCD camera, and the values ​​are determined by computer image analysis.

[0051] It should be noted that the grain size number, average grain size, and particle number mentioned above in the description of the blackheart malleable cast iron according to this embodiment are all values ​​measured for the metal structure of the blackheart malleable cast iron after the graphitization process is completed. The effects and benefits of this embodiment, such as suppression of grain growth and reduction of the time required for graphitization, mainly manifest during the intermediate stages of the graphitization process. However, it is difficult to numerically evaluate the metal structure at such intermediate stages of the process. Therefore, for convenience, the values ​​for the metal structure after the graphitization process is completed are used as substitutes.

[0052] <Component composition of black-heart malleable cast iron> The alloy composition (component composition) of the blackheart malleable cast iron according to this embodiment will be described below. In this specification, the content of each element is expressed in mass ratios. The symbol "ppm" means that the value to which this symbol is attached is a mass ratio expressed in parts per million. The symbol "%" means that the value to which this symbol is attached is a mass ratio expressed in percent.

[0053] In a preferred embodiment, the blackheart malleable cast iron according to this embodiment contains 2.0% to 3.4% carbon. When the carbon content is 2.0% or more, the melting point of the molten metal used for casting in the manufacturing process of blackheart malleable cast iron is 1400°C or lower, so it is not necessary to heat the raw materials to a high temperature to produce the molten metal, and large-scale melting equipment tends to be unnecessary. At the same time, the viscosity of the molten metal is also lower, so the molten metal flows more easily, and it tends to be easier to pour the molten metal into the casting mold. When the carbon content is 3.4% or less, molten metal tends to be less likely to form during casting and the subsequent cooling process. Therefore, it is preferable that the carbon content be 2.0% to 3.4%. A more preferable carbon content is 2.5% to 3.2%.

[0054] In a preferred embodiment, the blackheart malleable cast iron according to this embodiment contains 0.5% to 2.0% silicon. When the silicon content is 0.5% or more, the effect of promoting graphitization by silicon is obtained, and graphitization tends to be completed in a short time. When the silicon content is 2.0% or less, the effect of promoting graphitization by silicon is not excessive, and mottling tends to be less likely to occur during casting and the subsequent cooling process. Therefore, it is preferable that the silicon content be 0.5% to 2.0%. A more preferable silicon content is 1.0% to 1.7%.

[0055] The blackheart malleable cast iron according to this embodiment contains, by mass ratio, boron in an amount of 50 ppm or more and 100 ppm or less, and nitrogen in an amount of 65 ppm or more and 200 ppm or less. By simultaneously containing boron and nitrogen, the crystal grains of the ferrite matrix can be refined. When the boron content is 50 ppm or more, crystal grain refinement is more likely to occur in the presence of nitrogen. The higher the boron content, the finer the crystal grains become. The boron content is preferably 60 ppm or more. When the boron content is 100 ppm or less, a decrease in the strength and elongation of the blackheart malleable cast iron can be prevented. The boron content is preferably 90 ppm or less.

[0056] When the nitrogen content is 65 ppm or higher, grain refinement is more likely to occur in the presence of boron. The nitrogen content is preferably 80 ppm or higher. When the nitrogen content is 200 ppm or lower, graphitization is less likely to be inhibited. The nitrogen content is preferably 170 ppm or lower. In the prior art, it was believed that increasing soluble nitrogen significantly inhibited the progress of first-stage and second-stage graphitization. In this embodiment, conversely, by making nitrogen an essential element together with boron, the graphitization time was shortened.

[0057] In this embodiment, the following mechanism is considered to explain why the crystal grains of the ferrite matrix become finer when boron and nitrogen are included simultaneously. Note that the following mechanism is a hypothesis of the inventors based on the experimental results obtained and does not limit the technical scope of this disclosure.

[0058] When cast iron is heated to a temperature of 800°C to 900°C, the boron and nitrogen dissolved in the cast iron combine to form boron nitride. The boron nitride formed here is thought to be hexagonal boron nitride (h-BN), which is stable at atmospheric pressure. Hexagonal boron nitride has a crystal structure similar to graphite. Therefore, it is thought that during graphitization, this boron nitride acts as a nucleus, causing a large number of fine graphite particles to precipitate. As described in the explanation of the metal structure above, when graphite precipitates and grows into massive graphite, the pinning effect hinders the grain growth of the matrix. As a result, it is thought that a metal structure with a matrix grain size unique to the black-heart malleable cast iron according to this embodiment is more likely to form.

[0059] The boron content may also affect the microstructure of the cast iron before graphitization. This will be explained next. The microstructure of the cast iron before graphitization according to this embodiment has features not found in the microstructure of the cast iron according to the prior art. Figure 3 is an example of a photograph of the microstructure of the cast iron before graphitization according to this embodiment. The boron content is 68 ppm. Figure 4 is a photograph of the microstructure of the cast iron before graphitization according to the prior art. The boron content is 27 ppm. The nitrogen content is 110 ppm in both cast iron samples. The black areas in Figures 3 and 4 represent the phase in which austenite (γ-iron), which precipitated as the primary crystal during the casting process, transformed into pearlite during the subsequent cooling process. The light gray areas represent a phase called redebrite, which is a eutectic structure of austenite and cementite.

[0060] In the cast iron shown in Figure 4, the primary austenite forms a dendritic structure. In contrast, in the cast iron shown in Figure 3, which has a high boron content, the dendritic structure is finely fragmented and exists in a granular form. Furthermore, the redebrite region tends to be coarser in Figure 3 compared to Figure 4. The metal structure shown in Figure 3 is unique to the cast iron according to this embodiment and resembles the structure of white cast iron when the solidification rate is fast. The cast iron before graphitization with the component composition described in Patent Document 3 had the metal structure shown in Figure 4, and the metal structure shown in Figure 3 was not observed. Although the details are unclear, it is likely that these differences in the metal structure of cast iron due to the boron content have some influence on the subsequent refinement of the matrix after graphitization.

[0061] In a preferred embodiment, the blackheart malleable cast iron according to this embodiment satisfies the following formula (1) when the boron content is B (ppm) and the nitrogen content is N (ppm) by mass ratio.

[0062] N≧1.3B-10 (1)

[0063] When the amounts of nitrogen and boron in blackheart malleable cast iron satisfy equation (1), the resulting microstructure after graphitization, as illustrated in Figure 1, has a ferrite matrix and massive graphite contained within the matrix, and cementite tends to be less likely to remain. If the amount of boron exceeds the amount specified by equation (1), cementite tends to remain more easily at the grain boundaries of the matrix in the resulting microstructure, and the elongation of the blackheart malleable cast iron tends to decrease. The detailed reason for this is unknown, but it is thought that boron that did not participate in the formation of boron nitride inhibits the progress of graphitization, or that boron that is excessively dissolved in the matrix has an effect. The coefficient of 1.3 for boron content in equation (1) is equal to the value obtained by dividing the atomic weight of nitrogen (14) by the atomic weight of boron (10.8). The fixed value of 10 represents the amount of excess boron that is allowed relative to the stoichiometric ratio of boron nitride. The N-1.3B obtained by rearranging equation (1) may be, for example, greater than or equal to -7, greater than or equal to -5, greater than or equal to 0, or even less than or equal to 50, greater than or equal to 30.

[0064] In a preferred embodiment, when the blackheart malleable cast iron according to this embodiment further contains titanium, the values ​​of Ti, B, and N satisfy the following formula (2), where Ti (ppm) is the titanium content, B (ppm) is the boron content, and N (ppm) is the nitrogen content, in terms of mass ratio. When titanium is included, it is preferable to ensure good mechanical properties, particularly good elongation, by setting the boron and titanium content relative to nitrogen to satisfy the following formula (2). The N-1.3B-0.3Ti obtained by modifying formula (2) can be, for example, 100 or less, moreover 80 or less, and moreover 50 or less. N≧1.3B+0.3Ti-10 (2)

[0065] In a preferred embodiment, the blackheart malleable cast iron according to this embodiment satisfies the following formula (4) when the manganese content is Mn(%) and the sulfur content is S(%) by mass ratio. As shown in formula (4) below, it is thought that when (Mn-1.7S) is within the following range, manganese and sulfur, which are elements that inhibit graphitization, react to form manganese sulfide, and the graphitization process can be completed in a shorter time. 0.12≧(Mn-1.7S)≧0.35 (4)

[0066] The blackheart malleable cast iron according to this embodiment contains, in addition to the above elements, iron and unavoidable impurities as a remainder. One embodiment includes the above-mentioned carbon, silicon, boron, and nitrogen, with the remainder being iron and unavoidable impurities. Iron is the main element of blackheart malleable cast iron. In this disclosure, unavoidable impurities generally refer to impurities that are obvious to exist in the cast iron without intentional introduction during the manufacturing process to obtain the desired final cast iron product, and whose presence is unnecessary, but are present in trace amounts and do not necessarily adversely affect the properties of the cast iron, and are therefore left to exist. Specifically, trace metal elements such as chromium, sulfur (for example, when preparing molten metal in a cupola, the heat source coke contains sulfur), oxygen, and titanium that were originally contained in the raw materials, compounds such as oxides that are mixed in from the furnace wall during the manufacturing process, and compounds such as oxides that are produced by the reaction of molten metal with atmospheric gases are considered unavoidable impurities in this disclosure. These unavoidable impurities, even if present in a total of 1.0% by mass or less in blackheart malleable cast iron, do not significantly alter its properties. The preferred total content of unavoidable impurities is 0.5% by mass or less.

[0067] In this embodiment, it is permissible to include elements other than the above-mentioned carbon, silicon, boron, and nitrogen. In this case, the type and amount of elements included should be appropriately determined according to the desired properties, within a range that does not significantly hinder the expression of the effects in this embodiment. The elements other than the above-mentioned carbon, silicon, boron, and nitrogen may be added in the form of elements or compounds, or at least one of these forms. The type and amount of the elements and compounds can be appropriately selected according to the purpose of addition. However, it is preferable to determine the type and amount of elements to be added after confirming that they do not significantly hinder the expression of the effects in this embodiment. Manganese is a preferred element that can be included. For example, manganese may be included in a range of more than 0% and 1.0% or less. Furthermore, from the viewpoint of fully exhibiting the effects of boron and nitrogen, it is preferable to suppress the content of aluminum and titanium, which bond more easily with nitrogen than boron, as much as possible. For example, it is preferable to satisfy at least one of the following conditions by mass ratio: aluminum content of 0.10% or less and titanium content of 200 ppm or less.

[0068] The method for analyzing the carbon and sulfur content in blackheart malleable cast iron and cast iron is combustion-infrared absorption spectroscopy. Combustion-infrared absorption spectroscopy is preferred because it provides accurate analytical values ​​for carbon and sulfur. The method for analyzing the silicon, boron, manganese, titanium, and aluminum content in blackheart malleable cast iron and cast iron is inductively coupled plasma atomic emission spectroscopy. The method for analyzing the nitrogen content in blackheart malleable cast iron and cast iron is the ammonia distillation separation bis(1-phenyl-3-methyl-5-pyrazolone) (abbreviation: bispyrazolone) spectrophotometric method described in Annex 2 of Japanese Industrial Standard JIS G 1228 ("Iron and Steel - Method for Determination of Nitrogen", Japan Standards Association, revised August 20, 1997). The above methods for analyzing the amount of each element are also applicable to the analysis of molten metal obtained during the manufacturing process of blackheart malleable cast iron.

[0069] <Manufacturing method> A method for producing black-heart malleable cast iron according to this embodiment will be described.

[0070] In a second embodiment of the present invention, a method for producing blackheart malleable cast iron comprises the steps of: adjusting the component composition of molten metal obtained by melting raw materials; using the molten metal with the adjusted component composition, casting cast iron containing, by mass ratio, 2.0% to 3.4% or less of carbon, 0.5% to 2.0% or less of silicon, 50 ppm to 100 ppm or less of boron, 65 ppm to 200 ppm or less of nitrogen, with the remainder being iron and unavoidable impurities; and graphitizing the cast iron at a temperature exceeding 680°C. The content of each element specified herein represents the content contained in the final product after the casting and graphitization steps, as in the case of blackheart malleable cast iron according to this embodiment. The reasons for limiting the composition range of each element have already been stated, so an explanation is omitted here. In one embodiment, the composition of the cast iron may include, by mass ratio, 2.0% to 3.4% or less of carbon, 0.5% to 2.0% or less of silicon, 50 ppm to 100 ppm or less of boron, and 65 ppm to 200 ppm or less of nitrogen, with the remainder being iron and unavoidable impurities. The cast iron may further contain manganese in a mass ratio of more than 0% and 1.0% or less, similar to blackheart malleable cast iron. Furthermore, it is preferable that the content of aluminum and titanium in the cast iron is kept low, similar to blackheart malleable cast iron.

[0071] In the process of adjusting the composition of the molten metal obtained by dissolving raw materials, the content of carbon, silicon, boron, and nitrogen can be adjusted to the above range by adding metals or compounds to the molten metal, or by using raw materials that already contain the above elements, such as using steel scraps or recycled cast iron. Therefore, the raw materials used for casting to obtain cast iron may be individual elements of carbon, silicon, boron, nitrogen, and iron, or, for carbon, silicon, and boron, alloys of each element with iron, such as ferrocarbon, ferrosilicon, and ferroboron, may be used.

[0072] In this specification, "adding" the above-mentioned metals and / or compounds ("compounds, etc.") to the molten metal includes any of the following methods: adding the molten metal to the compound, etc., adding the compound, etc. to the molten metal, or adding the compound, etc. to the first molten metal described below, and then adding the second molten metal described below.

[0073] To add boron, a method can be used in which elemental boron and / or ferroboron are added to the molten metal. In a preferred embodiment, the method for producing black-heart malleable cast iron according to this embodiment includes a step of adjusting the component composition by adding at least one of ferroboron and manganese nitride to the molten metal before casting. Ferroboron is preferred because it dissolves more easily in molten metal than elemental boron. The proportion of boron contained in ferroboron is preferred because it is readily available in a mass ratio of 16% or more and 21% or less. In the method for producing black-heart malleable cast iron according to this embodiment, the molten metal before casting may contain boron close to the amount of boron in the black-heart malleable cast iron according to this embodiment, for example, 20 ppm or more and less than 50 ppm, or it may satisfy the amount of boron in the black-heart malleable cast iron according to this embodiment, as shown in the examples described later. A molten metal with a high boron content can be obtained, for example, by melting a material containing, as a raw material, at least one of cast iron produced using the method for manufacturing black-heart malleable cast iron according to this embodiment, and black-heart malleable cast iron, specifically, a material containing boron and nitrogen scrap generated in the implementation of the method for manufacturing black-heart malleable cast iron according to this embodiment. Once boron is added at the molten metal stage, its content hardly changes in subsequent processes.

[0074] To add nitrogen, methods such as bubbling nitrogen gas into the molten metal or adding nitrogen-containing compounds, which are compounds of nitrogen with other elements, can be used. For example, when an electric furnace is used to prepare the molten metal, the nitrogen content is often between 60 ppm and 70 ppm, so adjustment is usually necessary. As mentioned above, the method for analyzing the nitrogen content in blackheart malleable cast iron and cast iron is the ammonia distillation separation bis(1-phenyl-3-methyl-5-pyrazolone) (abbreviation: bispyrazolone) spectrophotometric method described in Annex 2 of Japanese Industrial Standard JIS G 1228 ("Iron and Steel - Method for Determination of Nitrogen", Japan Standards Association, revised August 20, 1997).

[0075] Among the methods for adding nitrogen, the method of bubbling nitrogen gas into the molten metal is preferred because it allows for the addition of nitrogen alone without altering other components. However, bubbling nitrogen may cause a rapid decrease in the temperature of the molten metal. Therefore, care must be taken to avoid bubbling too much nitrogen gas per unit time.

[0076] On the other hand, among the methods of adding nitrogen, the method of adding a compound of nitrogen with other elements (nitrogen-containing compound) is preferable to the method of bubbling nitrogen gas because it causes less of a temperature drop in the molten metal. Manganese nitride is preferred as the nitrogen-containing compound used for nitrogen addition because it has a relatively low melting point and is easy to add and mix. When a cupola is used to prepare the molten metal, as mentioned above, sulfur derived from the coke heat source is contained in the molten metal. Since sulfur is an element that inhibits graphitization, it is common practice to combine it with manganese to form manganese sulfide, which does not affect graphitization. In this case, ferromanganese is used for manganese addition. In this embodiment, when manganese nitride is used instead of ferromanganese, the manganese nitride decomposes in the molten metal, and the nitrogen is used to adjust the nitrogen content of the molten metal. Furthermore, manganese is preferable because it combines with sulfur to form manganese sulfide. However, if manganese nitride is added in an amount exceeding the amount that contributes to the formation of manganese sulfide, the excess manganese may inhibit graphitization, so care must be taken not to add too much manganese nitride.

[0077] As raw materials for iron, the aforementioned scrap steel can be used. Furthermore, the aforementioned cast iron can also be recycled. When using scrap steel as a raw material for iron, since carbon and silicon are already present in general steel materials, in many cases, these elements can be brought into the component composition range specified in this embodiment simply by dissolving the scrap steel. Scrap steel and recycled cast iron may contain large amounts of aluminum, titanium, etc., in addition to the carbon and silicon mentioned above. In this case, since aluminum, titanium, etc., react with nitrogen to form nitrides that are more stable than boron nitride, care must be taken to ensure that there is enough nitrogen necessary for the synthesis of boron nitride.

[0078] In a preferred embodiment, the method for producing blackheart malleable cast iron according to this embodiment includes, as described above, at least one of cast iron or blackheart malleable cast iron produced using the method for producing blackheart malleable cast iron according to this embodiment as raw materials used when melting the molten metal before casting. Here, "cast iron" refers to the product after casting but before graphitization, as described above. Cast iron and blackheart malleable cast iron include not only the product but also parts other than the product, such as sprues and runners, and so-called scrap such as products that have been found to be defective as a result of inspection.

[0079] The scrap (containing at least one of cast iron or black-heart malleable cast iron) generated by the manufacturing method of black-heart malleable cast iron according to this embodiment contains boron and nitrogen. If the majority of the boron and nitrogen present in the scrap were hexagonal boron nitride, there was concern as to whether it would be possible to melt and reuse it, since the melting point of hexagonal boron nitride is approximately 3000°C. Furthermore, it is known that graphitization is inhibited when boron is present in black-heart malleable cast iron above a certain amount. Therefore, there was concern that the boron contained in the scrap might inhibit graphitization. However, in the steel industry, it is known that boron nitride in steel decomposes into nitrogen and boron at a temperature sufficiently lower than the melting point of the hexagonal boron nitride mentioned above, and it is believed that the same applies to boron nitride in cast iron. Furthermore, according to the inventors' studies, it was found that even when raw materials consisting only of scrap are melted, graphitization is not inhibited if the boron and nitrogen content is properly controlled as shown in this embodiment, and it is possible to manufacture black-heart malleable cast iron according to this embodiment.

[0080] To prepare the molten metal by melting the raw materials, known melting equipment (melting furnaces) such as cupolas or electric furnaces can be used. In the method for producing black-heart malleable cast iron according to this embodiment, the carbon content is 2.0% by mass or more, so the temperature required for melting does not exceed 1400°C. Therefore, large-scale melting equipment with a reachable temperature exceeding 1400°C is not required.

[0081] One process for obtaining cast iron involves, for example, melting the raw materials in the melting furnace to prepare molten metal, then pouring the molten metal into a ladle, adjusting the composition and temperature of the molten metal in the ladle, and finally pouring the molten metal from the ladle into a mold to perform casting. As mentioned above, the composition can be adjusted in the ladle, or it can be adjusted by the raw materials used.

[0082] In the step of adjusting the component composition of the molten metal, when adjusting the amount of nitrogen in the molten metal by adding a nitrogen-containing compound such as manganese nitride to the molten metal, it is preferable to include a step of adding the nitrogen-containing compound to the first molten metal and then adding the second molten metal. According to this configuration, the decomposition and volatilization of the nitrogen-containing compound (preferably manganese nitride) when it comes into contact with the molten metal can be suppressed, and the amount of nitrogen in the molten metal can be easily adjusted. For example, when adjusting the component composition of the molten metal in a ladle, the remaining molten metal in the ladle after pouring into the mold the previous time can be used as the first molten metal, and a nitrogen-containing compound can be added to the first molten metal, and then molten metal dispensed from the melting furnace can be added as the second molten metal. The first molten metal may be the remaining molten metal in the ladle as described above, or it may be the unadjusted molten metal that was first dispensed from the melting furnace into the ladle. When the latter unadjusted molten metal is used as the first molten metal, the step of adding the nitrogen-containing compound to the first molten metal and then adding the second molten metal can be omitted.

[0083] To further enhance the effect of suppressing the decomposition of the nitrogen-containing compounds, it is preferable that the temperature of the first molten metal is lower than that of the second molten metal. For example, if the first molten metal is residual molten metal, its temperature can be in the range of approximately 1320 to 1350°C, and if the second molten metal is molten metal dispensed from a melting furnace, its temperature can be in the range of approximately 1460 to 1500°C. Furthermore, it is preferable that the mass of the first molten metal is less than that of the second molten metal. Having a smaller mass in the first molten metal makes it easier to achieve the temperature relationship between the first and second molten metals described above. For example, the amount of the first molten metal can be set to be 1% or more and less than 50% of the total molten metal by mass ratio.

[0084] The method for producing blackheart malleable cast iron according to this embodiment includes a step of obtaining cast iron by casting. In the manufacturing method according to this embodiment, known molds such as molded mold sand or metal molds can be used for casting. After casting, the cast iron is removed by dismantling the cooled mold. The cast iron is further separated into product parts and non-product parts such as sprues and runners. The non-product parts can be remelted as scrap.

[0085] The method for producing blackheart malleable cast iron according to this embodiment includes a step of graphitizing the cast iron at a temperature exceeding 680°C after casting. In the manufacturing method according to this embodiment, known heat treatment furnaces such as gas combustion furnaces and electric furnaces can be used as means for graphitizing.

[0086] Graphitization is a process unique to the manufacturing method of blackheart malleable cast iron. In the graphitization process, the cast iron obtained by casting is heated to a temperature exceeding 680°C and further exceeding 720°C, which corresponds to the A1 transformation point, thereby decomposing cementite and precipitating graphite. At the same time, the austenite matrix is ​​cooled to transform into ferrite, thereby imparting toughness to the cast iron. The graphitization process of cast iron is divided into a first-stage graphitization performed first and a second-stage graphitization performed after the first-stage graphitization. Preferably, the graphitization process includes a first-stage graphitization performed at a temperature exceeding 900°C and a second-stage graphitization with a starting temperature of 720°C or higher and 800°C or lower, and a completion temperature of 680°C or higher and 720°C or lower.

[0087] The first stage of graphitization is a process in which cementite in austenite is decomposed at a temperature range exceeding 900°C to precipitate graphite. In the first stage of graphitization, the carbon generated by the decomposition of cementite contributes to the growth of massive graphite. The temperature for performing the first stage of graphitization is preferably between 950°C and 1100°C. A more preferable temperature range is between 980°C and 1030°C.

[0088] In the method for producing blackheart malleable cast iron according to this embodiment, the time required for the first-stage graphitization can be significantly reduced compared to the conventional method due to the effects of this embodiment. The actual time can be appropriately determined depending on the size of the annealing furnace, the amount of cast iron to be processed, etc. While the time required for the first-stage graphitization in the conventional method ranged from several hours to tens of hours, in this embodiment it is sufficient at most 3 hours, typically less than 1 hour, and under certain conditions it can be completed in more than 30 minutes but less than 45 minutes.

[0089] The second-stage graphitization is a process in which cementite in pearlite is decomposed at a lower temperature than that used for the first-stage graphitization, thereby precipitating graphite and ferrite. To promote the growth of lump graphite and ensure the transformation from austenite to ferrite, it is preferable to perform the second-stage graphitization by gradually decreasing the temperature from the start temperature to the completion temperature. The average cooling rate from the start temperature to the completion temperature is more preferably 1.5°C / min or less, and even more preferably 1.0°C / min or less. From the viewpoint of lump graphite growth and transformation to ferrite, a slower average cooling rate is preferable, but from the viewpoint of ensuring productivity, the lower limit of the average cooling rate should be around 0.20°C / min.

[0090] The starting temperature for the second stage of graphitization is preferably 720°C or higher and 800°C or lower. A more preferred temperature range for the starting temperature of the second stage of graphitization is 740°C or higher and 780°C or lower. The completion temperature for the second stage of graphitization is preferably 680°C or higher and 720°C or lower, and is lower than the starting temperature of the second stage of graphitization. A more preferred temperature range for the completion temperature of the second stage of graphitization is 690°C or higher and 710°C or lower.

[0091] In the method for producing black-heart malleable cast iron according to this embodiment, the time required for the second-stage graphitization can be significantly reduced compared to the conventional method due to the effects of this embodiment. The actual time can be appropriately determined depending on the size of the annealing furnace, the amount of cast iron to be processed, etc. In the conventional method, the time required for the second-stage graphitization was several hours to tens of hours, similar to the first-stage graphitization, whereas in this embodiment, it is sufficient at most 3 hours, typically less than 1 hour, and under certain conditions, it can be completed in more than 30 minutes but less than 45 minutes.

[0092] In a preferred embodiment, the method for producing black-heart malleable cast iron according to this embodiment involves a graphitization step in which the cast iron is graphitized at a temperature exceeding 680°C for a total of 1 hour or more and 6 hours or less. In this specification, "time to graphitize cast iron at a temperature exceeding 680°C" refers to the sum of the time spent holding the cast iron at the first-stage graphitization temperature and the time spent holding it at the second-stage graphitization temperature. The total graphitization time is preferably 5 hours or less, and more preferably 3 hours or less. The above time is the time from when the temperature near the center of the cast iron reaches the above temperature range.

[0093] The method for producing black-heart malleable cast iron according to this embodiment is a method for producing black-heart malleable cast iron that possesses the above-described chemical composition (component composition) and metallic structure. The black-heart malleable cast iron produced by the method for producing black-heart malleable cast iron according to this embodiment, in particular the black-heart malleable cast iron after the graphitization process, has a ferrite matrix and lumpy graphite contained in the matrix, contains the above-described amounts of boron and nitrogen, and the grain size of the matrix is ​​8.0 or more and 10.0 or less in terms of grain size number, which is quantified by comparing a metallic photograph with a standard grain size diagram. Furthermore, in a preferred embodiment, the average particle diameter of the lumpy graphite is 10 micrometers or more and 40 micrometers or less.

[0094] <Other> This section describes the influence of the alloy composition (component composition) and manufacturing method on the microstructure of the blackheart malleable cast iron according to this embodiment.

[0095] The blackheart malleable cast iron according to this embodiment has a ferrite matrix and lumpy graphite contained in the matrix, and is characterized by its composition, containing boron in a mass ratio of 50 ppm to 100 ppm and nitrogen in a mass ratio of 65 ppm to 200 ppm. Furthermore, the matrix is ​​characterized by its metal structure, with a grain size number of 8.0 to 10.0, which is quantified by comparing a metallographic photograph with a standard grain size diagram. These characteristics are the minimum necessary to specify the first embodiment.

[0096] In order to produce blackheart malleable cast iron with the above characteristics, the manufacturing method must include a step of graphitizing the cast iron at a temperature exceeding 680°C. This condition is necessary for the implementation of this embodiment. In this embodiment, it is not necessary to perform the preheating step, which is performed at 275°C or higher and 425°C or lower, as described in Patent Document 3. Therefore, the time required for heat treatment can be shortened and manufacturing costs can be reduced compared to conventional technologies that require preheating. However, in this embodiment, heat treatment of cast iron at a temperature below 680°C is not excluded. [Examples]

[0097] <First Example> In the first embodiment, the effect of the amount of boron added to molten metal containing a certain amount of nitrogen on the microstructure and mechanical strength of blackheart malleable cast iron was investigated.

[0098] A molten metal mixture containing 3.0% carbon, 1.5% silicon, and the remainder being iron and unavoidable impurities by mass ratio was poured into a 700 kg ladle. Ferroboron containing 18% boron was added in six levels ranging from 56 g to 336 g, and after stirring, the mixture was immediately poured into a mold and cast to obtain six types of cast iron with different boron content. In addition to the above amounts of carbon and silicon, the obtained cast iron contained 110 ppm of nitrogen derived from the raw materials and 0.35% of manganese derived from the raw materials.

[0099] Next, the cast iron obtained by casting was heated from room temperature to 980°C over 45 minutes and held for 1 hour to perform the first stage of graphitization. Subsequently, the sample temperature was cooled to 760°C, and then the second stage of graphitization was performed while cooling from 760°C to 720°C over 1 hour, to prepare six types of black-heart malleable cast iron samples, No. 1 to 6. The carbon content of the prepared samples was analyzed by combustion-infrared absorption, and the silicon, boron, and manganese content was analyzed by inductively coupled plasma atomic emission spectrometry. All samples contained, by mass ratio, 3.0% carbon, 1.5% silicon, and 0.35% manganese derived from the iron raw material. The nitrogen content was measured using the ammonia distillation separation bis(1-phenyl-3-methyl-5-pyrazolone) (abbreviation: bispyrazolone) spectrophotometric method described in Annex 2 of Japanese Industrial Standard JIS G 1228 ("Iron and Steel - Method for Determination of Nitrogen," Japan Standards Association, revised August 20, 1997). The results of the analysis of boron and nitrogen, and the values ​​obtained by substituting the analysis results into N-1.3B derived from formula (1), are shown in Table 1. However, in Table 1, the estimated boron content of samples No. 2 and 3, estimated from the amount of ferroboron added, is indicated in parentheses.

[0100] [Table 1]

[0101] Next, the cut surfaces of the prepared samples were polished, and the grain boundaries were etched with nital. The metallographic structure of the cut surfaces was then observed using an optical microscope, and metallographic images were taken with a CCD camera mounted on the optical microscope. An example of the metallographic image of sample No. 4 is shown in Figure 1, and an example of the metallographic image of sample No. 1 is shown in Figure 2. In samples No. 1 through 6, the area ratio of ferrite in the metallographic structure was 80% or more in all cases.

[0102] Next, the grain size of the ferrite matrix was measured by comparing the captured metallographic images with the "Annex B (Normative) Measurement of Grain Size - Standard Grain Size Diagram" of the Japanese Industrial Standard JIS G 0551 ("Steel - Microscopic Test Method for Grain Size", Japan Standards Association, revised January 21, 2013). In the comparison, the massive graphite portions included in the metallographic images were ignored, and only the size of the grain boundaries of the ferrite matrix was considered. In addition, the presence or absence of residual cementite was determined from the metallographic images. Furthermore, test pieces for tensile testing were prepared from the obtained samples, and the strength (tensile strength) and elongation of the test pieces were measured using a tensile testing machine. These evaluation results are summarized in Table 1.

[0103] According to Table 1, samples No. 3 to 6 all met the boron and nitrogen content ranges according to this embodiment. Samples No. 1 and 2 had a boron content less than the lower limit of the composition range, which is 50 ppm. Furthermore, regarding the grain size of the ferrite matrix, samples No. 3 to 6 all met the grain size number ranges according to this embodiment. Samples No. 1 and 2 had grain size numbers smaller than the lower limit of 8.0, indicating a coarser grain size compared to samples No. 3 to 6. In other words, samples No. 3 to 6 listed in Table 1 correspond to the examples of black-heart malleable cast iron and its manufacturing method according to this embodiment.

[0104] From the first embodiment described above, it can be seen that, according to the black-heart malleable cast iron and its manufacturing method according to this embodiment, the graphitization time can be significantly reduced compared to the conventional technology, even without performing the preheating described in Patent Document 3.

[0105] As illustrated in Figure 1, in the metallographic structure of blackheart malleable cast iron samples No. 3 to 6, which correspond to embodiments of the present invention, many lumpy graphite particles were located at the grain boundary between two ferrite grains in the matrix, and / or at the triple point of the grain boundary between three ferrite grains. Lumpy graphite particles rarely spanned four or more grain boundaries in the matrix.

[0106] Furthermore, the bulk graphite was not concentrated in certain grains of the matrix, but rather evenly distributed across many grains of the matrix. In many grains of the matrix, bulk graphite was present at the grain boundaries between that grain and the surrounding grains, and only a few grains lacked bulk graphite at the grain boundaries. In other words, the bulk graphite was dispersed at the grain boundaries of the matrix.

[0107] In contrast, as illustrated in Figure 2 for sample No. 1, the metallographic structure of black-heart malleable cast iron samples No. 1 and 2, in which the boron content was less than the lower limit of the composition range of 50 ppm, showed that many massive graphite particles formed large lumps, and some of these massive graphite particles spanned four or more grain boundaries in the matrix. Furthermore, many massive graphite particles were concentrated in certain grain positions within the matrix, and numerous grains were observed where massive graphite particles were absent at grain boundaries.

[0108] In sample No. 3, one of the examples corresponding to the present invention, residual cementite was observed in the metal structure, suggesting that graphitization was not completely finished. No residual cementite was observed in sample No. 4, which had a higher boron content than sample No. 3. Small amounts of residual cementite were again observed in samples No. 5 and 6, which had even higher boron content. This is thought to be because graphitization was inhibited due to the excessive boron content relative to the nitrogen content.

[0109] Regarding the mechanical strength of samples corresponding to the embodiments of the present invention, as shown in Table 1, the strength and elongation were at their maximum values ​​in sample No. 4. Even in samples No. 3 and 5, where cementite residue was observed, the strength and elongation did not differ significantly from those of sample No. 4. As mentioned above, the improvement in strength and elongation is thought to be related to the finer grain size of the ferrite matrix. In sample No. 6, the mechanical strength decreased, and the elongation in particular decreased significantly compared to the other samples. The decrease in elongation in sample No. 6 cannot be explained solely by the residue of cementite. It is thought that an increase in the amount of boron dissolved in the matrix may have had an effect.

[0110] As described above, in the preferred embodiment, the blackheart malleable cast iron according to this embodiment satisfies formula (1) when the boron content is B (ppm) and the nitrogen content is N (ppm) by mass ratio. For sample No. 6 of the first example, the value on the right side of formula (1) is 120, which is greater than the nitrogen content of 110 on the left side, so formula (1) is not satisfied. For sample No. 5, the value on the right side of formula (1) is 107, which is slightly less than the nitrogen content of 110 on the left side. In other words, samples No. 3 to 5 correspond to the preferred embodiment of the present invention, and it can be said that the decrease in elongation caused by excess boron is suppressed in these samples.

[0111] <Second Example> In the second example, the effect of extending the graphitization time compared to the first example was investigated. The same cast iron samples as those used in the first example (samples No. 3 to 6) were heated from room temperature to 980°C over 1 hour and 10 minutes, and held for 1 hour and 20 minutes to perform the first stage of graphitization. Subsequently, the cast iron was cooled to 760°C, and then the second stage of graphitization was performed while cooling from 760°C to 720°C over 1 hour and 20 minutes to produce four types of black-heart malleable cast iron samples (samples No. 8 to 11). The prepared samples were evaluated in the same manner as in the first example. The evaluation results are summarized in Table 2.

[0112] [Table 2]

[0113] According to Table 2, samples No. 8 to 11, which satisfy both the boron and nitrogen content ranges according to this embodiment, also satisfy all of the particle size ranges according to this embodiment. In other words, samples No. 8 to 11 listed in Table 2 all correspond to examples of black-heart malleable cast iron and its manufacturing method according to this embodiment.

[0114] Furthermore, when comparing the results in Tables 1 and 2 for samples with the same boron and nitrogen content, the latter, which underwent a longer graphitization time, tended to have slightly larger grain size numbers in the ferrite matrix, as well as slightly improved strength and elongation. This is thought to be because the longer heating time from room temperature to the first-stage graphitization temperature of 980°C promoted the formation of boron nitride, resulting in finer ferrite grains.

[0115] On the other hand, a comparison of sample No. 11 with the other samples shows that, from the viewpoint of ensuring good elongation, it is preferable for the boron and nitrogen content to satisfy equation (1). Furthermore, from the results in Table 2 above, it can be seen that satisfying equation (1) is more effective in suppressing the decrease in elongation than extending the graphitization time in the manufacturing process to suppress the remaining cementite.

[0116] <Third Example> In the third embodiment, we investigated methods for adjusting the nitrogen content and whether the cast iron and blackheart malleable cast iron scraps according to this embodiment could be reused as raw materials. The target component composition of the sample was 3.0% carbon, 1.5% silicon, 70 ppm boron, and 160 ppm nitrogen by mass ratio. An electric furnace was used to melt the raw materials, and the mass of raw materials used for one melting was 50 kg.

[0117] For sample No. 12 of the third example, an alloy manufactured in an electric furnace was used as the raw material for melting. The boron content in the alloy was 33 ppm and the nitrogen content was 76 ppm. 50 kg of this alloy was melted, and in order to adjust the composition of the resulting molten metal, 16 g of ferroboron with a boron content of 16%, 100 g of manganese nitride with a nitrogen content of 28%, and 15 g of elemental bismuth were added to the molten metal. After stirring, the molten metal was immediately poured into a mold and cast to obtain cast iron. The obtained cast iron was graphitized under the same conditions as in the first example to obtain black-heart malleable cast iron of sample No. 12. The obtained black-heart malleable cast iron was evaluated in the same manner as in the first example.

[0118] For sample No. 13 of the third example, pre-graphitization cast iron (recycled cast iron) produced by the method for manufacturing black-heart malleable cast iron according to this embodiment was used as the raw material for melting. The boron content in the recycled cast iron was 69 ppm and the nitrogen content was 87 ppm. 50 kg of this recycled cast iron was melted, and in order to adjust the composition of the resulting molten metal, 100 g of manganese nitride with a nitrogen content of 28% and 15 g of elemental bismuth were added to the resulting molten metal. After stirring, the molten metal was immediately poured into a mold and cast to obtain cast iron. The obtained cast iron was graphitized under the same conditions as in the first example to obtain black-heart malleable cast iron sample No. 13. The obtained black-heart malleable cast iron was evaluated in the same manner as in the first example.

[0119] In the third example, sample No. 14 used black-heart malleable cast iron (recycled black-heart malleable cast iron) produced by the method for producing black-heart malleable cast iron according to this embodiment as the raw material for melting. The boron content in the recycled black-heart malleable cast iron was 68 ppm, and the nitrogen content was 82 ppm. In addition, the carbon content of the recycled black-heart malleable cast iron had decreased to 2.70% due to decarburization during the graphitization process. 50 kg of this recycled black-heart malleable cast iron was melted, and to adjust the composition of the resulting molten metal, graphite powder was added to increase the carbon content to 3.04%. Then, 100 g of manganese nitride with a nitrogen content of 28% and 15 g of elemental bismuth were added to the resulting molten metal and stirred. After stirring, the molten metal was immediately poured into a mold and cast iron was obtained by casting. The obtained cast iron was graphitized under the same conditions as in the first example to obtain black-heart malleable cast iron sample No. 14. The obtained blackheart malleable cast iron was then evaluated in the same manner as in the first example. Table 3 summarizes the carbon, boron, and nitrogen content (in units of mass ratio in % or ppm) and the evaluation results of the blackheart malleable cast iron for samples No. 12 to 14 before and after compositional adjustment.

[0120] [Table 3]

[0121] According to Table 3, the blackheart malleable cast iron samples No. 12 to 14 all met the boron and nitrogen content ranges specified in this embodiment after adjusting the composition, as well as the particle size range specified in this embodiment. In other words, samples No. 12 to 14 listed in Table 3 all correspond to examples of blackheart malleable cast iron and its manufacturing method according to this embodiment. Furthermore, no cementite residue was observed in any of the samples, and the mechanical strength was comparable to that of the first and second embodiments. The results of the third embodiment showed that manganese nitride can be used to adjust the nitrogen content. It was also found that even when cast iron and blackheart malleable cast iron scrap (recycled cast iron) according to this embodiment are melted and reused as molten metal, the effects of this disclosure can be achieved without any problems.

[0122] <Fourth Example> In the fourth example, the balance of elements other than nitrogen and boron was investigated. Molten metal formulated to contain 3.0% carbon, 1.5% silicon, and the remainder being iron and unavoidable impurities was poured into a 700 kg ladle. In sample No. 15 of the fourth example, 1400 g of an alloy containing 5% nitrogen, 70% manganese, and the remainder being iron, along with 135 g of ferroboron containing 18% boron, were added to the molten metal poured into the ladle and then stirred. After taking a sample to analyze the component composition of the molten metal, it was immediately poured into a mold and cast iron was obtained by casting.

[0123] In the fourth example, sample No. 16, 1100g of an alloy containing 10% nitrogen and 90% manganese, and 160g of ferroboron containing 18% boron were added to the molten metal poured into a ladle, and then the mixture was stirred. After taking a sample to analyze the component composition of the molten metal, it was immediately poured into a mold and cast iron was obtained by casting.

[0124] In the fourth example, for sample No. 17, 1400g of an alloy consisting of 5% nitrogen, 70% manganese, and the remainder being iron, along with 200g of ferroboron containing 18% boron, were added to the molten metal poured into a ladle and then stirred. After taking a sample for analysis of the molten metal's composition, the molten metal was immediately poured into a mold and cast iron was obtained by casting. Table 4 shows the results of the analysis of the composition of samples taken from the molten metals of samples No. 15 to 17, in the same manner as in the first example. The sulfur content was analyzed by combustion-infrared absorption spectroscopy.

[0125] [Table 4]

[0126] According to Table 4, the boron and nitrogen content in the molten metal components of samples No. 15 to 17 of the fourth example all fall within the range of this embodiment. From this, it can be inferred that the components of the blackheart malleable cast iron after graphitization also fall within the range of this embodiment. Furthermore, these samples contain manganese contained in the added alloy, sulfur contained in the coke used as a heat source, and trace amounts of titanium contained in the raw material iron.

[0127] Next, the cast iron obtained by casting was subjected to first-stage graphitization and second-stage graphitization under the same conditions as in the first example. The metal structure and mechanical strength of the samples after graphitization were observed and evaluated using the same method as in the first example. The evaluation results are shown in Table 5. In addition, Table 5 also shows the calculated values ​​of N-1.3B, N-1.3B-0.3Ti, and Mn-1.7S from the molten metal component analysis values ​​shown in Table 4, where nitrogen content is N (ppm), boron content is B (ppm), titanium content is Ti (ppm), manganese content is Mn (%), and sulfur content is S (%), in terms of mass ratio. Furthermore, the presence or absence of crystallized graphite when the fracture surface of the cast iron before graphitization was observed is also shown in Table 5.

[0128] [Table 5]

[0129] According to Table 5, in sample No. 15, no crystallized graphite was observed in the microstructure of the fracture surface after casting, and no cementite was observed in the microstructure after graphitization. Note that "crystallized graphite" refers to graphite that crystallizes when the molten metal cools inside the mold, as previously mentioned, and is distinct from the lumpy graphite that precipitates in the solid phase due to graphitization, such as the graphitized microstructure shown in Figure 1. Furthermore, the mechanical strength is comparable to that of other examples. This is thought to be because manganese and sulfur, elements that inhibit graphitization, react to form manganese sulfide, and the graphitization process was completed in a short time. On the other hand, in samples No. 16 and 17, crystallized graphite was observed in the fracture surface after casting, and a small amount of cementite was observed in the microstructure after graphitization. Furthermore, a comparison of sample No. 15 with samples No. 16 and 17 reveals that, from the viewpoint of reliably ensuring good mechanical strength, especially high elongation, it is preferable to satisfy equation (2) so that the amount of boron is not excessive relative to the nitrogen and titanium content.

[0130] <Fifth Example> In the fifth embodiment, one method of incorporating nitrogen was investigated: the addition of manganese nitride. In the fourth embodiment, manganese nitride was pre-set at the bottom of the ladle for all samples, and the molten metal was poured in and mixed. In contrast, in the fifth embodiment, 1100 g of an alloy containing 10% nitrogen and 90% manganese, and 135 g of ferroboron containing 18% boron were added to the surface of 90 kg of molten metal in the ladle without added manganese nitride, etc. (the remaining molten metal in the ladle after pouring into the mold in the previous instance, the first molten metal), and then mixed. The mixture was then held for the "holding time after adjusting the component composition" shown in Table 6 below. After that, the second molten metal was added to the ladle until the total weight of the molten metal reached 700 kg, and then stirred. Samples were taken to analyze the components of the molten metal. The manganese, nitrogen, and boron content was analyzed using the collected samples. The results of the analysis are shown in Table 6. Note that in Table 6 and Table 7 below, "original molten metal" refers to molten metal without the addition of manganese nitride, etc.

[0131] [Table 6]

[0132] According to Table 6, the above addition method resulted in manganese and nitrogen content equivalent to that of the fourth example. Furthermore, even when the second molten metal was added after holding it for 10 minutes following the addition of manganese nitride, no significant decrease in the content of each component was observed.

[0133] Next, 1100g of an alloy containing 10% nitrogen and 90% manganese, and 135g of ferroboron containing 18% boron were added to the surface of the remaining 50kg of molten metal in the ladle (the remaining molten metal in the ladle after the previous pouring into the mold, the first molten metal) and mixed. After letting it stand for 5 minutes, the second molten metal was poured into the ladle until the total weight of the molten metal reached 700kg, and the mixture was held for the "Holding time after adding the second molten metal" shown in Table 7 below. After that, samples were taken to analyze the components of the molten metal, and the manganese, nitrogen, and boron content of the samples was analyzed. The results of the analysis are shown in Table 7.

[0134] [Table 7]

[0135] According to Table 7, even after adding manganese nitride to a small amount of molten metal and then adding a second amount of molten metal, and letting it stand for a while, no significant decrease in the content of each component was observed. This method shows that even in continuous operation, it is not necessary to set manganese nitride at the bottom of the ladle, and it can be added to the surface of the molten metal. Furthermore, even after holding the molten metal with manganese nitride and ferroboron added for 10 minutes, there was no significant change in the composition of the molten metal, indicating that it does not interfere with casting.

[0136] The disclosures herein include the following aspects described in Japanese Patent Application No. 2020-038913, which forms the basis of the priority claim. (Aspect a1) A black-heart malleable cast iron having a ferrite matrix and massive graphite contained in the matrix, It contains, by mass ratio, boron at 50 ppm or more and 100 ppm or less, and nitrogen at 65 ppm or more and 170 ppm or less. The grain size of the aforementioned matrix is ​​between 8.0 and 10.0, as determined by a grain size number quantified by comparing a metallographic photograph with a standard grain size diagram. Black-heart malleable cast iron. (Aspect a2) When the boron content is B (ppm) and the nitrogen content is N (ppm), the values ​​of B and N satisfy the following equation (1): Blackheart malleable cast iron as described in embodiment a1. N≧1.3B-10 (1) (Aspect a3) The aforementioned lumpy graphite is dispersed at the grain boundaries of the matrix. Blackheart malleable cast iron according to either embodiment a1 or a2. (Aspect a4) The average particle size of the aforementioned lump graphite is 10 micrometers or more and 40 micrometers or less. Blackheart malleable cast iron according to any one of embodiments a1 to a3. (Aspect a5) The number of the aforementioned lump graphite particles per 1.00 square millimeter of cross-sectional area is between 200 and 1200. Blackheart malleable cast iron according to any one of embodiments a1 to a4. (Aspect a6) By mass, it contains 2.0% or more carbon and 3.4% or less, 0.5% or more silicon and 2.0% or less, with the remainder being iron and unavoidable impurities. Blackheart malleable cast iron according to any one of embodiments a1 to a5. (Aspect a7) A process of casting a casting containing, by mass ratio, 2.0% or more and 3.4% or less of carbon, 0.5% or more and 2.0% or less of silicon, 50 ppm or more and 100 ppm or less of boron, 65 ppm or more and 170 ppm or less of nitrogen, with the remainder being iron and unavoidable impurities. The process includes a step of graphitizing the casting at a temperature exceeding 680°C. A method for manufacturing black-heart malleable cast iron. (Aspect a8) In the graphitization process, the total time spent graphitizing the casting at a temperature exceeding 680°C is between 1 hour and 6 hours. A method for producing blackheart malleable cast iron as described in embodiment a7. (Aspect a9) The process involves adding at least one of ferroboron and manganese nitride to the molten metal before casting to adjust its composition. A method for producing blackheart malleable cast iron according to either embodiment a7 or a8. (Aspect a10) The raw materials used to melt the molten metal before casting include at least one of a casting or black-heart malleable cast iron produced using the method for producing black-heart malleable cast iron described in any of embodiments a7 to a9. A method for producing blackheart malleable cast iron as described in embodiment a9.

[0137] This application is based on a Japanese patent application, Japanese Patent Application No. 2020-038913, filed on March 6, 2020. Japanese Patent Application No. 2020-038913 is incorporated herein by reference.

Claims

1. A black-heart malleable cast iron having a ferrite matrix and massive graphite contained in the matrix, It contains, by mass ratio, boron in amounts of 50 ppm or more and 100 ppm or less, and nitrogen in amounts of 65 ppm or more and 200 ppm or less, and further contains, by mass ratio, carbon in amounts of 2.0% or more and 3.4% or less, and silicon in amounts of 0.5% or more and 2.0% or less, with the remainder being iron and unavoidable impurities. When the boron content is B (ppm) and the nitrogen content is N (ppm) in mass ratio, the values ​​of B and N satisfy the following equation (1): The grain size of the matrix is ​​between 8.0 and 10.0, as determined by comparing a metallographic photograph with a standard grain size chart. Black-heart malleable cast iron. N≧1.3B-10 (1)

2. The material further contains titanium, and when the mass ratio of titanium content is Ti (ppm), the boron content is B (ppm), and the nitrogen content is N (ppm), the values ​​of Ti, B, and N satisfy the following equation (2): Blackheart malleable cast iron according to claim 1. N≧1.3B+0.3Ti-10 (2)

3. The aforementioned bulk graphite is dispersed at the grain boundaries of the matrix. Blackheart malleable cast iron according to claim 1 or 2.

4. The average particle size of the lump graphite is 10 micrometers or more and 40 micrometers or less. Blackheart malleable cast iron according to any one of claims 1 to 3.

5. The number of the aforementioned lump-shaped graphite particles per square millimeter of cross-sectional area is 200 or more and 1200 or less. Blackheart malleable cast iron according to any one of claims 1 to 4.

6. It also contains manganese in a mass ratio of more than 0% and 1.0% or less. Blackheart malleable cast iron according to any one of claims 1 to 5.

7. A process of adjusting the component composition of the molten metal obtained by dissolving the raw materials, A process of obtaining cast iron by casting using molten metal with an adjusted composition, in which, by mass ratio, carbon is 2.0% or more and 3.4% or less, silicon is 0.5% or more and 2.0% or less, boron is 50 ppm or more and 100 ppm or less, and nitrogen is 65 ppm or more and 200 ppm or less, with the remainder being iron and unavoidable impurities, and when the boron content is B (ppm) and the nitrogen content is N (ppm), the values ​​of B and N satisfy the following formula (1): The process includes a step of graphitizing the cast iron at a temperature exceeding 680°C. A method for manufacturing black-heart malleable cast iron. N≧1.3B-10 (1)

8. In the process of graphitizing the cast iron, the total time spent graphitizing the cast iron at a temperature exceeding 680°C is between 1 hour and 6 hours. The method for producing blackheart malleable cast iron according to claim 7.

9. The step of adjusting the component composition of the molten metal is as follows: The process includes adding a nitrogen-containing compound to the first molten metal, followed by adding the second molten metal. A method for producing blackheart malleable cast iron according to claim 7 or 8.

10. The step of adjusting the component composition of the molten metal is as follows: The process includes adding at least one of ferroboron and manganese nitride to the molten metal before casting to adjust the composition of the metal. A method for producing blackheart malleable cast iron according to any one of claims 7 to 9.

11. The raw materials include at least one of cast iron or black-heart malleable cast iron produced using the method for producing black-heart malleable cast iron described in any one of claims 7 to 10. A method for producing blackheart malleable cast iron according to any one of claims 7 to 10.

12. The method for producing blackheart malleable cast iron according to any one of claims 7 to 11, wherein the cast iron further contains titanium, and when the mass ratio of titanium content is Ti (ppm), the boron content is B (ppm), and the nitrogen content is N (ppm), the values ​​of Ti, B, and N satisfy the following formula (2). N≧1.3B+0.3Ti-10 (2)

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