JP6019850B2 - Method of heat treatment of malleable cast iron and method of manufacturing casting - Google Patents

Description

  The present invention relates to a heat treatment method for malleable cast iron.

  Conventionally, castings made of this type of black core malleable cast iron are used for cast iron pipe joints such as gas pipes, for example. The cast iron pipe joint is, for example, an L-shaped joint for connecting two straight pipes arranged in a perpendicular direction, and has a galvanized layer on the surface thereof. Such a cast iron pipe joint is manufactured by the following process (Patent Document 1). That is, after producing a casting with a sand mold using a cast iron material, an annealing treatment is performed. Thereafter, it is manufactured by performing a shot blasting process at room temperature to remove the oxide film formed on the surface of the casting material, and then performing galvanization.

JP 5-98463

In the above method, in order to produce a good galvanized layer, it is necessary to remove the oxide film of the casting material after the annealing treatment. For this reason, in the conventional technique, a shot blast process is performed on the casting material, and then the oxide film is removed by pickling. However, since the shot blasting is a method for removing the oxide film by hitting a minute ball on the surface of the casting, a residual stress layer is easily generated on the surface of the casting. Such a residual stress layer has a problem that an iron / zinc alloy layer is excessively developed at the time of hot dip galvanizing treatment, the fluidity of the zinc phase is deteriorated, and the plating film tends to be uneven.
Another problem is that the zinc film becomes thicker than necessary, so that much zinc is consumed.

  In addition, as a means for suppressing the formation of an oxide film on the surface of the casting during the annealing process, a vacuum furnace that evacuates the atmosphere in which the casting is placed, or an inert gas such as nitrogen gas is charged in the furnace. A method using an atmospheric furnace for reducing the oxygen concentration is also known. However, these means have a problem that the facilities become large.

  SUMMARY An advantage of some aspects of the invention is to solve at least a part of the problems described above, and the invention can be implemented as the following forms or application examples.

(1) According to one form of this invention, the heat processing method of malleable cast iron is provided. This heat treatment method of malleable cast iron is one process for producing a cast in which a plating layer is formed on the surface of a cast material made of malleable cast iron. In the heat treatment method of malleable cast iron after casting the cast material, Using a heat treatment container that has a loading chamber that is airtight with respect to the outside air and is heated in a heating furnace, the casting material is charged into the loading chamber of the heat treatment container and annealed. A heat treatment method for malleable cast iron, characterized by comprising a cooling step of cooling at a cooling rate of 100 ° C./hour to 5000 ° C./hour until a temperature not higher than ° C. is reached. By this method, an oxide film with cracks and distortion that can be easily pickled is formed on the surface of the cast material after annealing, that is, an oxide film that hinders the formation of the plating layer can be suppressed, and a good plating layer can be formed. A casting material that can be produced can be obtained.

(2) The cooling rate may be 200 ° C./hour to 5000 ° C./hour.
(3) The cooling rate when the casting material is cooled from 575 ° C. to 300 ° C. or less may be 100 ° C./hour to 5000 ° C./hour.
(4) The cooling treatment may be a treatment performed by heat dissipation when the casting material is taken out from the heat treatment container.
(5) The cooling process may be a process of cooling from the outside of the heat treatment container.
(6) According to another aspect of the present invention, it includes a heat treatment method of the malleable iron, after the cooling step, the surface of the casting material comprises facilities to process the galvanized layer after pickling, casting A manufacturing method is provided.
(7) In the manufacturing method of the casting, the surface of the casting material in which the zinc-plated layer is laminated may be identical to the crystal structure of the interior of the template material material.

It is sectional drawing which shows a cast iron pipe joint roughly. It is explanatory drawing explaining a series of manufacturing processes of a cast iron pipe joint. It is explanatory drawing explaining the container for heat processing. It is explanatory drawing explaining the heat processing of a casting material. It is explanatory drawing which expands and shows the location of the cooling process of FIG. It is explanatory drawing explaining the relationship between the cooling rate of a sample, and an oxide film. It is explanatory drawing explaining the state of the zinc plating layer corresponding to a cooling rate. It is explanatory drawing explaining the method of calculating | requiring a plating defective area.

Hereinafter, a case where an embodiment of the present invention is applied to a cast iron pipe joint will be described.
(1) Outline of Cast Iron Pipe Joint 10 FIG. 1 is a cross-sectional view schematically showing the cast iron pipe joint 10. The cast iron pipe joint 10 is, for example, an L-shaped joint for connecting two straight pipes (not shown) arranged in a right angle direction. The joint main body 12 and a plating layer 14 applied to the surface of the joint main body 12. And. The joint body 12 is made of black core malleable cast iron, and is further manufactured through an annealing process. The wall thickness of the joint body 12 is 4 mm to 40 mm. The plated layer 14 is formed by immersing the joint body 12 in a molten zinc bath to form an alloy layer of iron and zinc and a zinc film. The thickness of the plating layer 14 is 80 μm to 200 μm.

(2) Heat Treatment of Cast Iron Pipe Joint 10 FIG. 2 is an explanatory diagram for explaining a series of manufacturing steps of the cast iron pipe joint 10. The cast iron pipe joint 10 is manufactured by sequentially performing each process of FIG. That is, this process includes a casting process, an annealing process, a pickling process, a flux process, a galvanizing process, and the like.

  The casting process is a normal process for producing a casting made of black core malleable cast iron. As a component of black core malleable cast iron, for example, an iron alloy containing C: 2.0 wt% to 3.3 wt% and Si: 0.9 wt% to 1.8 wt% is cast into this sand mold. Demold. Thereby, casting material 12A (Drawing 1) used as joint body 12 can be manufactured.

  The annealing process is a process in which the casting material 12A manufactured by the casting process is loaded into a heat treatment container and subjected to heat treatment in a heating furnace (not shown). Here, the heating furnace is a type of open-air furnace that is heat-treated in a vacuum atmosphere or an atmosphere not filled with an inert gas. FIG. 3 is an explanatory view illustrating the heat treatment container 20. The heat treatment container 20 is a pot-shaped container including a container main body 22 having a loading chamber 22S and a lid 24 that is mounted on the upper portion of the container main body 22 and is airtight with respect to the outside air. The annealing process is performed by charging the casting chamber 12S of the heat treatment container 20 with a large number of casting materials 12A after casting and heating the casting materials 12A in a heating furnace while being hermetically held by the lid 24. . By making the loading chamber 22S airtight, the heat treatment container 20 is prevented from intruding oxygen outside the heat treatment container 20 into the heat treatment container 20 during cooling after annealing, and the casting material 12A. Can be suppressed.

  FIG. 4 is an explanatory view for explaining the heat treatment of the casting material 12A. In FIG. 4, the heat treatment in the annealing process includes a temperature raising process (time t0 to t1), a first annealing process (time t1 to t2), a rapid cooling process (time t2 to t3), and a second annealing process (time t3). To t4) and a cooling process (after time t4). 4 is preferably measured on the surface of the casting material 12A. However, the temperature of the casting material 12A is substantially equal to the temperature of the heat treatment container and the heating furnace. Can be used.

  The temperature raising process is a process of raising the casting material 12A in the heat treatment container 20 from the normal temperature to the temperature of the first annealing process in a heating furnace. By the heating process, carbon is dissolved in the iron of the casting material 12A, and the transformation to austenite proceeds.

  The first annealing treatment is a treatment for dissolving a component such as carbon in iron to make the component concentration uniform by a diffusion phenomenon. For example, the casting material 12A is treated at a temperature range of 920 ° C. to 1000 ° C. Heat treatment for 0 hours to 25.0 hours. By the first annealing treatment, carbon in the casting material 12A diffuses in the casting material 12A, and the concentration in the casting becomes uniform.

  The second annealing treatment is a treatment that rapidly cools after the first annealing treatment to make pearlite, and then cementite in the pearlite is decomposed into ferrite and graphite, thereby causing the cast material 12A to be malleable. In this treatment, for example, the casting material 12A subjected to the first annealing treatment is heat-treated at a temperature range of 730 ° C. to 760 ° C. for 2.5 hours to 35.0 hours. The second annealing process is performed while gradually cooling so as to gradually lower the temperature.

  FIG. 5 is an explanatory diagram showing an enlarged portion of the cooling process of FIG. The cooling process is a process of cooling the temperature of the casting material 12A immediately after the second annealing process to 300 ° C. or less. For example, FIG. 5 shows a case where the casting material 12A is cooled at six cooling rates Cs1 to Cs6 of the samples S1 to S6. Such a cooling rate is obtained by using a machine such as a crane or a robot hand, and immediately after the annealing process is completed, the casting material 12A loaded in the heat treatment container is taken out from the heat treatment container and allowed to cool to the atmosphere, or the heat treatment is performed. It can be set by applying a fan, mist, etc. from the outside of the container. Due to the cooling process, different oxide films are generated on the surface of the casting material 12A due to the difference in cooling rate. The cooling rate, the form of the oxide film, and the evaluation thereof will be described later.

  In FIG. 2, the pickling process, the flux process, and the plating process are sequentially performed as normal processes after the cooling process. The pickling process is a process for removing the oxide on the surface of the casting material 12A with an acidic solution. For example, the casting material 12A is immersed in a mixed solution of hydrochloric acid and hydrofluoric acid for 5 to 60 minutes. Through the pickling process, the oxide film generated on the surface of the casting material 12A can be removed.

  Flux treatment means (a) prevention of formation of a substance that inhibits alloying of zinc and iron during plating treatment (temporary rust prevention), and (b) “a small amount of oxide that could not be removed by acid treatment” In order to perform “removal during plating”, a flux film is formed on the surface of the casting material. The flux treatment is performed by immersing the casting material in a mixed aqueous solution of ammonium chloride and zinc chloride for a predetermined time. The formed flux film on the surface of the material has the effect of (a) temporary rust prevention, that is, the formation of oxide on the surface of the casting material before plating. Furthermore, (b) when the casting material is immersed in a zinc bath during the plating process, the flux film and the oxide on the iron surface become a black melt by a chemical reaction due to heat and are removed from the surface of the material. . Thus, by removing the substance that inhibits alloying, the iron base on the surface of the casting material becomes active, which contributes to the promotion of alloying between zinc and iron during the plating process.

  The galvanizing step is a step of immersing the surface of the casting material 12A in a molten zinc bath to form a zinc film of an iron-zinc alloy layer on the surface of the casting material 12A. For example, the casting material 12A is converted into a zinc solution in a bathtub. And dipping for 40 to 60 seconds. A zinc plating layer of 80 μm to 200 μm is generated on the casting material 12A by the galvanizing process. Thereafter, the cast iron pipe joint 10 of FIG. 1 is obtained by washing with water and further drying.

(3) Evaluation of Cooling Process (3) -1 Evaluation of Oxide Film Next, the relationship between the cooling rates Cs1 to Cs6 of the samples S1 to S6 in the cooling process of FIG. 5 and the oxide film on the surface of the casting material 12A will be described. To do. When the casting material 12A is cooled after the second annealing treatment, an oxide film is generated on the surface of the casting material 12A due to oxygen contained in the atmosphere in which the casting material 12A is placed. There are mainly three types of oxides of iron-based materials, hematite (Fe ₂ O ₃ ), magnetite (Fe ₃ O ₄ ), and wustite (FeO), which transform according to temperature. Therefore, when the iron-based material immediately after the second annealing treatment is cooled, for example, from a temperature of 680 ° C. to 760 ° C., first, wustite is generated, and at 575 ° C. or less, the wustite is transformed into magnetite, and 300 ° C. The transformation stops. When cooling to the transformation completion temperature at a cooling rate equal to or higher than the transformation rate, not all of the wustite is transformed into magnetite, and a lot of wustite remains. However, in order to leave much wustite, it must be cooled at a very fast cooling rate (for example, 50 ° C./second to 100 ° C./second). In this example, immediately after the second annealing treatment, the thickness of the oxide film formed can be suppressed by cooling at a somewhat high cooling rate (100 ° C./hour to 3000 ° C./hour), and oxidation at the stage of wustite. An object can be easily cracked or distorted. Thereafter, even if the wustite transforms into magnetite, the cracks and strains formed at the stage of wustite are maintained, and the magnetite has many cracks and strains.

  In FIG. 5, the cooling rate of each sample is set as follows. That is, the cooling rate of the sample S1 is 3000 ° C./hour or more. This cooling rate can be set by taking out the casting material loaded in the heat treatment container from the heat treatment container and allowing it to cool to the atmosphere using a machine such as a crane or a robot hand. The cooling rates of the samples S2 to S5 are set to 200 ° C./hour, 180 ° C./hour, 125 ° C./hour, and 75 ° C./hour, respectively. Such a cooling rate can be set by applying a fan, mist or the like from the outside of the heat treatment container. The cooling rate of the sample S6 is 10 ° C./hour, which corresponds to the conventional technique, that is, a value obtained when the heat treatment container is taken out of the heating furnace and the casting material is naturally cooled in the heat treatment container. .

  FIG. 6 is an explanatory diagram for explaining the relationship between the cooling rate of the sample and the oxide film. In FIG. 6, the degree of sealing of the heat treatment container means that the lid 24 of the heat treatment container 20 shown in FIG. 3 is slightly opened (medium), slightly opened (minimal), and sealed (sealed). It means the case of setting each. In this experiment, the thickness of the oxide film corresponding to the cooling rate is obtained according to the degree of sealing. Here, the reason for examining the action and effect of the cooling rate in accordance with the degree of sealing of the heat treatment container 20 is as follows. If the loading chamber 22S of the heat treatment container is sealed with respect to the outside air, oxygen in the furnace does not enter the heat treatment container after the second annealing process, and the formation of an oxide film is suppressed. Therefore, the sealing degree of the loading chamber 22S was evaluated with respect to the thickness of the oxide film. In addition, the acid treatment removal degree is an evaluation of the workability using the acid amount as a parameter, the length of time for removing the oxide film by the pickling process, ◎ is good, ○ is slightly good, and x is unsuitable. Is shown.

As can be seen from FIG. 6, the evaluation of the oxide film described below could be obtained by changing the cooling rate.
a. It has been found that as the cooling rate increases, the oxide film becomes thinner and the removability of the acid treatment improves. In particular, when the sample S1 was 3000 ° C./hour, the oxide film was 1 μm or less under any condition, and the workability in the pickling process was also good.
b. When the sealing degree of the heat treatment container is increased, oxygen in the furnace does not enter the heat treatment container, so that the oxide film generated after the completion of the second annealing process becomes thin, and the oxide film is used in the pickling process. Workability was also good.

(3) -2 Evaluation of Plating Layer Next, the galvanized layer generated on the casting material 12A was examined. FIG. 7 is an explanatory diagram for explaining the state of the galvanized layer corresponding to the cooling rate. Here, the vertical axis represents the ratio (%) of the defective plating area, and the horizontal axis represents the cooling rate. FIG. 8 is an explanatory diagram for explaining a method for obtaining a plating defect area. The defective plating area was determined by the following method. The casting material after the cooling treatment is galvanized, and further, the casting material 12A is divided in half, a photograph is taken under a certain condition, the plating defective area Ad is obtained in the measurement area At, and the total of the defective areas is obtained. Was calculated. Then, the ratio of the defective area was determined by the ratio of the defective plating area Ad to the measurement area At.

  As can be seen from FIG. 7, by changing the cooling rate, that is, when the cooling rate is 200 ° C./hour or less, the defective area of plating is large, but when it exceeds 200 ° C., the defective area rapidly decreases. I understood that. This is presumed to be because many strains or cracks were generated in the generated oxide film, and it was easily peeled off by the pickling treatment.

(4) Functions and effects of the embodiment (4) -1 When the temperature of the casting material is 575 ° C. or higher, that is, in the stable region of wustite, and the cooling rate exceeds 100 ° C./hour, the initial stage of cooling Cracks and strains occur in the resulting wustite layer. Such cracks and strains in the wustite layer remain as cracks and strains in the magnetite layer even when the wustite is transformed into magnetite. A magnetite layer with many cracks and strains can be easily peeled off and removed by pickling because acid penetrates from the cracks and the mechanical strength of the layer itself is also small. Therefore, an oxide film (particularly a magnetite layer) that hinders the formation of a plating layer can be easily removed by pickling on the surface of the casting material, and a good plating layer can be obtained. Thus, in order to remove the oxide film (magnetite layer) by pickling in order to obtain a good plating layer, a cooling rate of 100 ° C./hour or more is necessary as can be seen from FIGS. Preferably, it is 200 ° C./hour or more. Further, as can be seen from FIG. 6, immediately after the completion of the heat treatment, for example, when a large cooling rate of 3000 ° C./hour is given, the temperature range in which the oxide film is generated passes very quickly. This is more desirable because almost no oxide film is generated, the oxide film can be completely removed by acid treatment, and the acid treatment time can be greatly shortened.

(4) -2 As shown in FIGS. 3 and 6, the loading chamber 22 </ b> S of the heat treatment container 20 increases the hermeticity, so that the oxygen outside the heat treatment container during the cooling process after the second annealing treatment is increased. Can be prevented from entering the heat treatment container, thereby suppressing the oxidation of the surface of the casting material. Therefore, even if the cooling rate is 100 ° C./hour, the thickness of the oxide film can be reduced and a good plating layer can be obtained.

(4) -3 According to the present heat treatment method, the oxide film that prevents the formation of a good plating layer can be suppressed only by setting the cooling rate of the casting material after the second annealing treatment. Such a mechanical process such as shot blasting for removing the oxide after the heat treatment is also unnecessary. That is, since the shot blasting process is not performed, the surface of the casting material is not work-hardened by the shot blasting process, and the structure of the surface of the casting material under the plated layer after plating is the crystal structure inside the casting material. It becomes the same casting material. Therefore, it is possible to cope with the trouble caused by the shot blasting treatment, that is, the complicated shape that is difficult to hit the shot ball, and the fluidity of the zinc phase of the plated alloy film is deteriorated by the stress of the cast iron material generated by the removal treatment. Without any problem, the generated plating film becomes uniform. Furthermore, the zinc coating does not develop excessively and the amount of zinc used does not increase.

(4) -4 In the above embodiment, since a heat treatment vessel is used as the casting material annealing step, there is no need to use a vacuum furnace or an atmospheric furnace as described in the prior art, and the equipment is simple. is there.

(5) Modifications In the above embodiment, the casting is applied to a cast iron pipe joint. However, the present invention is not limited thereto, and may be applied to various castings in which an oxide film hinders the formation of a plating layer after annealing. it can.

  The present invention is not limited to the above-described embodiments, examples, and modifications, and can be realized with various configurations without departing from the spirit thereof. For example, the technical features in the embodiments, examples, and modifications corresponding to the technical features in each embodiment described in the summary section of the invention are to solve some or all of the above-described problems, or In order to achieve part or all of the above effects, replacement or combination can be performed as appropriate. Further, if the technical feature is not described as essential in the present specification, it can be deleted as appropriate.

DESCRIPTION OF SYMBOLS 10 ... Cast iron pipe joint 12 ... Joint main body 12A ... Casting material 14 ... Plating layer 20 ... Container for heat treatment 22 ... Container main body 22S ... Loading chamber 24 ... Cover

Claims (7)

In the heat treatment method for malleable cast iron after casting the cast material, it is one process for producing a cast with a plating layer formed on the surface of the cast material made of malleable cast iron.
It has a loading chamber that is airtight against the outside air, and uses a heat treatment container that is heated in a heating furnace,
A cooling step of cooling at a cooling rate of 100 ° C./hour to 5000 ° C./hour until the casting material reaches a temperature of 575 ° C. or less after the casting material is charged in the loading chamber of the heat treatment container and annealed. A heat treatment method for malleable cast iron, characterized by comprising: In the heat processing method of the malleable cast iron according to claim 1,
The said cooling rate is a heat processing method of malleable cast iron which is 200 degreeC / hour-5000 degreeC / hour. In the heat treatment method of the malleable cast iron according to claim 1 or claim 2,
The heat processing method of malleable cast iron whose cooling rate when cooling the said casting raw material from 575 degreeC to 300 degrees C or less is 100 degreeC / hour-5000 degreeC / hour. In the heat treatment method of malleable cast iron according to any one of claims 1 to 3,
The heat treatment method for malleable cast iron, wherein the cooling treatment is a treatment performed by heat dissipation when the cast material is taken out from the heat treatment vessel. In the heat treatment method of malleable cast iron according to any one of claims 1 to 3,
The cooling treatment is a heat treatment method for malleable cast iron, which is a treatment for cooling from the outside of the heat treatment vessel. A method for manufacturing a casting,
A heat treatment method for malleable cast iron according to any one of claims 1 to 5,
The manufacturing method of a casting provided with the process of giving a galvanization layer after pickling to the surface of the said casting raw material after the said cooling process . In the manufacturing method of the casting according to claim 6,
Surface of the casting material in which the zinc-plated layer is laminated is the same as the internal crystal structure of the template object material, manufacturing method of the casting.

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