JP2013216950A - Cast iron billet for thixocasting and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cast iron billet for thixocasting, requiring no complicated temperature control or the like, and capable of being inexpensively produced without relying on continuous casting.SOLUTION: A cast iron billet for thixocasting includes a metal structure having a high carbon deposition phase having a hypoeutectic composition containing, by mass, 2.0-3.0% of C and 1.50-2.50% of Si and made of cementite (FeC) having a network-like distribution in a crystal grain boundary constituting an iron matrix, or the carbon deposition phase and a small amount of graphite having a fine point-like distribution.

Description

  The present invention relates to a cast iron billet for thixocasting and a method for producing the same.

  The semi-molten casting method, in which the casting material is heated and cast as a semi-molten state in which a solid phase and a liquid phase coexist, can be cast into a mold at a low melting temperature, so the heat load on the mold is normal casting. It has been attracting attention as a casting method of cast iron because it is reduced in comparison with the above, and the life of the mold is extended and is economical.

  The cast iron semi-molten casting method includes a rheocast method (for example, Patent Document 1) in which iron is cooled and held at a temperature between the liquidus temperature and the solidus temperature, and cast into a mold (die) as it is, There are two types of thixocasting methods, in which the target component, iron, is once solidified into a predetermined shape to form a solid, then made into a semi-molten state by high-frequency induction heating or the like, and cast into a mold.

  However, in the rheocast method of Patent Document 1, it is difficult to divide iron according to the weight of a part to be cast, and complicated temperature control is required, and stable production is difficult.

  On the other hand, the latter thixocasting method does not have such a problem and is expected as an economical and stable semi-molten casting method. The thixocasting of cast iron is performed with a hypoeutectic cast iron component as disclosed in Patent Document 2, for example.

  In addition, as a method for manufacturing a billet for thixocasting, a method using a horizontal continuous casting method (for example, Patent Document 3) or a method for semi-continuous casting using a continuous casting machine having a water-cooled copper mold (for example, Patent Document 4). )It has been known.

JP 2006-122971 A JP-A-2005-290420 JP 2003-290878 A Japanese Patent No. 4076155

  However, in the horizontal continuous casting method of Patent Document 3, since it is cooled in high-temperature air, a surface oxide film is generated, which may be mixed into the product during thixo injection molding. Therefore, in some cases, it is necessary to scrape the entire side surface, which increases the cost. In addition, the decarburized layer is cut at the cut surface, which affects the shape retention during billet heating.

  Furthermore, since the material is a hypoeutectic component, the temperature interval between the liquidus and solidus is wide, and the solidification time is long. Moreover, since cementite is crystallized, the shrinkage rate during solidification is large. For this reason, in continuous casting as in Patent Documents 3 and 4, solidification occurs in a dynamic state, that is, in a state where there is relative movement between the casting material and the mold, so that a unique defect called “cover” is formed on the surface. It is easy, and the surface mark is not uniform with the oscillation mark, which affects the shape retention. Moreover, it cannot be made a near net shape, resulting in high cutting costs.

  The present invention has been made in view of such circumstances, and provides a cast iron billet for thixocasting that does not require complicated temperature control and can be manufactured at low cost without using continuous casting, and a method for manufacturing the same. This is the issue.

  As a result of repeated investigations to solve the above problems, the present inventors have made continuous casting by appropriately controlling the structure when producing a cast iron billet as a material having a hypoeutectic composition for thixocasting. In order to complete the present invention, it has been found that a cast iron billet with a near net shape can be obtained without causing complicated control by casting in a stationary state using a normal mold. It came.

The present invention is based on such knowledge and provides the following (1) to (7).
(1) It has a hypoeutectic composition containing C: 2.0 to 3.0% and Si: 1.50 to 2.50% by mass%, and is reticulated at the crystal grain boundaries constituting the iron base. A cast iron billet for thixocasting, characterized by having a metal structure having a high carbon precipitation phase made of distributed cementite (Fe ₃ C), or the high carbon precipitation phase and a small amount of finely spot-distributed graphite.
(2) Further, Cr: 0.10 to 0.50%, B: 0.001 to 0.01%, Mn: 0.3 to 0.8%, Ni: 0.5 to 1.5% in mass% The cast iron billet for thixocasting according to (1), comprising one or more of them.
(3) The cast iron billet for thixocasting according to (1) or (2), wherein the surface has a decarburized layer having a thickness of 0.01 to 0.1 mm.
(4) It has a hypoeutectic composition containing C: 2.0 to 3.0% and Si: 1.50 to 2.50% by mass%, or in addition to this basic composition, Cr: 0.10 to 0.50%, B: 0.001 to 0.01%, Mn: 0.3 to 0.8%, Ni: 0.5 to 1.5%, one or more A cast iron billet for thixocasting, characterized in that a cast iron billet is produced by casting a molten metal containing selenium into a mold and solidifying it at a cooling rate greater than 20 ° C./min.
(5) The method for producing a cast iron billet for thixocasting according to (4), wherein a decarburized layer having a thickness of 0.01 to 0.1 mm is formed on a surface of the cast iron billet.
(6) The thixcasting according to (4) or (5), wherein the mold is a mold or a sand mold having a cavity, and the molten metal is solidified in a stationary state and a sealed state in the cavity. Manufacturing method of cast iron billet.
(7) The method for producing a cast iron billet for thixocasting according to any one of (4) to (6), wherein the cast iron billet after casting is a near net shape.

According to the present invention, the grain boundaries having a hypoeutectic composition containing C: 2.0 to 3.0% and Si: 1.50 to 2.50% by mass and constituting an iron base In the mold or sand mold cavity because it has a high carbon precipitation phase consisting of cementite (Fe ₃ C) distributed in a network form, or a metal structure having the high carbon precipitation phase and a small amount of finely distributed graphite. Thus, it can be completely solidified without causing a casting defect such as “cover” in a stationary and sealed state. For this reason, the cast iron billet for thixocasting can be obtained without the need for continuous casting and complicated control.

  In order to cool in such a closed mold, a thin decarburized layer is formed on the surface, and the thickness is set to 0.01 to 0.1 mm, so that even when it is in a semi-molten state when performing thixocasting The shape retention is good and the decarburized layer does not remain in the product as a non-molten product.

  Moreover, since it can cast with a sand mold and a metal mold | die, without using a continuous casting installation, installation cost can be reduced. Furthermore, since the cast iron billet as a raw material can be made into a near net shape, cutting is unnecessary.

It is sectional drawing which shows an example of the structure of the casting_mold | template used when manufacturing the cast iron billet of this invention. It is a metallographic photograph by the optical microscope of the billet cast using the gray iron mold of Example 1. It is a metallographic photograph by the optical microscope of the billet cast using the chromite sand mold of Example 3. 3 is a metallographic photograph of a billet cast using the quartz sand mold of Comparative Example 1 using an optical microscope.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
The cast iron billet for thixocasting of the present invention has a hypoeutectic composition containing C: 2.0 to 3.0% and Si: 1.50 to 2.50% by mass, and constitutes an iron base. It has a metal structure having a high carbon precipitation phase composed of cementite (Fe ₃ C) distributed in a network form at the crystal grain boundaries, or such a high carbon precipitation phase and a small amount of finely spot-distributed graphite.

  When C is 2.0 to 3.0%, cementite does not crystallize at less than 2.0%, so it does not become white cast iron. On the other hand, when it exceeds 3.0%, it is in the form of a piece instead of cementite. This is because graphite precipitates.

  Si is a promoting element of graphite. If it is less than 1.50%, the graphitization annealing time after thixoforming of cementite crystallized in the billet is long, and in some cases, it cannot be graphitized. This is because if it exceeds 50%, graphite crystallizes in place of cementite at the casting stage.

  In addition to these basic compositions, Cr: 0.10-0.50%, B: 0.001-0.01%, Mn: 0.3-0.8%, Ni: 0.5-1 It is preferable to contain one or more of 5%.

  Cr is an element for promoting cementite. By containing 0.10% or more, cementite can be easily formed. On the other hand, if it exceeds 0.50%, the graphitization treatment time becomes long, and in some cases, there is a possibility that graphitization cannot be performed.

  B is an element having the following double effect. That is, B is an element for promoting cementite in the solidification stage, and the addition of B can facilitate the formation of cementite. Moreover, since there exists an effect which accelerates | stimulates graphitization in the stage of an annealing process, graphitization annealing time can be shortened. Although these effects are effectively exhibited at 0.001% or more, the effects are saturated even when the content exceeds 0.01%.

  Mn, like Cr, is a cementite-promoting element and a base strengthening element. If the content is less than 0.3%, graphite is likely to precipitate, and the product strength of semi-molten thixo molding will be low. On the other hand, if it exceeds 0.8%, the graphitization treatment time becomes long, and in some cases, there is a possibility that graphitization cannot be performed.

  Ni is an element mainly toughening the iron base. When the content is less than 0.5%, the effect of toughening the product after semi-molten thixo molding is small. On the other hand, since Ni is an expensive element, if it exceeds 1.5%, the strength improvement effect sufficient to meet the cost increase cannot be obtained.

  These balances are Fe and inevitable impurities.

The metal structure of this cast iron billet material is divided into a high carbon precipitation phase composed of cementite (Fe ₃ C) distributed in a network at the grain boundaries constituting the iron base, or such a high carbon precipitation phase and a small amount of fine particles. The reason for having graphite distributed in the shape is as follows.

  Graphite in which high carbon crystallizes and grows from liquid as graphite in the casting stage tends to be flake graphite having a large size. Depending on the annealing, this graphite does not change, so the strength of the product is impaired. Therefore, in the solidification stage, it is basically necessary to crystallize all the carbon as cementite. On the other hand, the graphite precipitated with cementite crystallization is fine and does not adversely affect the properties of the product. It also has the effect of shortening the graphitization time of cementite by becoming the core of graphite during graphitization annealing. Therefore, the inclusion of a small amount of finely distributed graphite is acceptable.

  In this case, it is preferable that the volume fraction of graphite is less than 2% and the dimension thereof is 0.01 mm or less. When the volume fraction of graphite is 2% or more, the graphite becomes flakes when crystallized from the liquid, and the product strength of the semi-molten thixo molding is reduced. Moreover, when the dimension of graphite is larger than 0.01 mm, it becomes flakes, and the strength of the product is also lowered.

  In the process of producing a cast iron billet as a material for thixocasting, a decarburized layer with a small amount of C is formed on the surface, but the thickness of the decarburized layer may be in the range of 0.01 to 0.1 mm. preferable. When the thickness of the decarburized layer is less than 0.01 mm, the shape cannot be maintained when the cast iron billet is in a semi-molten state for thixocasting, and it becomes difficult to transport the cast iron billet to the mold. . On the other hand, if the decarburized layer exceeds 0.1 mm, it remains in the product as a non-molten product during casting and becomes a defect.

  Next, the manufacturing method of the cast iron billet used as the thixocasting material will be described.

  First, a cast iron melt having the above composition is prepared, and then the cast iron melt is poured into a mold (mold or sand mold) having a cylindrical or rectangular cavity and solidified completely in a stationary and sealed state in the cavity. .

  FIG. 1 is a sectional view showing an example of the structure of a mold used at this time. The mold 1 is configured as a sand mold or a mold, and a cavity 2 is formed therein. 3 is a runner, 4 is a feeder, and 5 is a heat insulation sleeve. When casting the molten metal, the molten metal is poured into the runner 3 and put into the cavity 2 from below. Then, the molten metal is completely solidified within the cavity 2 in a stationary and sealed state.

  At this time, it is solidified in the mold (in the cavity) so that the cooling rate is higher than 20 ° C./min. Thereby, the metal structure is formed. When the cooling rate is 20 ° C./min or less, flake graphite having a large size is crystallized and the strength of the product is lowered. The cooling rate in the present invention refers to the cooling rate immediately before the eutectic temperature point in the cooling curve.

  In order to achieve a cooling rate higher than 20 ° C./min, selection of the mold material is important. When the mold is a mold, pure copper, gray cast iron, carbon steel, etc. can be suitably used. In this case, chromite sand can be suitably used. However, in the case of a sand mold using quartz sand, since the thermal conductivity is low, the cooling rate is low and a desired structure cannot be obtained.

  At this time, since cooling is performed in a sealed mold, a thin decarburized layer is formed on the surface. The thickness of the decarburized layer can be controlled by the material (type) of the coating mold applied to the inside of the mold. it can. For example, the thickness of the decarburized layer can be controlled within a preferable range of 0.01 to 0.1 mm by applying an alumina-based coating mold inside the mold.

  Thereby, the cast iron billet used as the raw material for thixocasting can be obtained without using continuous casting and requiring complicated control.

  Thus, since it solidifies in a stationary state, a defect such as “cover” does not occur. Furthermore, since no continuous casting equipment is required and casting can be performed with a sand mold or a metal mold, equipment costs can be reduced. Furthermore, since the cast iron billet as a raw material can be made into a near net shape, cutting is unnecessary. Furthermore, complicated composition control as in the case of using the rheocast method is unnecessary.

Examples of the present invention will be described below.
(Example 1: Mold casting example 1)
As a mold, a cast iron mold having a capacity dimension of φ65 mm and a length of 130 mm and a hot water using a heat insulating sleeve provided at the top is prepared, preheated at 180 ° C., and an alumina-based mold is formed inside the cavity. A coating mold was applied.

  A cast iron billet as a thixocasting material was cast by injecting molten iron of C: 2.6% and Si: 1.6% by mass into such a mold cavity. The casting temperature was 1450 ° C. The cooling rate was 110 ° C./min.

  The obtained cylindrical billet was cut from the central portion in the length direction, and the quality of the cut surface was inspected. There were no casting defects such as shrinkage and pinholes on the cut surface, and the internal quality was good.

A sample was taken from the billet, etched with nital, and the metal structure was observed with an optical microscope. The metal structure photograph at that time is shown in FIG. 2 (observation position: bottom (1 / 2H) —15 mm (1/4 t) from the center). As shown in FIG. 2, the base was pearlite, carbon was present as a cementite (Fe ₃ C; indicated by C in the figure) distributed in a network form at the grain boundaries, and no graphite precipitation was observed. Further, as a result of investigating the surface of the billet, a decarburized layer having a thickness of 0.04 mm was uniformly distributed on the surface.

  When the obtained billet was heated to 1150 to 1180 ° C., which is a solid-liquid coexistence region, good shape retention was obtained. Using this semi-molten cast iron billet, thixocasting was performed with a thixocasting apparatus in an inert gas atmosphere.

  Of the obtained products, 10 samples were used in the sampling survey, and the center portion was cut, and the tensile properties, metal structure and internal quality were investigated. No internal defects were detected in any product, and it was a healthy product. As a result of the tensile test, a high strength value with a tensile strength of 600 MPa was obtained.

(Example 2: Mold casting example 2)
A cast iron billet as a thixocasting material was cast in the same manner as in Example 1 except that a pure copper mold was used as a mold. The casting temperature was 1460 ° C. The cooling rate was 290 ° C./min.

When the structure was observed in the same manner as in Example 1, the base was pearlite, the carbon was present as a cementite (Fe ₃ C) distributed in a network form at the grain boundaries, and no precipitation of graphite was observed. Further, as a result of investigating the surface of the billet, a decarburized layer having a thickness of 0.02 mm was uniformly distributed on the surface.

(Example 3: Sand casting example 1)
A cast iron billet as a thixocasting material was cast in the same manner as in Example 1 except that a chromite sand mold was used as a mold. The casting temperature was 1430 ° C. The cooling rate was 24 ° C./min.

When the structure was observed in the same manner as in Example 1, as shown in the metal structure photograph in FIG. 3, the base was pearlite, and the carbon was distributed in the form of a network at the grain boundaries (Fe ₃ C; C in the figure). No precipitation of graphite was observed. Further, as a result of investigating the surface of the billet, a decarburized layer having a thickness of 0.07 mm was uniformly distributed on the surface.

(Comparative Example 1: Sand casting example 2)
A cast iron billet as a thixocasting material was cast in the same manner as in Example 1 except that a quartz sand mold was used as a mold. The casting temperature was 1430 ° C. The cooling rate was 12 ° C./min. When the structure was observed in the same manner as in Example 1, coarse flake graphite (indicated by G in the figure) was precipitated as shown in the metal structure photograph of FIG.

  These results are summarized in Table 1.

1; mold 2; cavity 3; runner 4;

Claims (7)

A cementite having a hypoeutectic composition containing C: 2.0-3.0% and Si: 1.50-2.50% by mass, and distributed in a network at the grain boundaries constituting the iron base A cast iron billet for thixocasting, characterized by having a metal structure having a high carbon precipitation phase composed of (Fe ₃ C) or the high carbon precipitation phase and a small amount of finely distributed graphite.   Further, Cr: 0.10 to 0.50%, B: 0.001 to 0.01%, Mn: 0.3 to 0.8%, Ni: 0.5 to 1.5% in mass% The cast iron billet for thixocasting according to claim 1, further comprising two or more kinds.   The cast iron billet for thixocasting according to claim 1 or 2, wherein the surface has a decarburized layer having a thickness of 0.01 to 0.1 mm.   It has a hypoeutectic composition containing C: 2.0-3.0% by mass%, Si: 1.50-2.50%, or Cr: 0 by mass% in addition to this basic composition 10 to 0.50%, B: 0.001 to 0.01%, Mn: 0.3 to 0.8%, Ni: 0.5 to 1.5%, one or more A method for producing a cast iron billet for thixocasting, which comprises casting a molten metal in a mold and solidifying it at a cooling rate greater than 20 ° C / min to produce a cast iron billet.   The method for producing a cast iron billet for thixocasting according to claim 4, wherein a decarburized layer having a thickness of 0.01 to 0.1 mm is formed on a surface of the cast iron billet.   The cast iron billet for thixocasting according to claim 4 or 5, wherein the mold is a mold or a sand mold having a cavity, and the molten metal is solidified in a stationary state and a sealed state in the cavity. Production method.   The cast iron billet for thixocasting according to any one of claims 4 to 6, wherein the cast iron billet after casting is a near net shape.