KR102539284B1 - Nodular cast iron with excellent resistance to gas defects - Google Patents

Abstract

To provide spheroidal graphite cast iron having excellent gas defect resistance with fewer gas defects such as pinholes caused by glass N, and having mechanical properties and machinability equal to or better than those of the prior art. In terms of mass ratio, C: 3.3 to 4%, Si: 2 to 3%, P: 0.05% or less, S: 0.02% or less, Mn: 0.8% or less, Cu: 0.8% or less (excluding 0), Mg: 0.02 to 0.06%, Ti: 0.01 to 0.04%, V : 0.001 to 0.01%, Nb: 0.001 to 0.01%, N: 0.004 to 0.008%, the remainder substantially composed of Fe and unavoidable impurities, and having excellent gas defect resistance.

Description

Nodular cast iron with excellent resistance to gas defects

The present invention relates to nodular cast iron having excellent gas defect resistance.

Since nodular cast iron has excellent mechanical properties and good castability, it is widely used in various automobile parts and machine parts. In the production of spheroidal graphite cast iron, pig iron, snow (scrap), recovered scrap (casting return material), etc. are used as main raw materials. Among the above raw materials, pig iron for casting has conventionally been the main raw material for ductile cast iron, but recently, it is common to use snow as the main material instead of high-grade pig iron from the viewpoint of effective utilization of resources. has been Since snowfall occurs in large quantities with the growth of the automobile industry and is supplied at a low price, it is widely used as a raw material for cast iron in general, starting with nodular graphite cast iron.

Here, snowfall generated in the automobile industry is made from press-worked scraps for automobile bodies, and the share of high-strength steel sheet (Hiten) as such a steel material is increasing in recent years. For this reason, weight reduction is required for automobile bodies and the like from the viewpoint of environmental preservation by improving fuel efficiency, while securing strength and rigidity is also required from the viewpoint of securing safety for passengers in a collision. For this reason, it is required to achieve both weight reduction, high strength, and high rigidity for automobile bodies and the like, and in order to respond to this, high-strength steel sheets containing high concentrations of Mn, Cr, Mo, etc. tend to be widely used. If snow, which inevitably contains a large amount of Mn, Cr, Mo, etc., is used as a raw material, the crystallization of graphite is inhibited or carbide is generated when spheroidal graphite cast iron is used, reducing ductility (elongation characteristics). there is a problem with In order to solve problems caused by Mn, Cr, Mo, etc., which are inevitably included in raw materials, various proposals have been made to remove these elements from the molten metal in the melting process of nodular graphite cast iron.

By the way, some high-strength steel sheets contain N (nitrogen) in addition to Mn, Cr, and Mo to achieve high strength by precipitation hardening with carbonitride or nitriding treatment. Some of these high-strength steel sheets have a N content of up to several hundred ppm. In this way, when spheroidal graphite cast iron is manufactured using snowfall containing a large amount of N as a raw material, there is a possibility that the free N contained in the molten metal in which the raw material is melted increases. In the following description, "free N" refers to nitrogen atoms in a free state that do not constitute atoms constituting a solid phase or solid solution, and "N" refers to nitrogen as an element.

In addition, generally, since molten metal (raw water) melted in the melting process of nodular cast iron contains Si and Mn, it is deoxidized at the stage of the melting process, and then the raw metal is ladle. It is more strongly deoxidized by Mg added in the spheroidization treatment performed after transfer to the ladle or by Si added in the inoculation treatment. The deoxidized molten metal for spheroidal graphite cast iron releases N in the atmosphere when the molten metal comes into contact with the atmosphere in the pouring process from the melting furnace to the ladle or the pouring process from the ladle to the mold. It has the property of being easy to absorb or occlude.

As described above, in the molten metal for spheroidal graphite cast iron, which has a property of easily absorbing or occluding free N derived from the atmosphere, if the mixing of snow with a large amount of N, which inevitably contains a large amount of N as a raw material, is further increased, this snow The glass N in the molten metal increases due to the derived glass N, and the tendency of gas defects such as pinholes formed from nitrogen gas (N ₂ gas) caused by glass N increases in the cast product. Specifically, when molten metal containing excessive free N solidifies, free N that cannot be dissolved in the solid phase is released as nitrogen gas, which may cause gas defects such as pinholes made of nitrogen gas in cast products. . When gas defects occur in spheroidal graphite cast iron, there is a risk of causing not only appearance defects but also deterioration of mechanical properties such as strength and elongation due to fine pore defects.

As a technology for reducing the content of impurities containing N contained in cast iron such as nodular cast iron, Patent Document 1 has a mass ratio of carbon: 2.0 to 4.0%, silicon: 1.0 to 3.0%, sulfur: 0.02% or less, After the alloy composed of added iron and trace amounts of unavoidable impurities is put into a cold crucible melting furnace and melted, a spheroidizing treatment agent containing magnesium, calcium, and a spheroidizing promoting element composed of any one or two or more of rare earth elements is added thereto to promote spheroidization. By applying a manufacturing method in which elements are added to be 0.01 to 0.1% in the final composition, cooled without inoculation treatment to promote graphitization, and an alloy with iron is applied, the mass ratio of carbon: 2.0 to 4.0%, magnesium , Calcium, and a spheroidization accelerating element composed of any one or two or more of rare earth elements: 0.01 to 0.1%, silicon: 1.0 to 3.0%, sulfur: 0.02% or less, the balance being iron and unavoidable impurities, the Elements other than cobalt, copper, and nickel among unavoidable impurities are minimized in trace amounts, and the absorbed energy value in the Charpy impact test at -60 ° C or -80 ° C is 14 J / cm ² in the V-notch test piece There is a description of spheroidal graphite cast iron as above.

According to Patent Literature 1, after melting the raw materials and evacuating the inside of the melting furnace chamber, argon gas is introduced to make the inside of the melting furnace chamber an argon atmosphere, and at the same time, using a high-purity copper crucible cooled with water, Similarly, by using a cold crucible melting furnace, which is a device that melts (float melting) a copper crucible and molten metal in a non-contact state, mixing of impurities into molten metal from a crucible or gas phase (environment) like a conventional melting process is prevented. It is said that it is possible to manufacture high-purity materials. In addition, in the spheroidal graphite cast iron of Patent Document 1, by setting the elements other than cobalt, copper, and nickel among inevitable impurities to 0.003% or less in terms of mass ratio, respectively, inclusions can be further reduced and internal brittle parts can be reduced. It is said that Shikin can provide spheroidal graphite cast iron.

Japanese Patent Laid-open No. 2004-169167

In the manufacturing method of nodular graphite cast iron disclosed in Patent Document 1, since the raw material is melted in an argon atmosphere and solidified as it is to form nodular cast iron, the free N contained in the molten metal is reduced and the glass N generated in the nodular cast iron is attributed. There is a possibility that gas defects made by this can be suppressed. However, high-purity electrolytic iron, silicon for semiconductors, highly purified graphite, etc. of about 4N (99.99%, mass ratio) level are used as raw materials, and the starting materials themselves are high-purity raw materials. On the other hand, the production method is provided by using a cold crucible melting furnace, which is a special device made of a high-purity copper crucible that is water-cooled while setting the inside of the melting furnace chamber to an argon atmosphere. A cold crucible melting furnace is generally used for manufacturing high-purity materials such as manufacturing high-purity alloy ingots. In this way, when using high-purity raw materials or special equipment, the content of unavoidable impurities can be significantly reduced, but for the production of nodular cast iron applied to automobile parts and machine parts, the obtained cast products are extremely expensive, There is a problem from the point of view of economic rationality.

In addition, according to the manufacturing method of patent document 1, after melting|dissolving raw materials in a copper crucible and adding a spheroidizing treatment agent, it is said that cooling and solidification of molten metal are performed in a copper crucible. According to this manufacturing method, the obtained spheroidal graphite cast iron becomes a lumpy cast product that mimics the shape of the space of the crucible. In this way, it is not possible to enjoy the advantages of casting, a manufacturing method with a high degree of freedom in shape.

That is, in order to obtain cast products having free shapes, such as automobile parts and machine parts, pouring into a mold having a cavity formed by dividing the product shape directly from the melting furnace or through a ladle, and then cooling and solidifying. There is a need. In terms of industrial production, in order to manufacture cast products at a realistic and reasonable cost, it is important that the pouring process from the melting furnace to the ladle and the pouring process from the ladle to the mold can be performed in the air.

Here, in order to suppress melting (absorption or occlusion) of free N into the molten metal, it is also conceivable to place a ladle or a mold in the chamber and then tap or pour the metal after setting the chamber in an argon gas atmosphere or a vacuum. Equipment and equipment are special and large-scale, and the resulting cast products are more expensive, which is not only uneconomical but also unrealistic. In addition, there is no effective and practical method for removing free N from molten ductile cast iron, and even if you try to dilute it with a material with a low N content, for example, low-N raw materials such as high-purity pig iron or base metal are expensive and economical. not.

And in recent years, as described above, it is inevitable that the raw material of spheroidal graphite cast iron contains a large amount of N as an unavoidable content. In addition, as described above, in addition to snowfall and atmosphere, which are typical examples of sources of free N, other materials constituting raw materials and furnace materials of melting furnaces for melting raw materials, for example, can be dissolved in molten metal. There are numerous possible sources of free N. For this reason, for example, glass N derived from snowfall is not removed or diluted with expensive pig iron, etc., and gas defects such as pinholes caused by glass N are generated while the glass N is contained in the molten metal. If it is possible to obtain ductile graphite cast iron that is not processed, it is very effective industrially.

The present invention has been made in response to the above conventional problems, and its object is to have excellent gas defect resistance with less gas defects such as pinholes caused by glass N, and nodular graphite cast iron having mechanical properties and machinability equal to or better than the conventional ones. is in providing

That is, the nodular graphite cast iron having excellent gas defect resistance of the present invention,

in mass ratio,

C：3.3~4%,

Si: 2 to 3%;

P: 0.05% or less;

S: 0.02% or less;

Mn: 0.8% or less;

Cu: 0.8% or less (excluding 0);

Mg: 0.02 to 0.06%;

Ti: 0.01 to 0.04%;

V: 0.001 to 0.01%;

Nb: 0.001 to 0.01%;

N: 0.004 to 0.008%, and

The balance consists essentially of Fe and unavoidable impurities.

The spheroidal graphite cast iron of the present invention contains 0.015 to 0.045% of Ti, V and Nb in total in terms of mass ratio, while Ti, V, Nb and N are preferably contained so as to satisfy the following formula (1).

0.8 ≤ (0.29Ti + 0.27V + 0.15Nb)/N ≤ 2.0 ... (1)

However, the element symbol in the above formula (1) indicates the content (mass ratio (%)) of each element in nodular cast iron.

The spheroidal graphite cast iron of the present invention preferably contains P: 0.005% or more and S: 0.005% or more in terms of mass ratio, or/and Mn: 0.2% or more and Cu: 0.1% or more in terms of mass ratio it is desirable

The nodular graphite cast iron of the present invention preferably has a tensile strength of 600 MPa or more and an elongation of 12% or more.

The nodular cast iron of the present invention has excellent gas defect resistance with few gas defects such as pinholes caused by free N.

It is a schematic plan view of a flat test piece for measuring gas defects. It is a schematic side view of a flat test piece for measuring gas defects.

The configuration of the spheroidal graphite cast iron of the present invention will be described in detail below. On the other hand, the content of each element constituting the alloy is expressed as a mass ratio (%) unless otherwise specified. In the following embodiments, reduction of gas defects caused by free N derived from snowfall is described as one embodiment of the present invention, but the present invention is not limited to the following embodiments.

(1) C (carbon): 3.3 to 4%

C is an element that contributes to the fluidity of molten metal and the crystallization of graphite. If it is less than 3.3%, the fluidity during casting is reduced, and the number of graphite particles is reduced to easily generate chill (Fe ₃ C: cementite), and the elongation of spheroidal graphite cast iron is reduced. On the other hand, when C exceeds 4%, shrinkage voids are likely to occur, and at the same time, abnormal graphite is likely to occur, resulting in a decrease in strength. For this reason, C content becomes 3.3 to 4%. The lower limit of the C content is preferably 3.4%, more preferably 3.6%. The upper limit of the C content is preferably 3.9%, more preferably 3.8%.

(2) Si (silicon): 2 to 3%

Si is necessary for accelerating the crystallization of graphite or increasing the fluidity of molten metal. If Si is less than 2%, it is easy to generate chilled, and the machinability and elongation of nodular graphite cast iron are reduced. However, when Si exceeds 3%, the base of the nodular graphite cast iron becomes brittle, and the toughness (impact value) and elongation are extremely reduced, and the strength and machinability are deteriorated. For this reason, Si content becomes 2 to 3%.

The lower limit of the Si content is preferably 2.1%, more preferably 2.2%. The upper limit of the Si content is preferably 2.9%, more preferably 2.8%.

(3) P (phosphorus): 0.05% or less

P is an element that is unavoidably incorporated from raw materials. P inhibits the spheroidization of graphite and also dissolves in the base to embrittle the structure. For this reason, P content is made into 0.05 % or less. On the other hand, although the lower limit is not set, it is not economical to reduce the amount below the detection limit, so it is preferable to set the lower limit to about 0.005%. The upper limit of the P content is preferably 0.03%.

(4) S (sulfur): 0.02% or less

S is an element that is unavoidably incorporated from raw materials. S is an element that inhibits spheroidization of graphite, and its content is made 0.02% or less. On the other hand, although the lower limit is not set, it is not economical to reduce the amount below the detection limit, so it is preferable to set the lower limit to about 0.005%. The upper limit of the S content is preferably 0.01%.

(5) Mn (manganese): 0.8% or less

Mn is an element that is unavoidably incorporated from raw materials, but has an action of precipitating a pearlite phase as a pearlite phase stabilizing element. In the case of obtaining spheroidal graphite cast iron having improved strength by stabilizing and precipitating the paralite phase in the base structure, the Mn content is preferably 0.2% or more. On the other hand, when the content exceeds 0.8%, the generation of chilled becomes remarkable, deteriorating the toughness, elongation and machinability of nodular cast iron. For this reason, Mn content is made into 0.8 % or less. The upper limit of the Mn content is preferably 0.5%.

(6) Cu (copper): 0.8% or less (excluding 0)

Cu is a paralite phase stabilizing element, and is an effective element when obtaining nodular cast iron having improved strength by including a paralite phase in the base structure. For this reason, depending on the desired strength, it can be contained in an appropriate amount without including 0%. In order to stabilize and generate the paralite phase, it is preferable to set the Cu content to 0.1% or more. However, when Cu exceeds 0.8%, the hardness of the nodular cast iron becomes too high, and the nodularization of the graphite is inhibited, thereby reducing the elongation and toughness of the nodular graphite cast iron. For this reason, Cu content is prescribed|regulated to 0.8 % or less. The upper limit of Cu content is preferably 0.6%.

(7) Mg (magnesium): 0.02 to 0.06%

Mg is an element necessary for graphite nodularization, which not only improves mechanical properties such as strength and elongation of nodular cast iron, but also becomes important, but the effect of graphite nodularization is insufficient when its content is less than 0.02%. On the other hand, if the Mg content exceeds 0.06%, chilled or shrinkage voids are easily generated, and the machinability and toughness of nodular cast iron deteriorate. For this reason, the Mg content is made 0.02 to 0.06%. The lower limit of the Mg content is preferably 0.025%, more preferably 0.03%. The upper limit of the Mg content is preferably 0.05%, more preferably 0.04%.

(8) Ti (titanium): 0.01 to 0.04%, V (vanadium): 0.001 to 0.01%, Nb (niobium): 0.001 to 0.01%

Ti, V and Nb are the most important essential elements constituting the nodular cast iron of the present invention. From the viewpoint of the structural element of the nodular cast iron obtained from the molten metal, free N contained in the molten metal of nodular cast iron is finally (1) in the matrix phase, (2) in the nitride or carbonitride, (3) Among the gas defects formed in the nitrogen gas that are not fixed to the tissue elements of (1) and (2) and are released as nitrogen gas, it is considered that they exist mainly as N in these three tissue elements. On the other hand, the upper limit of the amount of N that can be contained in the matrix phase of (1) is the solubility limit of the austenite phase formed around 1000 ° C., which is the austenitization temperature.

Ti, V, and Nb are strong carbonitride-forming elements (hereinafter, Ti, V, and Nb are sometimes referred to as carbonitride-forming elements), and by containing these elements in a predetermined amount or more, glass in the molten metal It combines with N to form nitrides and carbonitrides (hereinafter, both are collectively referred to as carbonitrides). In addition, the crystallization temperature of the carbonitride formed by the carbonitride-forming element is higher than the crystallization start temperature of the austenite phase (matrix phase), and in the process of cooling and solidifying the molten metal, the carbonitride is formed faster than the matrix phase. formed over time As a result, free N dissolved in the molten metal in excess beyond the solubility limit of N in the austenite phase is fixed as a carbonitride. For this reason, even in spheroidal graphite cast iron formed using a raw material containing more than desired N, the release of nitrogen gas from the molten metal during solidification of free N exceeding the solubility limit in the austenite phase is suppressed, preventing the occurrence of gas defects. do.

In addition to the effect of preventing the generation of gas defects described above, the fixation of free N in the molten metal by the carbonitride-forming element suppresses the variation in the amount of N dissolved in the austenite phase, resulting in nodular graphite cast iron obtained. An effect of being able to reduce the variation in mechanical properties can also be mentioned. That is, N is a strong paralite phase stabilizing element together with Mn and Cu, and promotes the precipitation of the paralite phase from the austenite phase. And, when snowfall with a small amount of N is used as in the prior art, the amount of N contained in the raw material becomes relatively uniform between melting lots (charges), and there is little variation in free N in the molten metal in which the raw material is melted. For this reason, in order to obtain a desired paralite phase, it is possible to control the precipitated amount of the paralite phase by adjusting the addition amount of elements whose contents such as Mn and Cu are relatively easy to control.

However, when snowfall with a large amount of N is used as a raw material in addition to snowfall with a small amount of N, the amount of N contained in the raw material fluctuates between melting lots depending on the composition of the snowfall contained in the raw material, and the amount of free N in the molten metal fluctuations also increase. For this reason, the amount of N dissolved in the austenite phase fluctuates, and the amount of precipitation of the ferrite phase also becomes unstable, causing an imbalance in the mechanical properties (strength, elongation) of nodular cast iron between melting furnaces. On the other hand, in the present invention, ferrization by N is suppressed by reducing the amount of N dissolved in the austenite phase by fixing free N by the carbonitride-forming element as described above. For this reason, since it becomes possible to stably adjust the precipitation amount of the ferrite phase by controlling the contents of Mn and Cu, it is possible to reduce the variation in mechanical properties of nodular cast iron.

In order to obtain the fixing effect of free N by the carbonitride formed by the carbonitride-forming elements described above, the Ti, V and Nb contents need to be 0.01% or more, 0.001% or more, and 0.001% or more, respectively. On the other hand, when the content of Ti, V, and Nb exceeds 0.04%, 0.01%, and 0.01%, respectively, very hard carbides or nitrides are formed, and the machinability and mechanical properties (strength, elongation) of nodular graphite cast iron deteriorate. . For this reason, the Ti content is 0.01 to 0.04%, the V content is 0.001 to 0.01%, and the Nb content is 0.001 to 0.01%.

The lower limit of the Ti content is preferably 0.012%, more preferably 0.013%. The upper limit of the Ti content is preferably 0.035%, more preferably 0.025%.

The lower limit of the V content is preferably 0.002%. The upper limit of the V content is preferably 0.004%, more preferably 0.003%.

The lower limit of the Nb content is preferably 0.002%, more preferably 0.004%. The upper limit of the Nb content is preferably 0.006%, more preferably 0.005%.

As described above, by containing a predetermined amount of carbonitride-forming elements and fixing glass N as a carbonitride, excellent gas defect resistance with few gas defects such as pinholes caused by glass N is obtained, and at the same time, unevenness in mechanical properties is reduced. The formation of excessive carbonitrides is suppressed, and nodular graphite cast iron having mechanical properties (strength, elongation) and machinability equivalent to or higher than the conventional ones can be obtained.

In addition, one major feature of the present invention is that Ti, V and Nb are not contained individually, but these three elements are mixed and contained, and the content is regulated to an appropriate amount. In this way, by containing all of Ti, V, and Nb in the above numerical range, the total amount of these carbonitride-forming elements can be reduced more than when each alone or only any two of them are contained. Specifically, in forming the same amount of carbonitride, by compounding the three types of carbonitride-forming elements, the total amount of the carbonitride-forming elements to be contained can be suppressed compared to the case of containing alone or any two of them. can For this reason, while sufficiently exhibiting the fixing effect of glass N by the carbonitride formed by the above-mentioned carbonitride-forming element, the amount of carbonitride affecting mechanical properties and machinability is set within an appropriate range, and gas defect resistance and mechanical properties and It is possible to obtain nodular cast iron compatible with machinability.

(9) Ti, V and Nb: preferably 0.015 to 0.045% in total

The total amount of composite content of Ti, V, and Nb can take a range of 0.012 to 0.06% from the total amount of the upper limit and the lower limit of the content of each element. In the range of the N content described later, in order to suppress the occurrence of gas defects by fixing free N as a carbonitride and to further manifest the effect of reducing the imbalance in mechanical properties, Ti, V and Nb It is preferable to contain 0.015% or more in total. On the other hand, when the total amount of Ti, V and Nb exceeds 0.045%, the tendency to form hard carbides and nitrides increases, and the machinability and mechanical properties (strength, elongation) of nodular cast iron are significantly reduced. Therefore, the total content of Ti, V and Nb is 0.015 to 0.045%. The lower limit of the total content of Ti, V and Nb is preferably 0.02%. The upper limit of the total content of Ti, V and Nb is preferably 0.03%.

(10) N (nitrogen): 0.004 to 0.008%

N is an element mainly incorporated from snowfall such as high tensile steel sheet. In the molten metal of nodular cast iron obtained through the melting process using such snowfall as a raw material, free N is contained in an amount of about 0.008 to 0.015%. Even in nodular cast iron obtained by using a molten metal high in free N in this way, by containing a predetermined amount of carbonitride-forming elements as described above, free N in the molten metal is fixed to the carbonitride formed from the carbonitride-forming elements. As a result, N contained in nodular graphite cast iron, including N fixed (solid-dissolved) on the matrix, is 0.004% or more. On the other hand, if the N content is less than 0.004% even though the spheroidal graphite cast iron obtained by using molten metal high in free N, during casting, when the molten metal solidifies, excess N not dissolved in the solid phase is released as nitrogen gas, , there is a possibility of causing gas defects such as pinholes in nodular cast iron. For this reason, N content is made into 0.004 % or more. On the other hand, when N exceeds 0.008%, carbonitrides fixing N also increase, and there is a possibility that the machinability and mechanical properties (strength, elongation) of the nodular graphite cast iron obtained may decrease. For this reason, N content is made into 0.008 % or less. Therefore, the N content is set to 0.004 to 0.008%. The upper limit of the N content is preferably 0.007%, more preferably 0.006%.

(11) Equation (1): 0.8 ≤ (0.29Ti + 0.27V + 0.15Nb) / N ≤ 2.0

In the spheroidal graphite cast iron of the present invention, in order to further improve gas defect resistance and further improve mechanical properties (strength, elongation) and machinability, after satisfying the requirements of the above composition range, formula (1) It is desirable to satisfy On the other hand, the element symbol in Formula (1) represents the content (mass ratio (%)) of each element in nodular cast iron. Since Ti, V and Nb, which are carbonitride-forming elements, are bonded with N in an atomic number of one to one, the material amount (T) of the total carbonitride-forming elements shown in the following formulas (2) and (3) When the ratio of N to the substance amount (N), that is, the substance amount ratio (molar ratio) T/N, is in a predetermined range, the balance between the carbonitride-forming element and N is appropriate, and Ti, V and It becomes the total content of Nb. By setting the material amount ratio (molar ratio) T/N within a predetermined range, even in nodular graphite cast iron obtained by using a molten metal high in free N, Ti, V, and Nb are fixed as carbonitrides in N exceeding the solubility limit of the austenite phase. On the other hand, by suppressing the formation of excessive carbonitride, gas defect resistance, mechanical properties (strength, elongation) and machinability are further improved.

T = (Ti/48) + (V/51) + (Nb/93) ... (2)

N = N/14 ... (3)

(0.29Ti + 0.27V + 0.15 Nb)/N in Formula (1) am. The coefficient multiplied by the carbonitride-forming element is a coefficient obtained from the ratio of atomic weights of N and each element, where 0.29 is the atomic weight ratio of N and Ti (14/48), and 0.27 is the atomic weight ratio of N and V (14/51 ), 0.15 represents the atomic weight ratio (14/93) of N and Nb.

When the value of formula (1) is 0.8 or more, it contains an appropriate amount of carbonitride-forming elements with respect to N, so even in nodular cast iron obtained by using a molten metal with a large amount of free N, N exceeding the solubility limit of the austenite phase is carbonaceous The cargo-forming elements are fixed without excess or deficiency, and sufficient resistance to gas defects is obtained. On the other hand, when the value of formula (1) is 2.0 or less, formation of carbonitride is suppressed to a minimum, and mechanical properties (strength, elongation) and machinability are improved. Therefore, in the nodular cast iron of the present invention, it is preferable that the value of (0.29Ti + 0.27V + 0.15 Nb) / N is in the range of 0.8 to 2.0. Theoretically, when the material ratio of Ti, V, Nb, and N in nodular cast iron is 1, that is, when the value of formula (1) is 1, N is formed as a carbonitride without excess or deficiency, N to the austenite phase Although it is assumed that the solid solution of N is lost, two free N is required to suspend the formation of carbonitrides, promote precipitation of the paralite phase by solid solution of N to an appropriate degree in the austenite phase, or molecularize as nitrogen gas. Considering the need for (atoms), the range of 0.8 to 2.0 is actually very suitable. Moreover, as for the value of the left side of Formula (1), 1.0 is more preferable and 1.2 is most preferable, and, as for the value of the right side of Formula (1), 1.7 is more preferable and 1.5 is most preferable.

(12) mechanical properties

The spheroidal graphite cast iron of the present invention preferably has a tensile strength of 600 MPa or more and an elongation of 12% or more. Since ductile graphite cast iron having a tensile strength of 600 MPa or more and mechanical properties of 12% or more of elongation has mechanical properties equal to or higher than that of conventional ductile graphite cast iron, it is suitable for use in structural members and the like. The tensile strength is more preferably 610 MPa or more, and most preferably 620 MPa or more. Further, the elongation is more preferably 13% or more, and most preferably 14% or more. On the other hand, in order to achieve a tensile strength of 600 MPa or more and an elongation of 12% or more, it is preferable to adjust the addition amount of Mn and Cu, elements whose contents are relatively easy to control. Specifically, the Mn content is 0.2 to 0.5%. And, on the other hand, it is preferable to set the Cu content to 0.2 to 0.6%.

[Example]

The present invention will be described in more detail with reference to the following examples, but the present invention is not limited in any way by these examples. Also here, the content of each element constituting the nodular cast iron is expressed as a mass ratio (%) unless otherwise specified. In addition, examples to be described below are examples within the scope of the present invention, comparative examples are examples outside the scope of the present invention, and reference examples are examples showing the prior art level.

Collected scraps of pig iron, snow, and nodular cast iron as raw materials are melted in a high-frequency melting furnace with a capacity of 100 kg, and carbonaceous material, Fe-Ti, Fe-V, Fe-Nb, and Fe-Si alloys are added. Thus, the molten metal whose components were adjusted was melted. On the other hand, snow snow is a high-strength steel sheet with an N content of 0.05%, and the mixing ratio of snow snow is 100% of the blending ratio of raw materials totaling pig iron, snow snow, and collected scrap, in Examples 1 to 16 and Comparative Examples 1 to 20, which will be described later. In No. 11, it was 40%, and in the reference example, it was 0% without blending. Using this molten metal as a graphite spheroidizing agent, it was tapped at about 1500 ° C. to a ladle equipped with an F-Si-Mg alloy and a cover material made of iron plate scraps covering it, and spheroidizing by a sandwich method. On the other hand, the amount of N (free N) contained in the molten metal after performing the spheroidizing treatment was in the range of 0.005 to 0.009% in certain examples and comparative examples described below, and 0.003% in reference examples. The spheroidized molten metal is poured into a sand mold at about 1400°C, and cast into a plurality of 1-inch Y blocks, a flat test piece mold for evaluating gas defect area ratio described later, and a cylindrical test piece mold for machinability evaluation. did. On the other hand, in the case of pouring metal, inoculation was performed by adding Fe—Si alloy powder to the flow of molten metal.

In the above manner, nodular graphite cast iron having the composition shown in Table 1 was obtained. Examples 1 to 16 are nodular cast iron within the composition range specified in the present invention, and Comparative Examples 1 to 11 and Reference Examples are in the present invention. It is nodular cast iron outside the prescribed composition range. Comparative Example 1 and Comparative Examples 7 to 11 are nodular cast iron having too much content of any one or more elements of Ti, V, Nb and N, and Comparative Examples 2 to 6 and Reference Examples are any of Ti, V, Nb and N Ductile iron with too little content of one or more elements. On the other hand, the composition of the obtained spheroidal graphite cast iron was confirmed with a glow discharge mass spectrometer (manufactured by VG, trade name VG9000, hereinafter simply referred to as GDMS). On the other hand, GDMS cannot measure N contained in gas defects in 3) among the N present in each tissue element described above. Therefore, the amount of N (mass ratio) shown in Table 1 is the amount of N dissolved in the matrix of (1) and the amount of N fixed to the carbonitride of (2).

Test pieces were cut from each specimen of nodular graphite cast iron obtained by the casting described above, and the following evaluation was performed.

(1) Gas defect area ratio

In order to investigate the tendency of gas defects in spheroidal graphite cast iron of Examples and Comparative Examples, flat test pieces having a shape in which gas defects are more likely to occur than actual products were produced. For this reason, the measured value of the gas defect area ratio is significantly larger than that of the actual product. 1a and b are schematic views of a flat test piece for measuring gas defects, FIG. 1a is a plan view, and FIG. 1b is a side view. This flat test piece 10 is width: 60 mm, length: 150 mm, and thickness: 10 to 15 mm. Each flat test piece 10 is a flat test piece 10, a 45 mm diameter × 60 mm high riser 11, a spout (not shown), a width of 35 mm × a thickness of 3 mm Each molten metal, such as a 1-inch Y block, is poured into a sand mold in which a cavity 12b consisting of a runner 12a and a weir 12b with a width of 40 mm × a thickness of 9 mm is formed, After pouring from the spout at °C or higher, cooling and shape separation were carried out, the riser 11 was cut and separated, and shot blasting was performed to obtain the product.

In order to observe surface and internal gas defects, using a transmission X-ray imaging device (product name EX-260GH-3, manufactured by Toshiba Co., Ltd.), from above each flat test piece (vertical direction in the paper plane with respect to Fig. 1A), X-rays were irradiated under conditions of a voltage of 192 kV and an irradiation time of 3 minutes, and a transmission X-ray photograph was taken.

From each transmission X-ray photograph, only the surface and internal gas defects are extracted and tracked by visual inspection, and then image processing is performed using an image analysis device (manufactured by Asahi Kasei Co., Ltd., product name IP-1000) to determine the gas defect. The total area (mm ² ) was measured. The gas defect area ratio (%) was obtained by dividing the total area of gas defects by the total projected area of the flat test piece. Needless to say, the smaller the gas defect area ratio, the better the spheroidal graphite cast iron. Table 2 shows the measurement results of the gas defect area ratio. On the other hand, in Table 2, Examples 1 to 16, Comparative Examples 1 to 11, and Reference Examples of Ti, V, and the total amount of Nb and the value of Formula (1) are also written together.

As is clear from Table 2, the test pieces of Examples 1 to 16 in which the contents of Ti, V, Nb, and N are within the composition range of the present invention are comparative examples in which the content of any one or more elements of Ti, V, Nb, and N is too small. The gas defect area ratio was lower than that of the test pieces of 2 to 6. In this way, it was confirmed that by specifying the lower limits of the contents of Ti, V, and Nb, the tendency to generate gas defects can be reduced even in nodular cast iron obtained using a molten metal containing excessive free N. On the other hand, in the nodular graphite cast iron of the present invention, the gas defect area ratio is preferably 11% or less, more preferably 10.5% or less, and most preferably 10% or less.

(2) tool life

The cross section of a cylindrical test piece with an outer diameter of 100 mm, an inner diameter of 62 mm, and a length of 100 mm was lathe-processed under the following conditions using TiCN CVD-coated carbide insert P10 (JIS B 4053) .

Cutting speed 　： 180m/min

Transmission 　 　：0.25mm/turn

Amount of cutting: 2.0mm

Cutting fluid　　：Water soluble cutting water

In the flea cutting of each cylindrical test piece, when the amount of wear on the clearance surface of the carbide insert reached 0.3 mm, it was determined that the life had been reached, and the cutting time (minutes) leading to that point was taken as the tool life. Needless to say, the longer the tool life, the better the machinability.

Since the absolute value of tool life is affected by cutting conditions, test piece shape, etc., "tool life improvement rate" was used as an index of the machinability improvement effect not affected by these. The tool life improvement rate is a value (A/B) obtained by dividing the tool life A of the nodular cast iron of each example and each comparative example by the tool life B of the nodular cast iron of the reference example representing the prior art level. Table 2 shows the tool life improvement rates (times) of Examples 1 to 16, Comparative Examples 1 to 11, and Reference Examples.

As is clear from Table 2, Examples 1 to 16 within the composition range of the present invention all had tool life improvement rates in the range of 1.0 to 1.3 times. From the results of Examples 1 to 16, it can be seen that the spheroidal graphite cast iron of the present invention has machinability equal to or higher than the conventional one. On the other hand, in Comparative Example 1 and Comparative Examples 7 to 11 in which the content of any one or more of Ti, V, Nb, and N was too high, the tool life improvement rate was less than 1.0 times, and the machinability was poor. On the other hand, in the spheroidal graphite cast iron of the present invention, the tool life improvement rate is preferably 1.1 times or more, more preferably 1.2 times or more, and most preferably 1.3 times or more.

(3) Tensile test

A test piece of No. 14 A of JIS 2201 was prepared from a 1-inch Y block, and a tensile test was performed at room temperature with an Amsler tensile tester (AG-IS250kN manufactured by Shimatsu Co., Ltd.) according to JIS 2241, and a tensile test was performed. Strength 0.2% yield strength and elongation were measured. The results are shown in Table 2.

As shown in Table 2, Examples 1 to 16 within the composition range of the present invention all have tensile strengths of 600 MPa or more, 0.2% proof stress of 350 MPa or more, and elongations of 12% or more, all of which are equal to or higher than conventional equivalents shown in reference examples. It was confirmed to have mechanical properties. On the other hand, Comparative Examples 2, 5, 7, and 11 outside the composition range of the present invention all have low tensile strengths of less than 600 MPa, and also have too high content of any one or more elements of Ti, V, Nb, and N. 1 and Comparative Examples 7 to 11 had an elongation as low as less than 12%. Further, Comparative Examples 3, 4, and 6 having a tensile strength of 600 MPa or more and an elongation of 12% or more had mechanical properties, but the gas defect area ratio was as high as 12.7% or more.

As described above, it was confirmed that the nodular graphite cast iron of the present invention is nodular graphite cast iron having mechanical properties and machinability equivalent to or better than those of the prior art, and also having excellent gas defect resistance.

10 flat test piece
11 pressure
12a tangdo
12b weir

Claims (7)

As a nodular cast iron having excellent gas defect resistance,
in mass ratio,
C：3.3~4%,
Si: 2 to 3%;
P: 0.05% or less (not including 0),
S: 0.02% or less (not including 0),
Mn: 0.8% or less (excluding 0);
Cu: 0.8% or less (excluding 0);
Mg: 0.02 to 0.06%;
Ti: 0.01 to 0.04%;
V: 0.001 to 0.01%;
Nb: 0.001 to 0.01%;
N: 0.004 to 0.008%, and
consisting of the balance Fe and unavoidable impurities,
Nodular graphite cast iron characterized by containing 0.015 to 0.045% of Ti, V and Nb in mass ratio in total.
According to claim 1,
Nodular cast iron characterized by containing Ti, V, Nb and N so as to satisfy the following formula (1).
0.8 ≤ (0.29Ti + 0.27V + 0.15Nb)/N ≤ 2.0 ... (1)
However, the element symbol in the formula (1) indicates the content (mass ratio (%)) of each element in nodular cast iron.
According to claim 1 or 2,
In terms of mass ratio, nodular graphite cast iron, characterized in that P: 0.005% or more, S: 0.005% or more.
According to claim 1 or 2,
In terms of mass ratio, nodular cast iron with Mn: 0.2% or more and Cu: 0.1% or more.
According to claim 1 or 2,
Spheroidal graphite cast iron, characterized in that the tensile strength is 600 MPa or more, and the elongation is 12% or more.
According to claim 1 or 2,
The gas defect area rate is 11% or less,
The gas defect area ratio is a value obtained by dividing the total area of the gas defects by the total zero area of the flat test piece.
According to claim 1 or 2,
The tool life improvement rate is more than 1.0 times,
The tool life improvement rate, the tool life of the target nodular cast iron, in mass ratio, C: 3.61%, Si: 2.41%, P: 0.021%, S: 0.009%, Mn: 0.41%, Cu: 0.39%, Mg: 0.045% Ti: 0.004%, V: 0.003%, Nb: 0.0008%, N: 0.003%, balance Fe and unavoidable impurities Divided by the tool life of nodular cast iron as a reference example Nodular graphite cast iron, characterized in that .

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