KR100296253B1 - Manufacturing Method of Osstemford Spheroidal Graphite Iron by Cooling Rate Control - Google Patents

Abstract

The present invention relates to a method for producing osstem-hard nodular cast iron by cooling rate control, and more particularly, to austenite by charging spherical graphite iron (Ductile Cast Iron) to which a special alloy element is not added in a salt bath or in an atmosphere furnace. After maintaining isothermal in the state, by controlling the cooling rate in the furnace to 800 ℃, the temperature at which the pearlite transformation occurs by cooling to suppress the nucleation of pearlite, to produce spheroidal graphite cast iron having a high tensile strength and excellent elongation Ostemford Spheroidal Graphite Cast Iron by Cooling Speed Control that can be used as a substitute for forgings or rolled products, and can be applied to a wide range of fields such as automobile parts, construction, mechanical design, industrial machinery, etc. It relates to a manufacturing method of.

According to the present invention, a method for preparing ostempered nodular cast iron by controlling the cooling rate is performed by charging the nodular cast iron in a salt bath maintained at 870 to 930 ° C. or in an atmosphere furnace to maintain heating for 60 to 180 minutes. Austenitic austenitic step (10) and the austenitic step (10) after the austenitic nitrite system is maintained at 300 to 400 ℃ by taking out austenitic spheroidal cast iron in a salt bath or atmosphere furnace After the quenching step (30) to quench in a salt bath of the, and the quenching step (30), isothermal maintenance for 30 to 90 minutes isothermally maintained in a nitrite-based salt bath of quenched spheroidal graphite iron After the step 40 and the isothermal holding step 40, the austenite in the air cooling step 50 to remove the nodular cast iron from the nitrite-based salt bath to cool at room temperature Saei of the quenching step (10) and the quenching step (30) are heated and maintained in a furnace at a temperature of 870 ~ 930 ℃ to austenitized spheroidal graphite cast iron at 800 ℃ at a cooling rate of 0.02 ℃ / sec ~ 0.5 ℃ / sec It is characterized by passing through the furnace cooling step 20 to slowly cool down.

[Index]

Nodular cast iron, austempering, austenite, pearlite, bainite, ferrite

Description

Marking process of austempered ductile cast iron utilized control of colling speed

The present invention relates to a method for producing osstem-hard nodular cast iron by cooling rate control, and more particularly, to austenite by charging a ductile cast iron (Ductile Cast Iron) to which a special alloy element is not added in a salt bath or in an atmosphere furnace. After maintaining isothermal in the state, by controlling the cooling rate in the furnace to 800 ° C, which is a misleading pearlite transformation by cooling, it suppresses the nucleation of pearlite, thereby providing spherical graphite cast iron having high tensile strength and excellent elongation. Ostemford spheroidal graphite by cooling rate control to manufacture and use forged or rolled products, cost reduction and light weight, applicable to a wide range of fields such as automobile parts, construction, mechanical design, industrial machinery It relates to a method for producing cast iron.

In general, in order to provide high strength and toughness and to prevent deformation or cracking due to quenching, first of all, spheroidal black cast iron is charged into a heating furnace to correspond to room temperature. Spheroidal graphite is heated by heating at a temperature of 900 ° C, corresponding to austenite and point B, and maintaining the heated nodular cast iron in the furnace for about 120 minutes under the electric temperature (B → C). After quenching the cast iron, the austenitic nodular graphite cast iron was removed from the heating furnace and quenched by immersion in a salt bath maintained at a temperature of 350 ° C. corresponding to the point D of the bainite region. , C → D), and 350 ℃ is maintained in the salt bath for a certain time (D → E), and then the nodular cast iron is taken out of the salt bath to the point A corresponding to room temperature. It is used in the tempering agarose (Austempering) in a process of air-cooling.

However, in the conventional method of manufacturing osstempered nodular cast iron, the tensile strength of the ostempered nodular cast iron can be improved to a relatively high level, but the elongation ratio affecting the ductility of the material is very high. In addition to lowering the mass effect, the copper (Cu) is described in Finnish Patent No. 10116390 for the purpose of increasing the mass effect (the degree of heat treatment effect varies depending on the mass and cross-sectional dimension of the material). ), A separate alloying element such as molybdenum (M0), nickel (Ni) or the like, or molybdenum (Mo) is added to about 0.1 to 0.26%, there was a problem that the ostamping treatment by a general method.

As described above, when alloying elements such as copper, molybdenum, and nickel are added for the purpose of increasing the mass effect in the process of producing spheroidal graphite cast iron by the conventional ostempering treatment, due to the addition of alloying elements As the number increases and the cost of expensive alloy element is added, the production cost of spheroidal graphite cast iron is increased. In addition to increasing the production cost of the various products to be, there was a problem such that the cutting scrap after the machining of the aforementioned nodular cast iron has to be managed separately.

In order to solve the above-mentioned problems, the present invention charges agglomerated graphite iron into a salt bath or an atmosphere furnace and heats it to austenitized region at 870 to 930 ° C to austenitize the nodular graphite iron. After controlled cooling in a furnace at a rate of 0.02 ° C./sec to 0.5 ° C./sec from 930 ° C. to 800 ° C., where the pearlite transformation occurs by cooling, it is rapidly cooled and isothermally maintained in a nitrite salt bath maintained at 350 ° C. By air-cooling at room temperature, it is possible to manufacture spheroidal graphite cast iron with high tensile strength and excellent elongation, so that it can be used as a substitute for forged or rolled products. Addition of expensive alloying elements and increase of the number of processes To prevent an increase in production costs and at the same time and according to the object of the present invention that it can provide Osstem Preferred nodular cast iron excellent in machinability.

1 is a schematic diagram of a conventional heat treatment cycle.

Figure 2 is a process block diagram showing a method for producing ostemford spherical graphite iron by the cooling rate control according to the present invention.

3 is a schematic diagram of a heat treatment cycle according to the present invention;

Figure 4 is a graph showing the strength and elongation according to the change in cooling rate of the specimen subjected to the osmosis treatment according to the present invention.

5 is a graph showing the strength-ductility balance value and the residual austenite content according to the cooling rate change of the Ostempered specimen according to the present invention.

6 is a TTT (isothermal transformation) curve of nodular cast iron

7 is a graph showing the relationship between the strength-ductility balance value and the elongation according to the retained austenite content.

Figure 8 is a scanning electron micrograph of the specimen subjected to the osmosis treatment according to the present invention.

Figure 9 (a) and (b) is a transmission electron micrograph of the specimen subjected to the ostampering process by the conventional method and the method of the present invention.

In order to achieve the above object, the present invention provides austenitization step of austenitizing spheroidal graphite iron by charging spheroidal graphite iron in a salt bath or atmosphere furnace maintained at 870-930 ° C. and maintaining it for 60 to 180 minutes. And, after the austenitic step (10), the quenching step of taking the austenitic nodular cast iron in a salt bath or in an atmosphere furnace and quenching it in a nitrite-based salt bath maintained at 300 to 400 ° C. 30) and an isothermal holding step 40 for isothermally maintaining the quenched spheroidal graphite iron for 30 to 90 minutes in a nitrite-based salt bath at 300 to 400 ° C. after passing through the quenching step 30, and the isothermal After undergoing the holding step 40, in the air cooling step 50 to remove the nodular graphite iron from the nitrite-based salt bath to cool at room temperature, between the austenitizing step (10) and the quenching step (30) On It is characterized in that it passes through a furnace cooling step (20) of gradually heating the austenitized spheroidal graphite cast iron at a temperature of 870 to 930 ° C to 800 ° C at a cooling rate of 0.02 ° C / sec to 0.5 ° C / se. .

Hereinafter, an embodiment of the present invention will be described in detail with reference to the accompanying drawings, and the specimen used in one embodiment of the present invention is a kind of spherical graphite cast iron that is generally used GCD 45 (KS standard ferrite and pearlite a base having a mixed tissue, dissolving the Brinell hardness is 143~217H _B) KS _B 0801 4 test specimen (a metal material tensile test specimen according to the Korea industrial standards, the gauge length (L) 50mm, length of parallel portion (P) About 60mm, diameter (D) 14mm, shoulder radius (R) round bar having a dimension of about 15mm or more) was used and the chemical composition of the specimen is as shown in Table 1.

[Table 1]: Chemical Composition of Specimen

In addition, one specimen is austenitized for 2 hours in a nitrogen gas (N ₂ ) atmosphere maintained at 900 ℃, then quenched immediately in a nitrite-based salt bath maintained at 350 ℃ isothermally maintained Ossampling treatment and the other specimens were austenitized for 2 hours in a nitrogen gas (N ₂ ) atmosphere maintained at 900 ° C., and then 0.02 ° C./sec to 5 ° C./sec from 900 ° C. to 800 ° C. After the controlled cooling at different cooling rates within the cooling rate range, and quenched in a nitrite-based salt bath maintained at 350 ° C. to maintain isothermal treatment, the osmampling treatment was carried out by the method according to the present invention. The same was followed, and isothermally maintained in a salt bath, followed by air cooling.

The process of the embodiment according to the present invention is a nitrogen gas (N ₂ ) to maintain the spherical graphite cast iron, which is primarily a specimen at 900 ℃, as shown in the process block of Figure 2 and the schematic of the heat treatment cycle of Figure 3 Charged into an atmosphere furnace and heated to a temperature of 900 ° C. corresponding to the point B of the austenitized region at room temperature A point (A → B) to maintain the nodular cast iron in the atmosphere for about 2 hours (B → C) through the austenitization step (10) of austenitic nodular cast iron, and through the austenitic step (10) described above, carbon present in graphite and carbide inside the nodular cast iron is diffused into ferrite. As a result, the carbon concentration of the matrix structure becomes uniform and the carbon concentration in the austenite is increased.

After the austenitic step (10) as described above, the spheroidal graphite cast iron as a specimen is maintained at a temperature of 900 ℃ corresponds to the point F before the pearlite transformation occurs at the point C of the austenitized state The furnace is subjected to the furnace cooling step 20 of gradually cooling the furnace at 0.02 ° C./sec to 5 ° C./sec in the furnace up to 800 ° C., and the change in the cooling rate in the furnace cooling step 20 described above is the strength of the spheroidal graphite cast iron. In order to investigate the effect on the elongation and elongation, the specimens in the normal casting state without the osmosis treatment, and the cooling rate range of 0.02 ° C / sec to 5 ° C / sec in three stages during cooling from 900 ° C to 800 ° C Specimens according to the present invention, which were divided and controlled cooled, and specimens according to the conventional method, which were rapidly cooled at a cooling rate of 10 ° C./sec from 900 ° C. to 800 ° C., were respectively installed in an Instron 4210 tensile tester. And, a slip strength, yield point and elongation were measured with a tensile speed of the tensile tester to 50mm / min.

As shown in the graph of FIG. 4, the experimental results show that the tensile strength and the yield strength are more than two times higher than those of the cast state under all of the ostamping treatment conditions. Yield strength in the case of is not significantly affected by the cooling rate during control cooling, but shows a similar tendency, and tensile strength also shows a similar tendency to yield strength, but at a cooling rate of 0.02 ℃ / sec, 1040MPa, 10 ℃ / The cooling rate of sec shows a tendency to increase slightly as the cooling rate increases to 1088MPa, and in the case of the elongation, the cooling rate decreases rapidly when the cooling rate increases from 0.02 ℃ / sec to 2 ℃ / sec, in particular 0.02 The elongation of the specimen initially cooled slowly at a cooling rate of ℃ / sec was 15.6%, and the elongation of the specimen rapidly cooled at a cooling rate of 10 ° C / sec from 900 ° C to 800 ° C was 6.0%. It was about 2.6 times.

In addition, in the graph of FIG. 5 showing the strength-ductility balance value and residual austenite content according to the cooling rate, the strength-ductility balance value showed the maximum value at 1656 kgf / mm ^2. % At 0.2 ° C / sec, which was the slowest cooling rate. It can be seen that as the cooling rate increases after the increase, the strength-ductility balance value described above is equivalent to 802kgf / mm ² . Compared to 666.1kgf / mm ^2. %, Which corresponds to the specimen rapidly cooled from 10 ℃ / sec to 10 ℃ / sec, it is more than doubled in each case, and the residual austenite content is also the highest. The maximum reached 39.7% at slow 0.02 ° C / sec.

Therefore, the strength-ductility balance value depends very much on the content of the austenitic austenite, and the content of the retained austenite depends very much on the cooling rate, and as the supercooling decreases, that is, the slowing down the cooling rate, The content of knight is increased, this cause is described with reference to the isothermal transformation curve (TTT curve) of the nodular graphite cast iron shown in FIG.

As shown in Fig. 6, the Ps lines (1, 2) serving as the starting line for pearlite change depending on the content of alloying elements, and in the spheroidal graphite iron without addition of alloying elements such as nickel, molybdenum or copper, In this case, if the cooling area is not very high, in this case, if the cooling rate is not very high, the cooling line overlaps the Ps line, which is the formation curve of pearlite, as in the cooling curve c, and pearlite is inevitably generated. Slow cooling is continued until the temperature is 800 ° C. to allow the upper metastable austenite region to pass slowly, followed by quenching, thereby inhibiting nucleation of pearlite and moving the Ps line to the right (2 → 1).

As a result, even when a separate alloy element is not added for the purpose of increasing the mass effect, the same effect can be obtained when the alloying element is added. As a result, residual austenite and bainitic ferrite can be obtained. ) As the amount of residual austenite increases, as shown in the graph of 7, the strength-ductility balance value and elongation of the austem-treated spheroidal graphite extract are increased, Tensile strength is increased by tripping, and bainitic ferrite is to increase toughness more than bainite containing carbides. In the cooling step according to the present invention, the residual austenite The optimum cooling rate is 0.02 based on the strength-ductility balance value and elongation according to the content. Restricted to /sec~0.5℃/sec.

After the furnace cooling step 20 as described above, by immersing the nodular graphite iron in the nitrite-based salt bath maintained at 350 ℃ in the atmosphere furnace, the spheroidal graphite iron in the bainite formation region at 800 ℃ corresponding to the F point A quenching step (30) is performed to quench (F → G) up to 350 ° C., which belongs to the G point, and as shown in FIG. 6, the Ps line corresponding to the pearlite nose part is contacted with the cooling line as shown in FIG. This prevents pearlite nucleation.

After passing through the quenching step 30, the quenched nodular cast iron is subjected to an isothermal holding step 40 of isothermal holding (G → H) for 1 hour in a salt bath of 350 ° C nitrite. After the ferrite and a large amount of retained austenite are produced, the present invention is subjected to an air-cooling step (50) in which spheroidal graphite cast iron, which has undergone the aforementioned isothermal holding step (40), is taken out of a nitrite-based salt bath and cooled in air at room temperature. The manufacturing method of the ostempered nodular cast iron by the cooling rate control according to this is completed.

Scanning electron micrographs of the ostamped nodular cast iron prepared by the method of treating the osmosis according to the present invention as shown in Figure 8, as shown in Figure 8, the above picture is austenitized at 900 ℃ to 800 ℃ Scanning electron micrograph of the specimen which was slowly cooled at 0.02 ℃ / sec and treated with ossampling at 350 ℃, it can be seen that it is formed by the synthetic structure of bainitic ferrite and retained austenite. In the process of undergoing the isothermal maintenance step 40, the supersaturated carbon is discharged into the austenite as the ferrite nucleates and grows at the austenite grain boundary and the austenite and graphite interfaces.

In addition, as shown in (a) and (b) of FIG. 9, plate bainite as a transmission electron micrograph of a specimen subjected to austenitization at 900 ° C. and immediately austerized in a 350 ° C. isothermal salt bath. In the photograph corresponding to the dark field (dark field, dark field of view) of the bainite area, carbides are not observed inside bare night, but cooling rate of 0.02 ° C / sec from 900 ° C to 800 ° C is observed. In the picture (b) corresponding to the bright field (bright field) of the retained austenite as a transmission microscopic image of the specimen initially cooled by slow cooling, it can be seen that the retained austenite exists between the bainite plates.

As described above, compared to the conventional cast spheroidal graphite cast iron which is not subjected to the osmosis treatment, the osmosis spherical graphite cast iron produced by the manufacturing method of the present invention has elongation of 16.2% and 15.6%, respectively. Although the spheroidal graphite cast iron treated by the present invention is somewhat reduced compared to the structural graphite cast iron in general casting state, in terms of tensile strength, the spheroidal graphite cast iron in the general cast state is 485 MPa. 1040 MPa, which is more than doubled, and compared with the conventional ostempering treatment method that cools directly from 900 ° C to 350 ° C, in the case of tensile strength, 1088MPa is cooled directly and 1040MPa is slowly cooled to 800 ° C. In this case, the value changes abruptly depending on the cooling rate. Especially, it is slowly cooled to 0.02 ℃ / sec The elongation increased by 6.0 times from 6.0% to 15.6% compared to the case where ostemperized nodular cast iron was continuously cooled to the isothermal treatment temperature under the same treatment conditions.

As described above, the osstem-hard nodular cast iron manufactured by the manufacturing method according to the present invention has high tensile strength and excellent elongation, so that it may be used as a substitute for forging or rolled products, and in order to increase the mass effect during the osstempering treatment, Even if the alloy element is not added, the same effect as that of the alloy element can be obtained, thereby preventing the increase of the cost of the expensive alloy element and the increase in the number of processes, and at the same time, the excellent processability of Osstemford It is possible to manufacture nodular cast iron, which can be applied to a wide range of fields such as automobile parts, construction, mechanical equipment, and industrial machinery.

As described above, according to the present invention, the method for preparing ostemford spheroidal graphite cast iron by cooling rate control is carried out in a salt bath or in an atmosphere furnace by adding spheroidal graphite iron to which no special alloy element is added, and isothermally maintained in an austenite state. By suppressing the nucleation of pearlite by controlling the cooling rate in the furnace to the temperature at which the pearlite transformation occurs due to cooling, alloying elements such as copper, molybdenum, nickel, etc. are not added for the purpose of increasing the mass effect during the ostempering treatment. Even if the alloying element is added by simple improvement of heat treatment method, it can have the same effect as the case of adding alloy element, and it can reduce the production cost due to the addition of expensive alloy element and the number of processes due to the addition of alloy element. There is an effect that can be reduced.

In addition, it is possible to use Ostemford Spheroidal Graphite Cast Iron, which has high tensile strength and excellent drawing efficiency, so that it can be used as a substitute for forgings or rolled products, and improves the machinability of Ostemford Spheroidal Graphite Cast Iron. In addition, it is possible to reduce the production cost of the product, and at the same time, to solve the inconvenience of separately managing the cutting scrap generated after the machining of the aforementioned nodular cast iron.

Claims (1)

An austenitizing step (10) of austenitizing nodular cast iron by charging Ductile Cast Iron in a salt bath or atmosphere furnace maintained at 870 to 930 ° C and heating for 60 to 180 minutes; After the austenitization step (10), the quenching step of quenching the austenitic nodular cast iron in a salt bath of a nitrite system maintained at 300 to 400 ℃ in a salt bath or atmosphere furnace, and After the quenching step 30, the isothermal holding step 40 for isothermally maintaining the quenched graphite cast iron isothermally maintained for 30 to 90 minutes in a nitrite-based salt bath of 300 to 400 ℃, After passing through the isothermal holding step (40), in passing through the air-cooling step (50) to remove the spheroidal graphite iron in the nitrite-based salt bath, and cooled at room temperature, Between the austenitic step 10 and the quenching step 30, the austenitic nodular cast iron is heated in a furnace at a temperature of 870 to 930 ° C. at a cooling rate of 0.02 ° C./sec to 0.5 ° C./sec. A method for producing osstemford spheroidal graphite iron by cooling rate control, characterized in that it passes through a furnace cooling step (20) to gradually cool down to 800 ° C.