KR850001162B1 - Ductile cast iron roll - Google Patents

Abstract

The present invention relates to a ductile cast iron roll, which is excellent in its resistance to breaking and which has a chemical composition comprising 3.0-3.8% C, 1.5-2.5 % Si, 0.2-1.0% Mn, 0.01- 0.2 % P, less than 0.06% S, 0.7-3.0% Ni, 0.1-0.6% Cr, 0.1-0.8% Mo, 0.02-0.1% Mg, the balance in iron and unavoidable impurities. The base structure has a fine two-phased structure of ferrite mingled with pearlite.

Description

Ductile cast iron for hot rolled rolls

1 is a diagram showing the transformation state when the ductile iron is heated as the amount of expansion.

2 is a table showing the heating situation in the heat treatment method according to the present invention.

3 is a chart showing the relationship between the heat treatment temperature and the mechanical properties of the resulting rolls.

4 is a cross-sectional view of an essential part showing an example of a roll for attaching a caliber.

5 is a photograph of 2000 times magnification showing the casting structure of ductile cast iron.

6 is a photograph at 2000 times magnification showing the structure of a roll using ductile cast iron of the present invention.

7 (a), 7 (b) to 10 (a), and 10 (b) show another example of a roll structure using ductile cast iron of the present invention.

7 (a) is a photographic figure with a magnification of 100 times.

Figure 7 (b) is a photographic figure with a magnification of 400 times.

The present invention relates to a ductile cast iron for hot rolled roll excellent in fracture resistance.

In the rough rolling stand in a hot rolling facility, the rolling material is hot and the rolling speed is slow, so that the heat load on the roll is extremely high.

Therefore, the rough rolling roll is likely to be cracked due to thermal fatigue, and the roll loss due to the diffusion of the crack was often caused under severe rolling load conditions.

Conventionally, rolls made of ductile cast iron have been used in hot rolling equipment, and rolls made of ductile cast iron have a structure in which spherical graphite is dispersed in the base, and the graphite suppresses the spread of the cracks and forms cracks. It has a relatively good crack resistance since it is dispersed in the

However, the roll using the conventional ductile cast iron still lacks crack resistance against severe use conditions of heat and load as in the rough-rolled stand in hot rolling described above, and it is not easy to cause a breakage accident.

Of course, the occurrence of a roll breaking accident does not only increase the amount of roll consumption, but also leads to a decrease in rolling efficiency and a decrease in productivity.

Imparting excellent fracture resistance to the roll must improve resistance to crack generation and diffusion, that is, crack resistance.

In order to improve this crack resistance, it is necessary to improve the toughness, that is, the elongation value in the tensile test and the amount of curvature in the fracture resistance test.

As such a method, there is a method of reducing the amount of cementite in the tissue or increasing the toughness of the basic ground.

Since the former method has been conventionally performed, excessive reduction of cementite leads to a decrease in wear resistance. Therefore, when a constant wear resistance is required, such as a rolling roll for the present invention, toughness is required. There is a limit to improvement.

As a result of various studies focused on the latter method, the present invention maintains good abrasion resistance by specifying chemical composition and structure as described below, and improves resistance to occurrence of thermal cracking and its development. It is equipped with the fracture resistance which surpasses material.

SUMMARY OF THE INVENTION In view of the above problems, the present invention provides a ductile cast iron for hot rolled rolls having sufficient cut resistance to withstand harsh use conditions.

The present invention is about 2.0-3.8% of C (% by weight, the same below), Si about 1.5-2.5%, Mn about 0.2-1.0%, P about 0.01-0.2%, S about 0.06% or less, Ni about 0.7-3.0 %, Cr about 0.1-0.6%, Mo about 0.1-0.8%, Mg about 0.02-0.1%, the remainder has the composition of components consisting of iron and inevitable impurities, and also as the basic structure, the two phases of ferrite and perlite (相) It is an object to provide ductile cast iron for hot rolled rolls with excellent fracture resistance that has a structure.

Hereinafter, the component limitation of the roll material using the ductile cast iron of this invention is demonstrated.

C is directly involved in the formation of cementite and graphite in the base. If the amount of C is less than about 3.0%, the amount of cementite and graphite is insufficient, resulting in insufficient thermal cracking dispersal effect, and thus the characteristics as ductile cast iron cannot be sufficiently exhibited.

However, if the amount is large, it causes weakening of the material, so the upper limit is about 3.8%.

Si determines the amount of cementite, and if it is less than about 1.5%, the amount of cementite becomes excessively high, causing weakening, or the coexistence range of ferrite and austenite is lost.

In the present invention, the two-phase structure of ferrite and ferrite is obtained by using the ferrite and austenite coexisting ranges at the vacancy transformation temperature in the heat treatment as described later.

When the Si becomes insufficient due to lack of the ferrite austenite coexistence range, the above-described predetermined basic structure cannot be obtained.

The transformation state in heating two types of cast iron (a) and (b) which differ in component composition from FIG. 1 is shown as expansion amount. (Heating rate: 194 ° C) However, (a) refers to cast iron containing C 3.38%, Si 2.08%, Mn 0.49%, Ni 1.76%, Cr 0.18%, and Mo 0.16% (range of the present invention), ( b) is cast iron containing C 3.41%, Si 1.08%, Mn 0.72%, Ni 2.41%, Cr 0.62% and Mo 0.48% (lack of Si content, excess Cr content).

The small amount of vacant transformation in (a) is due to coexistence of ferrite and austenite due to the precipitation of ferrite, whereas the large amount of vacancy transformation in (b) with small amount of Si is completely austenitized Indicates that austenite is in one phase.

Therefore, the amount of Si is made about 1.5% or more in order to avoid complete austenitization and to coexist with ferrai and austenite.

On the other hand, when the content exceeds about 2.5%, the amount of graphite becomes too large to be softened and the basic ground tissue becomes weak, and the transition temperature is increased, which is not preferable.

Mn is effective to suppress the ratio of S incorporated as impurities. For this reason, about 0.2% or more is added.

In addition, Mn has the effect of increasing the hardness of the base, but if the content is too high, the material becomes vulnerable, so the upper limit is about 1.0%.

P weakens the material, so it should be about 0.2% or less.

In addition, P has a good flowability of the melt and increases wear resistance, so that the upper limit is about 0.2%, and about 0.01% or more may be added.

S, like P, weakens the material and hinders the spheroidization of graphite, so it is about 0.06% or less.

Ni has the effect of increasing the graphitization and the hardness of the base. However, if the content is small, the amount of graphite is insufficient, and the hardness of the base is also lowered.

For this reason, it shall be about 0.7% or less.

However, when it exceeds about 3.0%, the amount of cementite decreases, and it becomes easy to become bainite or martensite whose base ground is thermally unstable, which is not suitable as a roll material for hot rolling.

Cr is a cementite stabilizing element, but less than about 1% of cementite almost disappears, resulting in a lack of wear resistance.

On the other hand, when it exceeds about 0.6%, the amount of cementite becomes excessively large, and it becomes difficult to obtain a basic base structure composed of two phases of ferrite and pearlite for the purpose of the present invention, in addition to weakening of the material. Mo has the effect of stabilizing cementite and increasing the hardness of the base.

However, at less than about 0.1%, the amount of cementite is small, and the hardness of the base is also insufficient.

On the other hand, if it exceeds about 0.8%, the cementite becomes too large, and the hardness improvement effect of the base is also saturated, which is not economical.

Mg is required for graphite spheroidization.

If the amount is less than about 0.02%, the effect is insufficient. On the other hand, if the amount is more than about 0.1%, defects in the casting are likely to occur, resulting in weakening of the material due to an increase in the amount of cementite.

The present invention roll ash may include Al and rare earths in addition to all the above-mentioned elements.

Since Al has a graphitization effect, it is good to contain it according to the use of a roll, and it is especially effective in the case of the roll material used for the stand | strict heat and load conditions.

However, since graphite spheroidization is hindered, the upper limit is about 3%.

Rare earth elements such as La and Cel are effective as graphite spheroidizing stabilizers. However, if the total amount of one or two or more kinds exceeds about 0.05%, the effect is substantially saturated, so the upper limit is set to about 0.0%.

The present invention roll made up of the above-mentioned ingredient composition has one phase of perlite (see Fig. 5) until casting, or a perlite having a two-phase base structure of ferrite and perlite as shown in Fig. 6 by heat treatment. The part imparts good wear resistance required as a roll for rolling, while the ferrite part contributes to remarkably improved fracture resistance by suppressing the diffusion of cracks and increasing toughness.

In order to acquire this ferrite and pearlite two-phase structure, it is good to make into a coexistence structure of a ferrite and austenite once, and to transform austenite into a pearlite in this state.

According to the present invention, the roll of the composition of the above components is maintained at a temperature of about 780-850 ° C. for about 20 hours or less, and then a heat treatment is performed to cool down to about 600 ° C. at a cooling rate of about 50-300 ° C./Hr.

Referring to the heat treatment method shown in Figure 2 the heat treatment method of the present invention is as follows.

Parts (A) and (B) are temperature rise processes.

The heating rate may be appropriately adjusted according to the capacity of the heat treatment furnace so that no damage such as damage due to thermal deformation occurs, and there is no particular limitation other than this.

Part (C) is the most important process in the heat treatment method of the present invention. That is, in this process, the two-phase structure of the ferrite austenite is formed, and then the target two-phase structure of the ferrite and perlite is formed by performing austenite transformation of the austenite in the cooling step of the next part (D). Get

The heat treatment temperature in this process greatly affects the material properties of the finally obtained roll.

3 shows the relationship between the heat treatment temperature and the mechanical properties of the rolls. Curve (i) shows tensile strength (kg / mm ² ) and (ii) shows elongation (%).

In addition, any maintenance holding time in the same heat processing was 2 hours, and the cooling rate in the process (D) after heat processing was 70 degreeC / Hr.

The dimensions of the rolls and the chemical composition of the rolls provided for the test were as follows. The test pieces were taken at a position of 140 mm from the manifestation of the center of the body part.

Roll dimension: 780mmψ × 2200ml (body part)

Chemical composition: C 3.38%, Si 2.03%,

Mn 0.49%, P 0.078%,

S 0.012%, Ni 1.76%,

Cr 0.18%, Mo 0.16%,

Mg 0.051%,

The rest is iron and inevitable impurities.

As shown in the drawing, the tensile strength gradually increases as the heat treatment temperature increases, while the elongation rapidly changes at its peak around 820 ° C.

In other words, if the heat treatment temperature is less than about 780 ° C, C in the base is adsorbed on the graphite, and thus the basic base hardness of the finally obtained roll material becomes low, and the perception of the periphery of the graphite becomes insufficient. .

On the other hand, when it exceeds about 850 degreeC, a structure will become an austenite 1 phase, and the basic base of the target ferrite and pearlite 2 phase will not be obtained, and toughness will fall slightly.

Therefore, this heat treatment temperature is about 780-850 degreeC.

7, 8, and 9 show the microscopic structure of the roll obtained when the heat treatment temperatures in the test described above were 780 ° C, 820 ° C, and 840 ° C, respectively. (B) is 400).

All of them show two-phase structure of ferrite and pearlite, and it can be seen that the structure is excellent in toughness and wear resistance.

In addition, the formation and material properties of the two-phase basic ground tissue of the present invention roll are dependent on the chemical composition and the interrelationship between the three factors of the salting temperature and the holding time in the above-mentioned process (C). As a tendency, the toughness is the highest at the low temperature at the low Si% -high Cr% composition, and at the high temperature at the high Si% -Cr% low 800 ° C composition.

In addition, the longer the holding time, the higher the temperature characteristics are displayed, for example, the same hardness and toughness as the case of holding for 10 hours at 800 ℃ and less than 1 hour at 820 ℃.

In the above-described temperature range of the component aid of the present invention, the total holding time does not need to exceed about 20 hours, and it is uneconomical, so it is appropriately selected within the range of about 20 hours or less depending on the composition and the set temperature. Do it.

When the austenite is transformed into pearlite in the cooling step in the following part (D) after the biphasic structure of the petite austenite in the above (C) process, the cooling rate is about 50 ° C. Hardness decreases because the pearlite granulates in the vicinity of about 720 ° C during the cooling of tinnitus less than / Hr.

As the cooling rate is increased, the produced pearlite becomes dense, and thus high hardness and excellent wear resistance are obtained.

FIG. 10 is a microscopic structure of a roll obtained by treating at a heat treatment temperature of 820 ° C. and a holding time of 2 hours using a roll of the same composition as in the test of FIG. 3 described above, followed by cooling at 300 ° C./Hr. 10 times (a) is 100 times, and 10 times (b) is 400 times).

Compared with the structure of FIG. 8 (the cooling rate is 70 ° C./Hr) treated as the same heat treatment condition, it can be confirmed that the pearlite portion is densified.

In this regard, the mechanical properties of both are compared with the first table.

[Table 1]

As shown in the above table, it can be confirmed that the strength and hardness (wear resistance) are improved without decreasing the toughness as the cooling rate is increased.

However, the cooling rate exceeding about 300 DEG C / Hr is not only difficult to obtain in actual work, but also the base is excessively hard, resulting in a decrease in toughness.

Therefore, the cooling rate is about 50-300 ° C / Hr.

In addition, the final temperature of this cooling process (D) is about 600 ℃ or less, because the transformation is required to complete the end.

Part (E) following the above cooling process is the deformation cooling is a salt cooling process, and the deformation and internal stresses of the rolls resulting from the aforementioned heat treatment are maintained by maintaining them at about 450-650 ° C. according to a conventional method. Remove

The retention time does not need to exceed about 20 hours.

In the case of using a carrier attachment roll, it is common to carry out the carver processing after heat treatment.

However, since the cooling speed of the roll in the cooling process of the above-mentioned part (D) is gentle on the inside compared with the roll surface, the density of the pearlite produced inside is low compared with the surface.

Therefore, the internal hardness becomes lower than that of the surface, and as a result, the wear resistance of the bottom portion of the carver formed by processing is also low.

This may be carried out prior to the heat treatment.

4 shows an example in which a box-shaped carry 3 is formed on the roll body 1. The solid line l is the shape before the heat treatment carried out by the carver processing leaving the thickness of the remaining part, and (m) indicates the shape finished in the predetermined dimension by the cutting process after the heat treatment.

The thickness (n) of the remainder is usually about 1-20 mm. In this way, if the heat treatment according to the present invention is carried out after the carver processing, the cooling rate of the vicinity of the lower part of the carrier 2 is sufficiently large, and the pearlite is also densified.

As a result, the hardness of the bottom portion of the carver is about 1-5 Hs higher than the hardness at the same diameter position as the bottom portion of the flange portion 4, and the wear resistance of the roll is further improved.

In addition, the roll of the present invention may be used as a double structure composed of an outer layer having the composition and structure of the present invention prescribed by the present invention and an inner layer of a suitable material, if necessary.

As described above, the roll using the flexible cast iron of the present invention has a basic base structure composed of two phases of tough ferrite and dense and hard pearlite. You will have

Therefore, when it is used for rolls for rough rolling stands, etc. in a hot rolling facility, it withstands severe use conditions well and contributes to the improvement of the rolling efficiency and the productivity.

Claims (1)

C 3.0-3.8% (weight% or less), Si 1.5-2.5%, Mn 0.2-1.0%, P 0.01-0.2%, S 0.06% or less, Ni 0.7-3.0%, Cr 0.1-0.6%, Mo 0.1-0.8%, Mg 0.02-0.1%, the remainder is composed of iron and inevitable impurities, and the soft cast iron for hot rolled rolls, characterized in that the basic base structure is a two-phase structure of ferrite and pearlite.

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