KR850000433B1 - Method product of ductile cust iron roll - Google Patents

Abstract

A manufacturing method of ductile cast-iron roll for hot rolling is characterized by maintaining cast-iron roll containing carbon:3.0- 3.8%, silicon:1.5-2.5%, manganese:0.2-1.0%, phosphorus:0.01-0.2%, sulphur:below 0.06%, nickel:0.7-3.0%, chrome: 0.1-0.6%, molybdenum:0.1-0.8%, magnesium:0.02-0.1%, iron and inevitable impurities within 20 hours at the temperature range of 780≰C-850≰C, cooling it to 600 ≰C in the speed of 50-300≰C/hour and then preserving it within twenty hours at the temperature range of 450≰C-650≰C.

Description

Preparation of Ductile Cast Iron Rool for Hot Rolling

Each drawing is a drawing marked for implementing the manufacturing method of the present invention.

1 is a diagram showing the expansion of the transformation state when heating the dichroic cast iron.

2 is a diagram specifically showing the heat pattern of 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 olol.

4 is a cross-sectional view of an essential part showing an example of a calibrator attachment rool.

5 is a photograph of a magnification of 2,000 times showing the casting structure of the dilution tile.

6 is a photograph of a magnification of 2,000 times indicating operation of a dichroic cast iron rool produced by the manufacturing method of the present invention.

7 (a), 7 (b) to 10 (a), and 10 (b) are photographs showing another example of the duck tile cast iron roll tissue manufactured by the manufacturing method of the present invention. ,

(a) degree is photograph photograph of magnification 100 times. (b) is a photograph of the magnification 400 times.

The present invention relates to a method of manufacturing a hot rolled dilution tile cast iron roll, and more particularly, to a hot rolled dilution tile cast iron furnace which can prevent the occurrence of cracking due to thermal fatigue during hot rolling. It relates to the manufacturing method of the oar.

In general, the rough rolling stand in a hot rolling facility has a high temperature of the rolled material and a low rolling speed, so the heat load on the roll is extremely large. For this reason, cracks due to thermal fatigue tend to occur in the rough rolled rolls, and under the strict rolling load conditions, the breakage accidents of the rolls due to the diffusion of the cracks often occur.

Dirktile cast iron rool has good crack resistance because it has a dispersed structure of spheroidal graphite in the basic ground, but it also suffers frequent breakage accidents in the strict rough rolling stand under thermal conditions. Therefore, the amount of cementite was suppressed in the dilute tile cast iron rool for rough rolling stands up to now.

However, the cementite has a problem that reducing the amount of cementite in order to improve wear resistance at high temperatures reduces the wear resistance.

The present invention is to solve the drawbacks and problems of the rool of the rough rolling stand in the conventional hot rolling described above, a state in which a certain amount of cementite is present, the tissue is improved to crack resistance, fracture resistance It is a method of manufacturing a dilution dike tile cast iron olol for hot rolling. Characterized in the manufacturing method of the hot-rolled duck tile cast iron olol according to the present invention, C 3.0-3.8% (same by 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 about 0.02-0.1%, the remainder of cast iron rool consisting of iron and unavoidable impurities has a temperature of 780 ℃ -850 ℃ After holding for 20 hours or less, the solution is cooled to 600 ° C or lower at a cooling rate of 50-300 ° C / Hr, and then maintained for 20 hours or less at 450-650 ° C.

The rool by the manufacturing method of the hot rolling dike cast iron rool which concerns on this invention has a mixed structure of ferrite and perlite, and this can be obtained by heat-processing at the temperature of 780 degreeC-850 degreeC. More specifically, the rool according to the manufacturing method of the hot rolled dilution cast iron rool according to the present invention is a ferritic structure in the vicinity of cementite, and is mixed with ferrite and perlite at a distance from the cementite. I have an organization. Hereinafter will be described in detail with respect to the manufacturing method of the hot-rolled duck tile cast iron rool of the present invention.

When explaining the components and composition ratios of the rool material in the hot-rolled dilution cast iron rool of the present invention, when the C content is less than about 3.0%, the amount of semanite and graphite is insufficient and the dispersion effect of thermal cracking is insufficient. It cannot fully exhibit the characteristics as a dilute tile cast iron. However, if the amount is large, it causes weakening of the material, so the upper limit is about 3.8%.

Si depends on the amount of cementite, less than about 1.5%, and the amount of cementite becomes excessive, leading to fragility or losing the coexistence range of ferrite and austenite. As for the rool manufactured by the manufacturing method of this invention, a two-phase structure of a ferrite and a pearlite is obtained by using the ferrite and austenite coexistence range in a vacancy transformation temperature in heat processing as mentioned later.

When the ferrite and austenite coexisting ranges disappear due to the lack of Si, 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: 194C / Hr) 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 amount, excess Cr amount).

The small amount of vacancy transformation shrinkage in (a) is due to the coexistence of ferrite and austenite due to the precipitation of ferrite, whereas the large amount of vacancy transformation shrinkage of (b) with small amount of Si is due to complete austenitization. It shows that it is in the stenite unitary state. Therefore, in order to avoid complete austenitization and to coexist with ferrite and austenite, the amount of Si is made into about 1.5% or more. On the other hand, exceeding about 2.5%, the amount of graphite is excessively softened and softened, the base tissue becomes fragile, and the transition temperature is increased, which is undesirable. Mn is effective to suppress the ratio of S mixed as an impurity.

For this reason, about 0.2% or more is added. In addition, Mn is rich in the effect of increasing the hardness of the base, but if the content is too high, the material becomes weak, so the upper limit is about 1.0%.

S is made to be weaker like P and prevents spheroidization of graphite, so it is about 0.06% or less. Ni has the effect of increasing the graphitization and the hardness of the ground, but if the content is small, the amount of graphite is insufficient, and the hardness of the ground is also lowered.

For this reason, it shall be about 0.7% or more. However, when it exceeds about 3.0%, the amount of cementite decreases, and the basic ground is likely to become thermally unstable bainite or martensite, which is not suitable as a hot rolled olol material. Cr is a cementite stabilizing element, but if it is less than about 0.1%, cementite is almost lost, resulting in a lack of wear resistance. On the other hand, when it exceeds about 0.6%, the amount of cementite is also excessive, and it becomes difficult to obtain a basic base structure composed of two phases other than ferrite and perlite, in addition to weakening of the material. Mo has the effect of stabilizing cementite and increasing the hardness of the base. However, the amount of cementite is less than about 0.1% and the hardness of the base is also insufficient. On the other hand, if the content exceeds 0.8%, the cementite becomes excessive, and the hardness improvement effect of the base is also saturated, which is uneconomical.

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 higher than about 0.1%, defects in the casting are likely to occur. The rool produced by the manufacturing method of the present invention made from the above-described composition of the composition is a single phase of ferrite (see Fig. 5) until casting, or a base of two phases of ferrite and perlite as shown in Fig. 6 by heat treatment. The ferrite portion retaining the structure imparts good abrasion resistance required as a rolled rollol, while the ferrite portion inhibits the diffusion of cracks and contributes to remarkably improved fracture resistance by increasing the toughness.

In order to acquire this ferrite and pearlite two-phase structure, it is good to make a tissue into a coexistence structure of ferrite and austenite once, and in this state, austenite is transformed into a pearlite. According to the production method of the present invention, the above-mentioned rool of the composition is maintained at a temperature of about 780-850 ° C. for about 20 hours or less, and then heat-treated to cool 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 are as follows.

Part (A) and (B) are the temperature rising process. The heating rate may be appropriately adjusted according to the capacity of the heat treatment furnace so as not to cause damage such as breakage due to thermal displacement, and there is no particular limitation other than that. Part (C) is the most important process in the heat treatment method of the present invention. That is, in this process, the tissue is made into two phases of ferrite and austenite, and then the target two-phase tissues of ferrite and perlite are formed by performing austenite transformation of austenite in the cooling process of the next part (D). Get The heat treatment temperature in this process finally obtains the desired two-phase structure of ferrite and ferrite. The heat treatment temperature in this process greatly affects the material properties of the ultimately obtained roll.

3 shows the relationship between the heat treatment temperature and the mechanical properties of the rool produced by the manufacturing method of the present invention.

Curve (i) shows the extensional strength (kg / mm ² ) (ii) shows the elongation (%).

In addition, any maintenance holding time in the same heat treatment was 2 hours, and the cooling rate in (D) process after heat processing was 70 C / Hr. The dimensions and chemical composition of the rool provided for the test were as follows. The test pieces were taken at a position of 140 mm from the surface of the center of the body part.

Roll dimensions: 780mm × 2200mm (body)

Chemical Adjustment: 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 figure, the extension strength gradually increases as the heat treatment temperature is increased, while the elongation rapidly changes with crystals around 820 ° C. That is, when the heat treatment temperature is less than about 780 ° C, since the C in the base is adsorbed by smoking, when the base hardness of the finally obtained olol material is low, the toughness is not sufficient due to ferrite of the peripheral portion of the graphite. .

On the other hand, if the temperature exceeds about 850 ° C, the structure becomes austenite monolithic, and the base of the desired ferrite and perlite two phases cannot be obtained. Therefore, this heat treatment temperature is about 780-850 degreeC.

7, 8 and 9 show the microstructure of the rool obtained when the heat treatment temperatures in the above test were set to 780 ° C, 820 ° C and 840 ° C, respectively. (The ratio in each drawing is 100 times (a) and 400 times (b).) In all cases, it shows that the two-phase structure of ferrite and pearlite is excellent, and it can be seen that the structure is also excellent in toughness and wear resistance. In addition, the formation and the material properties of the two-phase base structure of the rool prepared by the manufacturing method of the present invention are related to the relationship between the chemical composition, three factors of heating temperature and maintenance time in the above-mentioned process (C). As a basic tendency, as the basic tendency, the low Si% -high Cr% composition has the highest toughness at the low temperature side, and the high Si% -low Cr composition at the high temperature side. In addition, the longer the maintenance time, the higher temperature characteristics are displayed. For example, the hardness and toughness are approximately equal to those when the maintenance time is maintained at 800 ° C. for 10 hours and when the maintenance time is maintained at 820 ° C. for less than 1 hour.

In the treatment in the above-described temperature range of the component composition of the present invention, the holding time does not have to exceed about 20 hours, and it is uneconomical, so it is suitable within the range of about 20 hours or less depending on the component composition and the set temperature. It is good to choose. After forming the two-phase structure of ferrite and austenite in the above-mentioned process (C), in the cooling process in the following part (D), when the austenite is transformed into pearlite, the cooling rate is about 50 ° C. If less than / Hr, the hardness is lowered because the granite is granulated at about 720 ° C during cooling. As the cooling rate is increased, the produced pearlite becomes dense, and thus high hardness and excellent abrasion resistance are obtained.

9 is a microscopic structure of a rool obtained by cooling at 300 ° C / Hr after treatment with a heat treatment temperature of 820 ° C. and a holding time of 2 hours using a rool having the same composition as that in the test of FIG. 3 described above. ((A) is 100 times and (b) is 400 times). Compared with the structure of FIG. 7 (but 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 relation to this, the mechanical properties of both are shown in Table 1 below.

[Table 1]

As shown in the above table, it can be confirmed that strength and hardness (abrasion resistance) are improved without deteriorating toughness by the criticality of the cooling rate. However, the cooling rate exceeding about 300 DEG C / Hr is not only difficult to obtain in actual work, but also the base ground becomes too hard, resulting in a decrease in toughness. Therefore, the cooling rate is about 50-300 ° C / Hr. In addition, since the final temperature of this cooling process (D) requires the transformation to be completely completed, the final temperature is about 600 ° C. or less.

Part (E) following the cooling process is a displacement elimination and heating and cooling process, and is maintained at about 450-650 ° C. according to a usual method to remove displacement and internal stress of the rool resulting from the aforementioned heat treatment. The correction holding time does not need to exceed about 20 hours. In the case of producing a carburized attachment rool, it is common to carry out carburizing after heat treatment. However, since the cooling rate of the rool in the cooling process of the above-mentioned (D) is gentler on the inside side than the surface of the rool, the density of the pearlite produced therein is lower than 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 carrier formed by processing is also low. This may be done by pre-carburizing before heat treatment.

4 is an example in which a box-shaped carrier 3 is formed in a rool body 1 produced by the method of the present invention. The solid line l is the shape before the heat treatment carried out by the carver processing leaving the thickness of the remaining portion, and (m) indicates the shape completed after the heat treatment by the predetermined dimension by the cutting process.

The thickness n of the surplus portion 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 near the bottom of the carver is large enough, so that the density of the pearlite roll is increased. As a result, the hardness of the bottom of the carver becomes about 1-5 Hs higher than the hardness at the same diameter position as that of the bottom of the flange 4, and the wear resistance of the rool is further improved. As described above, the manufacturing method of the hot rolled dichroic cast iron rool according to the present invention has the configuration as described above, so that the manufactured rool does not generate cracks due to thermal fatigue, and even if cracks occur. No progress has been made. Therefore, it has excellent performance as a hot rolled rool without fear of loss of the rool.

Claims (1)

C 3.0-3.8% (same by 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%, Mg0. 02-0.1%, the remainder of the cast iron rool consisting of iron and unavoidable impurities is maintained at a temperature of 780 ℃-850 ℃ for less than 20 hours, then cooled to 600 ℃ or less at a cooling rate of 50-300 ℃ / Hr, and then A method for producing hot rolled diluent cast iron rool, characterized by maintaining at 450-650 ° C. for 20 hours or less.

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