KR100972190B1 - Milling roll and fabrication method thereof - Google Patents

Abstract

The present invention relates to a rolling roll and a manufacturing method thereof, and an object thereof is to provide a rolling roll excellent in toughness and wear resistance. To this end, in the present invention, C: 3.0-3.35% by weight, Mn: 0.1-0.5% by weight, Si: 1.8-2.2% by weight, Cu: 0.6-0.8% by weight, Ni: 2-8% by weight, Mo: 0.1-1 Preparing a molten metal having a composition in which: wt%, Cr: 0.3-2 wt%, Mg: 0.05-0.06 wt%, the balance is Fe and other unavoidable impurities; And casting the molten metal, casting the casting so that the time required for cooling the casting from 750 ° C to 300 ° C is 100-300 minutes, thereby sequentially obtaining a rolling roll, thereby producing 15-45% of cementite. Manufacture a rolling roll whose base structure is bainite merchant.

Rolled Roll, Cementite, Heat Treatment

Description

Rolling roll and its manufacturing method {Milling roll and fabrication method

1 is a micrograph showing the microstructure of a conventional rolling roll,

2 is a graph showing the hardness change according to the heat treatment temperature and heat treatment time for the rolling roll according to the present invention,

3 is a scanning electron micrograph (100 magnification) of Specimen 1 in Example 3,

4 is a scanning electron micrograph (2000 magnification) of Specimen 1 in Example 3,

5 is a scanning electron micrograph (100 magnification) of Specimen 4 in Example 3,

6 is a scanning electron micrograph (2000 magnification) of Specimen 4 in Example 3,

7 is a scanning electron micrograph (100 magnification) of Specimen 7 in Example 3,

8 is a magnification of a scanning electron micrograph 2000 of specimen 7 in Example 3,

9 is a scanning electron micrograph showing the wear surface of the existing material,

10 is a scanning electron micrograph showing the wear surface of the specimen 4 in Example 3.

The present invention relates to a rolling roll and a method for manufacturing the same, and more particularly to a rough rolling roll used in the rolling for the production of wire rods and a method for manufacturing the same.

At present, the rough rolling roll mainly used in the wire rod mill is NCI (Nodular Indefinite Chilled Iron).

NICI is economical and mechanically superior to other cast irons, and NICI contains a large amount of coarse graphite, which acts as a hole in the occurrence of cracks and part of the rolled material on the roll surface. It acts as a lubrication to prevent sticking, and it also has the effect of suppressing fire cracking due to thermal shock.

In particular, since the graphite contained in the NICI is spherical, and more resistant to crack generation and propagation than when present in a flaky form as in other cast irons, the NICI has a firm position as a roll material in this field.

1 is a micrograph showing the microstructure of a NICI roll. As shown in FIG. 1, the matrix phase of the NICI roll is pearlite, and contains a large amount of spheroidal graphite.

In addition, cementite (cementite: Fe ₃ C) is present in NICI, and cementite improves wear resistance.

The presence of cementite distinguishes NICI from ductile cast iron (DCI), which is used for general structural purposes.

In other words, the NICI differs from the DCI material by intentionally producing cementite in order to impart the required wear resistance as a roll material, whereas cementite degrades the strength and ductility, thereby causing excessive roll wave and thermal fatigue cracks. It is becoming the main person.

However, in recent years, as the rolling conditions are advanced according to the high quality of the rolled products, many studies on the development of new rolling rolls and improvement of physical properties such as abrasion resistance, strength, fracture toughness and thermal fatigue characteristics of the rolling rolls are being conducted.

In order to improve the physical properties, the austempering heat treatment is used to make a matrix austenite single phase in a constant austenite temperature zone, and then rapidly cool it to bainite transformation temperature, and then incubate the substrate structure at this temperature to bainite. It goes through a two-step process of cooling to room temperature.

Although the mechanical properties can be improved by performing such a complicated heat treatment method, the manufacturing cost is increased, so it is not effective for practical application.

In the austenitic treatment, cementite decomposes over time, which adversely affects wear resistance. In order to obtain a sufficient amount of cementite, more than 60% of cementite should be obtained in a kitchen state, which is a very fragile material because it needs to generate overprocessed cementite.

On the other hand, in order to improve the service life of conventional rolled steel rolls, expensive high-speed coated steel and high chrome cast steel were tested. Unlike hot plate rolling, the lubrication deterioration and sticking due to adsorption of rolled material on the roll surface This situation does not solve the chronic problems.                         

Even if the high speed steel rolls are successfully applied on-site, in terms of roll price, the high speed steel rolls are five times more expensive than the conventional NICI rolls, and cracks due to thermal stress are applied to the large rolls of rolls 1-5. Due to the characteristics of high-speed steel rolls vulnerable to the possibility of development, there is a demand for the development of high-quality rolls having low cost and excellent thermal fatigue crack resistance.

As a result, the use of NICI materials, which are still inexpensive and does not cause sticking phenomenon, is increasing the amount of cementite gradually to improve wear resistance, but the wear resistance is improved as the amount of cementite is increased, but excessive thermal fatigue crack And the risk of roll failure is increased.

Therefore, in order to improve the life of the roll using the existing NICI material, securing the toughness and wear resistance of the material is most important, but it can be seen that there is a limit because it decreases as the amount of carbide increases.

Accordingly, the present invention has been made to solve the above problems, the object of the present invention is to provide a roll material for rough rolling of the wire rod with improved toughness and wear resistance through the development of an alloy that satisfies both sufficient hardenability and mechanical properties at the same time. .

In order to achieve the above object, the present invention proposes an alloy composition design and optimized heat treatment conditions of a rolling roll excellent in toughness and wear resistance.

That is, the rolling roll according to the present invention is C: 3.0-3.35% by weight, Mn: 0.1-0.5% by weight, Si: 1.8-2.2% by weight, Cu: 0.6-0.8% by weight, Ni: 2.2-8% by weight, Mo : 0.1-1% by weight, Cr: 0.3-2% by weight, Mg: 0.05-0.06% by weight, and the balance is composed of Fe and other unavoidable impurities.

At this time, it is preferable that 15-45% of cementite is produced in the rolling roll, and the matrix structure of the rolling roll is bainite.

In addition, the manufacturing method of the rolling roll according to the present invention, C: 3.0-3.35% by weight, Mn: 0.1-0.5% by weight, Si: 1.8-2.2% by weight, Cu: 0.6-0.8% by weight, Ni: 2-8 Preparing a molten metal having a composition in which% by weight, Mo: 0.1-1% by weight, Cr: 0.3-2% by weight, Mg: 0.05-0.06% by weight, the balance is Fe and other unavoidable impurities; And casting the molten metal, casting the casting so that the time required for cooling the casting from 750 ° C to 300 ° C is 100-300 minutes to obtain a rolling roll.

When casting molten metal, the molten metal of the composition mentioned above may be melted with a casting mold, cooled to an austenite temperature, and then the casting mold may be disassembled and air cooled.

In addition, after the casting step may further comprise the step of heat treatment for 2-16 hours at 400-600 ℃.

Hereinafter, the present invention will be described in detail.

In general, during large roll heat treatment, it is difficult to control temperature and thermal gradient over time, and because the heat treatment time becomes long, it is difficult to apply ostempering after austenitization treatment. Accordingly, in the present invention, by adding an alloying element, cementite is produced in an easier manner by about 15 to 45%, and a rolling structure having a matrix structure of bainite is intended.

One way is to obtain bainite texture in the kitchen by adding alloying elements. However, after melting, cooling should not result in a perlite transformation, and the nose near the bainite transformation should be large in the CCT-curve.

It is difficult to complete the transformation because the bainite transformation is performed by continuous cooling, so that pearlite, martensite, and residual austenite are present in the final structure, thereby decreasing strength and toughness. Thus, controlled cooling treatment was performed to effectively transform the matrix to bainite transformation.

Controlled cooling is an alternative method of ostempering to the production sites of most nodular cast irons that do not have the equipment to perform osstempering.

In order to reduce scratch wear, it is necessary to control the carbide phase. In the case of NICI materials, which are currently commercially available, the carbides used in large quantities for the improvement of wear resistance are coarse cementite (hardness (Hv): 800 to 1000), and thus thermal fatigue. And very vulnerable to impact.

Thermal fatigue and wear resistance are remarkably improved for the NICI, if a high hardness fine carbide (hardness (Hv): 2400, 300 µm or less) can be present in the NICI roll material for wire rods instead of the coarse cementite in the dispersed phase. It can be.

The composition of the rolling roll, which has a matrix structure of bainite phase and produces about 15-45% of cementite, is not significantly different from that of commercially available nodular cast iron.                     

In order to determine the composition of the rolling roll, firstly, the DCI material or cementite is produced in the range of 15 to 45% and the rolling structure is known as bainite, so that the formation of non-spherical graphite, carbides or inclusions, or shrinkage holes are encouraged. Consideration should be given to the limiting quantities of elements that do not adversely affect the composition.

Next, the amount of carbon, silicon, and other major alloying elements that control the hardenability of iron and the properties of the transformed microstructure must be considered. In addition, in order to make cast iron having a sufficient hardenability to secure the physical properties of the surface layer use part essential for large rolling rolls, it is designed as an alloy having excellent hardenability by adding an appropriate alloy.

Accordingly, the alloy of the present invention is C: 3.0-3.35% by weight, Mn: 0.1-0.5% by weight, Si: 1.8-2.2% by weight, Cu: 0.6-0.8% by weight, Ni: 2.2-8% by weight, Mo: 0.1-1% by weight, Cr: 0.3-2% by weight, Mg: 0.05-0.06% by weight, the balance consisting of Fe and other unavoidable impurities.

Hereinafter, the reason which limited the component composition of this invention material to the said range is demonstrated.

First, manganese (Mn) is both beneficial and harmful. Mn greatly increases austemperability, but segregates at the cell boundary during the solidification process to form carbides and retard the bainite reaction.

As a result, the spheroidization rate is lowered in castings larger than about 20 mm and the possibility of shrinkage pores, carbides and unstable austenite formation at the cell boundary is increased. These microstructural defects and nonuniformities reduce machinability and mechanical properties.

In order to increase the mechanical properties and reduce the sensitivity to casting size and nodularity, it is necessary to limit the content of manganese to 0.5% by weight or less. Therefore, it is preferable that the addition range of Mn is 0.1-0.5 weight%.

Next, copper (Cu) increases the hardenability of the rolling roll having a cementite of 15-45% and a matrix structure of bainite up to 0.8 wt%. Copper has no significant effect on the tensile properties but increases the elongation at heat treatment below 350 ° C. Therefore, it is preferable that the addition range of Cu is 0.6-0.8 wt%.

Next, as the addition amount of nickel (Ni) increases, the synergistic effect of hardenability increases. By adding a large amount of Ni to more than 2.2% by weight of the existing material can significantly increase the ostempering hardenability. Data on the effect existed until the addition of 4% by weight, but no experimental results were found at higher contents.

Ni is less segregated than manganese or molybdenum and does not segregate at cell boundaries. Therefore, more Ni may be added.

In addition, Ni under ossferring below 350 ° C slightly decreases tensile strength and increases elongation and fracture toughness. And Ni acts as a graphitization aid to reduce the amount of carbide produced. Therefore, when Ni is used together with elements such as copper or chromium rather than alone, tensile strength and hardness can be improved.

Therefore, it is preferable that the addition range of Ni is 2.2-8 weight%.

Next, molybdenum (Mo) acts as the most powerful hardenability agent in rolling rolls with about 15-45% cementite production and bainite matrix, and can be used to inhibit pearlite growth in large castings.                     

However, the tensile strength and the elongation decrease with increasing Mo content under the required curing ability. This deterioration of mechanical properties is caused by segregation and carbide formation at the cell boundary. Therefore, it is preferable that Mo addition range is 1 weight% or less.

Next, chromium (Cr) forms a stable complex carbide as a strong carbide former during the solidification of cast iron. Therefore, the addition of Cr can increase the hardness of cast iron.

In the cast iron containing 2% silicon, the difference between the graphite process temperature and the carbide process temperature is about 35 ° C. When the Cr content is increased to 1.1% by weight, the difference between the two process reaction temperatures decreases to about 14 ° C. The carbide process reaction occurs even under supercooling.

And Cr increases the hardenability of cast iron, the effect is known to be similar to manganese. Therefore, it is possible to reduce the amount of graphite promoted by nickel and increase the amount of carbides in the nickel-added cast iron, to improve the tensile strength and hardness, and to obtain the strengthening effect by the hardenability.

Therefore, it is preferable that the addition range of Cr is 2 weight% or less.

The function and scope of application of the heat treatment process of the present invention are as follows.

Controlled cooling treatment was performed to effectively transform the matrix to bainite. Controlled cooling heat treatment is a heat treatment method as an alternative to austempering to obtain high strength bainite structures. Controlled cooling should add sufficient alloys and air cooled after dismantling the mold at high temperatures to avoid perlite transformation.                     

To this end, after manufacturing the molten metal of the above-described composition, when casting to obtain a rolling roll, the cooling rate in the temperature range of the casting 750-300 ℃ so that the time required for cooling from 750 ℃ to 300 ℃ 100-300 minutes It is important to control.

In this way, if the casting rate is controlled to obtain a rolling roll by controlling the cooling rate to take 100-300 minutes in the temperature range of 750-300 ° C., 15-45% of cementite is produced as desired, and a rolling structure having a bainite phase can be obtained.

In order to control the cooling rate as described above, the molten metal of the composition of the present invention described above may be tapped into a mold and cooled to an austenite temperature, and then the mold may be disassembled and air cooled.

In addition, the step of heat-treating for 2-16 hours at 400-600 ℃ for the stress relief for the rolling roll obtained by the method as described above may be further performed.

Table 1 shows the alloying elements according to scrap iron, pig iron (COREX 4.6% C), Fe-Si-Mg (45% Si, 5% Mg) and its components to determine the hardness change when the Ni content is 0-7% by weight. After the charging, the molten metal was dissolved in a high frequency induction furnace, and the tapping temperature of the molten metal was 1380 to 1400 ° C., which shows the chemical composition of the specimen prepared by injection into a CO 2 sand mold.

Sample C Si Mn Mo Ni Cr Cu Mg One 3.28 2.01 0.3 0.16 0.00 0.32 0.79 0.055 2 3.26 2.02 0.28 0.17 2.22 0.49 0.80 0.056 3 3.28 2.16 0.26 0.23 4.61 0.32 0.80 0.053 4 3.34 1.94 0.27 0.17 6.98 0.43 0.80 0.058

Table 2 shows the volume fraction and hardness value of each phase in the microstructure according to the Ni content shown in Table 1.                     

Sample black smoke
volume(%) Cementite Volume (%) Bainite Volume (%) Pearlite Volume (%) Hardness (HRc) One 1.37 26.4 0.0 69.7 46.3 2 1.59 24.8 38.0 33.2 51.0 3 2.12 23.9 73.2 0.0 55.7 4 3.17 21.2 75.3 0.0 52.5

As shown in Table 2, the Ni content of up to about 4% by weight (Samples 1 and 2) decreases the fraction of pearlite and the fraction of bainite as the Ni content increases, resulting in the cementitization effect of Ni. Although the amount of was decreased, the hardness of the whole specimen was increased.

This is because the fine hardness (Hv) of bainite is 550, which is higher than the pearlite of 350, and the hardness of specimen 3 to which 4.6% by weight of Ni is added is composed mostly of bainite. It showed much higher hardness than specimen 1 consisting of specimen 1 and specimen 2 containing 33% pearlite.

However, Specimen 4 with Ni added at about 7% by weight dropped further to 52.5 because some retained austenite was present in the matrix.

From these results, it was confirmed that the specimen 3 added with Ni about 4.6% by weight exhibited the best hardness.

Next, in order to find out the hardness change with heat treatment temperature and time, the chemical composition of the Ni content of 4% by weight (C: 3.2% by weight, Si: 2.0% by weight, Mn: 0.3% by weight, Mo: 0.2% by weight, Ni: 4.0% by weight, Cr: 0.5% by weight, Cu: 0.8% by weight, Mg: 0.06% by weight) was prepared and maintained at 980 ° C. for 1 hour, and then changed to 400-600 ° C., followed by furnace cooling after heat treatment for 2-16 hours.                     

Figure 2 shows the change in hardness over time and the heat treatment temperature of the prepared specimen. As shown in FIG. 2, when the heat treatment temperature is 400 ° C., the hardness increases from 43 HRc to 46 HRc as time is increased up to 8 hours, which is high in carbon content of bainite part as shown in the microstructure photograph. It is thought that some of the cementite precipitated as the austenite decomposed.

However, as time increased further, decomposition accelerated, leading to a decrease in hardness after 16 hours.

Since the decomposition rate of bainite was faster at the heat treatment temperature of 500 ° C. than at 400 ° C., the hardness gradually decreased with increasing time.

At 600 ° C of heat treatment, pearlite decomposes together with bainite, so the hardness decreases rapidly to 35 HRc with increasing time.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example

Table 3 shows the chemical composition of the specimen prepared in order to determine the mechanical properties according to the content of the main alloying elements Ni, Mo, Cr, together with the chemical composition of the existing material.

Example 1-3 is a specimen prepared by fixing Ni content and adding 0.2-1% by weight of Mo, and Example 4-6 is a specimen prepared by changing Cr content to 0.8-2% by weight. Finally, Examples 1, 3 and 7 are specimens prepared to determine the effect of Ni by changing the content of Ni to 2-8% by weight. All the prepared specimens were heat treated at 400 ° C. for 10 hours after maintaining 1-2 hours at 980 ° C.

C Si Mn Mo Ni Cr Cu Mg Existing Material 3.20 1.8 0.45 0.5 2.0 0.6 - 0.06 Example 1 3.26 2.02 0.28 0.17 2.22 0.49 0.80 0.056 Example 2 3.13 1.94 0.31 0.49 2.21 0.30 0.80 0.053 Example 3 3.21 1.97 0.33 0.98 2.29 0.74 0.79 0.055 Example 4 3.07 1.80 0.39 0.18 3.99 0.82 0.80 0.059 Example 5 3.15 1.81 0.30 0.29 3.78 1.52 0.80 0.056 Example 6 3.17 1.98 0.33 0.26 3.94 1.90 0.78 0.053 Example 7 3.11 1.80 0.38 0.22 7.42 0.80 0.74 0.061

3 to 8 show scanning electron micrographs of Examples 1, 4, and 7, which are representative specimens of the prepared specimens, and FIGS. 3 and 4 show the microstructures of Example 1, and FIGS. 5 and 6 Is a microstructure of Example 4, Figures 7 and 8 shows a microstructure of Example 7.

Referring to FIGS. 3 and 4, graphite, cementite, pearlite, and bainite are used. The volume fraction of each phase is 1.6% graphite, 32% cementite, 32% pearlite, and 34% bainite.

When 2.0 wt% of Ni is added, it seems that pearlite remains in the final structure because the hardenability is not sufficient yet.

Referring to FIGS. 5 and 6, graphite, cementite, and bainite are used. The volume fraction of each phase is 2% of graphite, 28% of cementite, and most of the matrix structure is bainite.

No pearlite was observed here because the addition of 4.0% by weight of Ni provided sufficient hardenability.                     

Referring to FIGS. 7 and 8, the phase ratios of constituents were 3.2% graphite and 21% cementite, and a large amount of residual austenite coexisted between bainite laths.

This is judged to be due to 7.0 wt% of Ni providing excessive hardenability and Ni acting as an austenite stabilizing element.

Table 4 shows the mechanical properties of the existing and manufactured invention.

tenacity
(10 ^-3 J / ㎣) Wear resistance
(kJ / ㎣) Hardness
(HRc) The tensile strength
(kg / ㎣) Elongation
(Mm / mm) Existing Material 52 3.19 40.2 40 0.2 Example 1 130 5.14 54.4 42.3 0.4 Example 2 135 5.22 54.9 45.0 0.39 Example 3 142 5.35 55.4 49.1 0.37 Example 4 183 6.60 57 60.9 0.5 Example 5 167 6.82 58.2 55.8 0.47 Example 6 154 6.96 59.8 53.4 0.43

Looking at Table 4, it can be seen that all the specimens prepared according to Examples 1-6 of the present invention have superior mechanical properties than existing materials. In particular, toughness was up to 3.5 times better than existing materials and abrasion resistance was more than twice better.

Looking at the mechanical properties of the prepared invention for each additional element in detail as follows. First, when comparing Examples 1, 2, and 3, which are specimens to determine the influence of Mo, as the Mo content increases from about 0.2% to 1% by weight, all mechanical properties such as toughness, abrasion resistance, tensile strength, and elongation except hardness are The property is slightly reduced.

When comparing Examples 4, 5, and 6, which are specimens for determining the influence of Cr, wear resistance and hardness slightly increased as the amount of Cr increased from about 0.8 wt% to 2 wt%. However, toughness, tensile strength and elongation decreased. In particular, the toughness decreased significantly from 180 10 ^-3 J / mm ³ to 154 10 ^-3 J / mm ³ .

Looking at the specimens Examples 1 and 4 to determine the effect of the Ni content, the mechanical properties such as toughness, abrasion resistance, hardness, tensile strength, elongation, etc. are increased as the Ni content increases to approximately 4% by weight (Sample 4). Increased significantly. In particular, the toughness and wear resistance were increased.

However, as Ni content increased, when the Ni content was added about 2% by weight (Example 1), the mechanical properties were slightly better, but when the addition of about 4% by weight (Example 4), the mechanical properties were significantly better. .

9 and 10 are photographs observed to determine the wear trend of the existing material and the invention after the wear resistance test. 9 shows the wear surface of the existing material and FIG. 10 shows the wear surface of specimen 4 having the best wear resistance among the prepared specimens.

Looking at the wear surface of the existing material in Figure 9, the base part is almost eliminated due to extreme wear, cementite remains and protrudes, and some cementite is destroyed by the base dropout. This is because the base structure of the existing material is made of pearlite having a low hardness, so that the wear resistance is poor and not strong enough to prevent the cementite from falling out.

Looking at the wear surface of Example 4 of Figure 10, it can be seen that the drop of matrix tissue is hardly observed as compared with the existing material. Harder lower bainite has better wear resistance than pearlite, so it does not differ significantly from cementite's wear rate, resulting in a low propensity to protrude. And it can be seen that bainite's toughness also prevents cementite from falling out.

As described above, when the rolling roll is manufactured under the alloy composition and heat treatment condition according to the present invention, there is an effect of providing a roll for rough rolling of the wire with improved toughness and wear resistance.

Claims (5)

delete delete C: 3.0-3.35 wt%, Mn: 0.1-0.5 wt%, Si: 1.8-2.2 wt%, Cu: 0.6-0.8 wt%, Ni: 2-8 wt%, Mo: 0.1-1 wt%, Cr: Preparing a molten metal having a composition of 0.3-2% by weight, Mg: 0.05-0.06% by weight, the balance being Fe and other unavoidable impurities; Casting the molten metal, casting the casting so that the time required for cooling the casting from 750 ° C to 300 ° C is 100-300 minutes to obtain a rolling roll; And Heat treatment for 2-16 hours at 400-600 ° C. after the casting step Method for producing a rolling roll comprising a. The method of claim 3, wherein When casting the molten metal, the molten metal of the composition is melted with a mold and cooled to an austenite temperature, and then the mold is dismantled and air-cooled. delete

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