KR101304602B1 - Method for improving scale detachment property - Google Patents

Abstract

Heating the steel to 900 to 1150 ° C. in an oxidizing atmosphere to form a scale on the surface of the steel and cooling while maintaining an oxygen partial pressure of 1 to 2 vol% or less, so that the surface temperature of the scale is 0.6 of the surface temperature of the steel. A method is provided for improving the peelability of a scale comprising adjusting greater than twice and smaller than 0.8 times.
According to the present invention, it is possible to reduce the surface flaw generated on the steel material due to the scale remaining without peeling during rolling.

Description

How to improve peelability of scale {METHOD FOR IMPROVING SCALE DETACHMENT PROPERTY}

TECHNICAL FIELD This invention relates to the method of improving the peelability of a scale as a pretreatment of the hot rolling process of steel materials.

Conventional techniques for improving the peelability of scales in furnaces are mainly related to nozzle angle and water pressure. The direct influence on the descaling is the impact pressure generated when the high pressure water impinges on the scale surface, and the discharge pressure at the tip of the nozzle most directly affects the impact pressure. Therefore, it is divided into a technique of controlling the collision pressure and a technique of controlling the discharge pressure. The discharge pressure is usually controlled to 300 kg / cm ² or less (some of 300 kg / cm ² or more are also present), and the collision pressure is controlled to about 30 kg / cm ² or less. In the nozzle discharge pressure control, the height of the nozzle is very important, and in most inventions, the condition is controlled to 150 mm or less. The discharge pressure or the collision pressure at the time of secondary scale removal is higher than at the time of primary scale removal.

The problem of the prior art is that the surface temperature of the steel drops sharply due to the form of spraying water having a certain amount or more of water pressure to peel the scale. In the case of hot rolled sheet, this difference can be overcome by double heat due to the large heat capacity. However, in the case of bloom or billet material, the heat capacity is not large compared to the plate, so if a certain amount or more is applied to the surface, the surface temperature may change. And hot rolling brings unbalanced rolling force. The present invention has been devised to solve this problem.

An aspect of the present invention is to provide a method for improving scale peelability as a pretreatment of a hot rolling process of steel to reduce scale generated in a heating furnace which is a heating process facility for hot rolling.

However, the problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, one aspect of the present invention, heating the steel to 900 to 1150 ℃ in an oxidizing atmosphere to form a scale on the surface of the steel and cooling while maintaining the oxygen partial pressure of 2 vol% or less Thus, it provides a method of improving the peelability of the scale comprising the step of adjusting the surface temperature of the scale is greater than 0.6 times and less than 0.8 times the surface temperature of the steel.

According to one aspect of the present invention, it is possible to reduce the surface scratches generated on the steel material due to the scale remaining without peeling during rolling.

1 is a graph measuring the change in adhesion of the steel type scale according to an embodiment of the present invention
Figure 2 is a graph showing the peel test results according to the oxygen partial pressure change, according to an embodiment of the present invention.
Figure 3 is a graph showing the peel test results according to the change in oxygen partial pressure and water spray time, according to an embodiment of the present invention.

Hereinafter, with reference to the accompanying drawings will be described in detail a method of improving the peelability of the scale of the present invention to be easily implemented by those skilled in the art.

The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments and examples described herein.

Iron grows when it is exposed to a high temperature oxidizing atmosphere to form a multilayer coating of FeO, Fe ₃ O ₄ and Fe ₂ O ₃ . At 570 ° C. or less, FeO (wustite) is not formed, and a two-layer coating of Fe ₃ O ₄ and Fe ₂ O ₃ is formed, wherein Fe ₃ O ₄ is formed on the metal surface side. At a temperature of 570 ° C. or higher, an oxide film is formed and grown in the order of FeO, Fe ₃ O ₄ , and Fe ₂ O ₃ . FeO has a scale generation mechanism in which Fe moves through the oxide film, and at 1000 ° C., FeO _.95 to FeO _.88 departs from stoichiometry very much. As such, when the concentration of anion vacancy increases, the movement of metal ions and electrons is accelerated by vacancy and electron holes. Magnetite Fe ₃ O ₄ has an inverse spinel structure, except at very high temperatures, the crystal structure hardly deviates from stoichiometry. Hematite Fe ₂ O ₃ has α-Fe ₂ O ₃ having a rhombohedral structure and γ-Fe ₂ O ₃ having a cubic structure. However, in the oxidation reaction, α-Fe ₂ O ₃ mainly produced by oxidation of Fe ₃ O ₄ above 400 ° C is mainly observed.

Scaling refers to the process of forming a layer of high temperature oxidation product (Busite-FeO, Magnetite-Fe ₃ O ₄ , Hematite-Fe ₂ O ₃ ) on the metal surface at high temperature. These are formed by the reaction:

O ₂ + 2Fe = 2FeO

FeO + Fe ₂ O ₃ = Fe ₃ O ₄

O ₂ + 4Fe = 2Fe ₂ O ₃

The scaling process begins with the formation of iron oxide on the metal surface and proceeds as oxygen diffuses to the iron / iron oxide interface. This diffusion process is a function of temperature and time.

The scale generation formula generally follows the following formula (1).

h, w, L = thickness, width, and length of the slab, respectively

M = scale weight loss per unit weight of slab (ton)

t = scaling time

T = slab temperature (℉)

Ρ = density of slabs (lbs / in ³ )

a, b = constant depending on the type of iron and the atmosphere of the furnace

As shown in the above equation, the scale generation amount is proportional to the power of 1/2, and is proportional to the exp (-1 / T) of the temperature, and thus is more affected by the temperature.

As discussed above, the present invention has been devised by focusing on the property of temperature dependence in scale formation.

That is, one aspect of the present invention, by heating the steel to 900 to 1150 ℃ in an oxidizing atmosphere to form a scale on the surface of the steel and cooling while maintaining the oxygen partial pressure of 2 vol% or less, the surface temperature of the scale is It provides a method for improving the peelability of the scale comprising the step of adjusting the surface temperature of the steel than greater than 0.6 times and less than 0.8 times.

In the present invention, it is easy to remove the scale generated from the heating furnace, which is a heating process facility for pre-rolling, using the thermal stress generated between the steel sheet and the scale, and then perform the rolling process. When the steel is heated to 900 to 1150 ° C. in an oxidizing atmosphere, scales of FeO, Fe ₃ O ₄ , Fe ₂ O ₃ , FeSiO ₄ , (Fe, Cr) ₂ O _3, and the like are formed on the steel surface. This scale acts as an impurity in the rolling process after heating, causing a flaw in the steel surface. Therefore, in order to remove this scale before rolling, water or the like is sprayed immediately after heating the steel to lower the surface temperature of the generated scale.

The heating temperature is limited to 900 to 1150 ℃ because scale generation is greatly suppressed below 900 ℃, but the rolling load in the rough rolling section is rapidly increased, making it difficult to realize in wire rod rolling. Above 1150 ℃, scale generation increases rapidly. However, the Fe-Si scale, which greatly affects the scale peelability, melts into a liquid state. As a result, the Fe-Si-based scale having good adhesion at the interface between the steel surface (base phase) and the scale is pulled out and the adhesiveness of the scale is drastically deteriorated, so that the peelability is rather improved without applying the present invention. It is meaningless to do.

The partial pressure of oxygen is limited to 2 vol% or less. If the oxygen partial pressure exceeds 2 vol%, the scale thickness grows sufficiently thick, and the resulting cracks and stress deviations cause cracks in the scale, which makes it easy to remove the scale even with ordinary descaling equipment.

The smaller the oxygen partial pressure, the thinner the scale, the better the adhesiveness, and the lower the peelability. (See Table 3 and FIG. 2) Therefore, the thinner the scale, the harder it is to be removed by a general physical method. Even if the oxygen partial pressure is 0 vol%, some scale is produced and it is not easy to remove the scale. Therefore, the lower limit setting of the said partial pressure of oxygen is meaningless.

A coolant, such as water, is sprayed onto the scale surface to lower the scale surface temperature.

Cooling the scale surface through water spraying induces rapid thermal contraction of the scale and reduces the density of the scale structure by thermal stress to maximize peelability between the scale and the steel surface.

When the scale is formed on the surface of the steel, FeO is generated first, followed by Fe ₃ O ₄ and then Fe ₂ O ₃ as the temperature increases. Therefore, FeO is located at the inner side closest to the surface of the steel, and FeSiO ₄ may be formed at the bottom thereof. The FeSiO ₄ is difficult to remove by a conventional physical method, it has conventionally adopted a method of melting at a high temperature of more than 1150 ℃.

The present invention is a revolutionary method that can avoid such a cumbersome process, by spraying a refrigerant, such as water, to cool only the surface of the scale to a certain temperature, to change the density of the scale structure to easily peel off from the steel surface.

The refrigerant spray process, such as water, is adopted as the easiest way to lower the surface temperature of the scale, and is not intended to exclude other means for lowering the surface temperature.

The surface temperature of the scale is adjusted to be greater than 0.6 times and less than 0.8 times the surface temperature of the steel.

When the surface temperature (Tscale) of the scale is higher than 0.8 times the surface temperature (Ts) of the steel, the thermal stress difference is small and the effect of improving the peelability is insignificant. This is because the rolling load can be increased during rough rolling by dropping to the surface temperature of the steel.

In order to confirm this, a peel test was performed while changing the value of Tscale / Ts at an oxygen partial pressure of 1 vol%.

Tscale / Ts Adhesion between material and scale (psi) Condition 1 1.0 443 Condition 2 0.95 398 Condition 3 0.9 412 Condition 4 0.8 250 Condition 5 0.7 197 Condition 6 0.6 155

In an exemplary embodiment, the surface temperature of the steel after the completion of cooling and before the start of rolling may be greater than 0.9 times the initial temperature of the scale surface and smaller than the initial temperature of the scale surface, but is not limited thereto.

After completion of cooling (eg water spraying) the surface temperature before rolling starts again rises somewhat due to the latent heat of the sample. This is because when the surface temperature (Ts) of the steel is maintained at 0.9 times or more of the initial temperature (T ₀ ), the thermal stress difference can be exhibited, thereby improving the peelability. Where T ₀ is the surface temperature of the steel when the material emerges from the furnace and the surface temperature of the scale. This is because when the material is extracted from the furnace, the surface temperature of the steel and the surface temperature of the scale are the same.

Hereinafter, the present invention will be described in detail with reference to Examples. However, the following examples are only for illustrating the present invention in more detail and do not limit the scope of the present invention.

[ Example ]

In order to evaluate scale peelability, carbon steel (SWRCH45F), boron steel (SCr420B), and alloy steel (SCM435) having the composition shown in Table 2 were prepared. Hereinafter, it is necessary to pay attention that the content unit of each component is weight% unless otherwise stated.

Steel grade C Mn Si P S Cr Mo B Carbon steel 0.46 0.74 0.2 0.015 or less 0.015 or less - - - Alloy steel 0.36 0.77 0.19 0.015 or less 0.015 or less 1.0 0.18 - Boron River 0.22 0.8 0.19 0.015 or less 0.015 or less 1.0 - 0.002

During the hot rolling of the alloy steel was maintained for 90 minutes at 1080 ℃ which is the middle value of 1000 ~ 1150 ℃ commonly used during hot rolling and scale generation in the heating furnace in accordance with the oxygen partial pressure 1, 2, 3 vol% change. At this time, water spraying was not performed. As the specimen size, a cube specimen with a length of 20 mm was used. After completion of the heat treatment process, water cooling was performed to remove the primary scale, and a peel test was performed as a residual scale. Peelability test was measured with a coating adhesion tester (SEVASTIAN IV) to test the adhesion of the object and the coating. When the pull bar was attached to the scale surface and the pull bar was fixed to the measuring device, the measuring device measured the force when the scale peeled from the specimen and is shown in Table 3 and FIG. 2.

Oxygen partial pressure Adhesion between steel surface and scale (psi) Average Deviation 1 vol% 547 219 2 vol% 376 193 3 vol% 148 91

The above figures indicate the adhesion between the steel surface (base) and the scale, and the higher the value, the better the adhesion and the lower the peelability. As the oxygen partial pressure increases, the adhesion decreases (the peelability increases) and the dispersion is also small. At 1 vol% of oxygen partial pressure, adhesion to the specimen was high and dispersion was large. In the case of an oxygen partial pressure of 3 vol%, the numerical value is 200 or less, and the numerical value may be excluded from the scope of the present patent because it has adhesiveness that can be sufficiently removed even by a general descaling equipment.

Table 4 and Figure 1 is a measurement result of the change in adhesion of the steel type scale.

Carbon steel shows the highest adhesion and the largest dispersion, alloy steel shows the next, and boron steel shows the smallest. The reason why the scale adhesion of the alloy steel is smaller than that of the carbon steel is that an oxide having a dense structure such as (Fe, Cr) ₂ O ₃ or FeSiO ₄ is formed at the lower end of the scale to prevent the diffusion of Fe ⁺ ions. In the case of boron steel, boron is diffused at the lower end of the scale to form a thickening layer on the scale or the specimen to inhibit scale adhesion.

Steel grade Adhesion between steel surface and scale (psi) Average Standard Deviation Boron River 1 150.5 54.24 Boron River 2 194.5 54.34 Alloy Steel 1 173.5 65.32 Alloy Steel 2 229.5 61.26 Carbon Steel 1 294.5 88.35 Carbon steel 2 269.5 89.59

Boron steel 1 and boron steel 2 in Table 4 are the same, but extracted from different billets. The same applies to alloy steel 1 and alloy steel 2, carbon steel 1 and carbon steel 2.

Water spraying was carried out under the same heating conditions (1080 ° C., 90 min) to increase the peelability at the given conditions. The test grades were made of alloy steel with good adhesion to the scale (SCM435), and the water spray conditions were wet for 30 seconds, 60 seconds, and 90 seconds. After the test, the scale peelability (inferred by measuring the adhesion between the steel surface and the scale) of each condition was measured with a peelability tester. The measurement results are shown in Table 5 and FIG. 3.

division Oxygen partial pressure, spray time Adhesion between steel surface and scale (psi) Average Standard Deviation 1-30 1 vol%, sprayed for 30 seconds 515.0 292.5 1-60 1 vol%, sprayed for 60 seconds 150.3 142.0 1-90 1 vol%, sprayed for 90 seconds 129.8 106.5 2-30 2 vol%, sprayed for 30 seconds 308.4 212.4 2-60 2 vol%, sprayed for 60 seconds 336.4 150.9 2-90 2 vol%, sprayed 90 seconds 293.6 259.6 3-30 3 vol%, sprayed for 30 seconds 181.0 155.7 3-60 3 vol%, sprayed for 60 seconds 135.4 113.8 3-90 3 vol%, 90 sec spray 203.8 139.8

As shown in Table 5 and Figure 3 it was found that the adhesion strength is reduced by about 50% or more. The decrease in adhesion strength resulted in thermal stress on the scale surface due to water spraying on the scale surface, which resulted in cracking of the scale layer, thereby reducing the density. Even without special pressurization, scale surface water spraying could improve scale peelability.

Claims (2)

Heating the steel to 900 to 1150 ° C. in an oxidizing atmosphere to form a scale on the surface of the steel; And
Cooling while maintaining an oxygen partial pressure of 2 vol% or less, thereby adjusting the surface temperature of the scale to be greater than 0.6 times and less than 0.8 times the surface temperature of the steel. The method of claim 1,
And the surface temperature of the steel after the completion of the cooling and before the start of rolling is greater than 0.9 times the initial temperature of the scale surface and smaller than the initial temperature of the scale surface.

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