KR20220055272A - Hot rolled steel coil and method of manufacturing the same - Google Patents

Abstract

The present invention provides a hot-rolled steel coil with suppressed formation of internal oxide layer. According to one embodiment of the present invention, the hot-rolled steel coil has an internal oxide layer having a depth of more than 0 to 8 μm or less in a medium area of the total width, and comprises: 0.70-0.80 wt% of carbon (C), more than 0 to 0.40 wt% of silicon (Si), more than 0 to 0.50 wt% of manganese (Mn), more than 0 to 0.02 wt% of aluminum (Al), more than 0 to 0.02 wt% of phosphorus (P), more than 0 to 0.02 wt% of sulfur (S), and the remaining of iron (Fe) and inevitable impurities.

Description

Hot rolled steel coil and method of manufacturing the same

The technical idea of the present invention relates to a steel material and a method for manufacturing the same, and more particularly, to a hot-rolled coil steel material in which the formation of an internal oxide layer is suppressed and a method for manufacturing the same.

Steel materials used in automobile bearings require uniform properties from the surface layer to the core. The presence of an internal oxide layer in the surface layer shortens the lifespan of automobile bearing parts that are continuously subjected to repeated loads. This internal oxidation causes oxygen to diffuse into the inside of the steel to form a metal oxide inside the steel, and it changes the composition of the surface to reduce the final physical properties of the steel, or removes the interface to cause premature fracture. Therefore, in manufacturing bearing steel, it is necessary to suppress the formation of such an internal oxide layer.

The prior art for suppressing the internal oxide layer is largely divided into the following two. The first is a method of suppressing the formation of an oxide layer due to the addition of an alloying element that suppresses carbon diffusion. However, the alloying elements may have a negative effect on the final heat treatment, and the use of these alloying elements may violate environmental regulations.

The second method is to change the hot rolling process conditions such as finishing rolling temperature and winding temperature. However, in this case, coil pass-through properties and shape control are difficult, quality problems such as edge cracking may occur, and the depth of the internal oxide layer in the width direction of the steel may have a large deviation.

Korean Patent Application No. 10-2017-0124747

The technical problem to be achieved by the technical idea of the present invention is to provide a hot-rolled coil steel material in which the formation of an internal oxide layer is suppressed and a method for manufacturing the same.

However, these tasks are exemplary, and the technical spirit of the present invention is not limited thereto.

According to one aspect of the present invention, there is provided a hot-rolled coil steel material and a manufacturing method in which the formation of an internal oxide layer is suppressed.

According to an embodiment of the present invention, the hot-rolled coil steel is, by weight, carbon (C): 0.70% to 0.80%, silicon (Si): more than 0% to 0.40%, manganese (Mn): more than 0% to 0.50%, aluminum (Al): more than 0% to 0.02%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.02%, and the balance being iron (Fe) and unavoidable impurities and may include an internal oxide layer having a depth of greater than 0 μm and less than or equal to 8 μm in the middle region of the entire width.

According to an embodiment of the present invention, the hot-rolled coil steel may include an internal oxide layer having a depth of 4 μm to 8 μm in the middle region of the total width.

According to an embodiment of the present invention, the hot-rolled coil steel may include an internal oxide layer having a depth of more than 0 μm to 3 μm in the edge region of the full width.

According to an embodiment of the present invention, the method for manufacturing the hot-rolled coil steel is, by weight, carbon (C): 0.70% to 0.80%, silicon (Si): more than 0% to 0.40%, manganese (Mn): >0% to 0.50%, Aluminum (Al): >0% to 0.02%, Phosphorus (P): >0% to 0.02%, Sulfur (S): >0% to 0.02%, and balance iron (Fe) and reheating the slab containing unavoidable impurities at a temperature of 1,180°C to 1,220°C; hot rolling the reheated slab to a finishing temperature of 860°C to 900°C; winding the hot-rolled slab at a winding temperature of 590° C. to 630° C. to form a hot-rolled coil steel; and cooling the wound hot-rolled coil steel material.

According to an embodiment of the present invention, the hot-rolled coil steel may include an internal oxide layer having a depth of greater than 0 μm and less than or equal to 8 μm in the middle region of the total width.

According to the technical idea of the present invention, in the method for manufacturing the hot-rolled coil steel, the cooling rate can be increased by winding by reducing the winding temperature, and thus the formation of the internal oxide layer can be suppressed. Therefore, the present invention can implement the control of the internal oxide layer stably and easily.

The above-described effects of the present invention have been described by way of example, and the scope of the present invention is not limited by these effects.

1 is a process flowchart schematically illustrating a method of manufacturing a hot-rolled coil steel material according to an embodiment of the present invention.
2 is a scanning electron microscope photograph showing the depth of the internal oxide layer of Example and Comparative Example manufactured by the method for manufacturing a hot-rolled coil steel material according to an embodiment of the present invention.

Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more completely explain the technical idea of the present invention to those of ordinary skill in the art, and the following examples may be modified in various other forms, The scope of the technical idea is not limited to the following examples. Rather, these embodiments are provided so as to more fully and complete the present disclosure, and to fully convey the technical spirit of the present invention to those skilled in the art. In this specification, the same reference numerals refer to the same elements throughout. Furthermore, various elements and regions in the drawings are schematically drawn. Accordingly, the technical spirit of the present invention is not limited by the relative size or spacing drawn in the accompanying drawings.

The following Wagner's relational expression regarding the depth of the internal oxide layer is shown.

<Relationship>

Here, C _o : surface oxygen concentration, C _s : solute concentration, D _o : oxygen diffusion coefficient, v: the quantity of oxygen forming an oxide, T: temperature, and t: time.

According to the above relational expression, the factors affecting the depth of the internal oxide layer are the surface oxygen concentration, the solute concentration, the oxygen diffusion coefficient, the amount of oxygen forming the oxide, temperature, time, and the like. Among them, the surface oxygen concentration, the solute concentration, the oxygen diffusion coefficient, the quantity of oxygen forming the oxide, etc. are almost impossible to change during the manufacture of the hot-rolled steel material. Therefore, it is possible to reduce the oxidation time by reducing the manufacturing temperature or increasing the cooling rate as a method of reducing the internal oxidation layer in the manufacture of hot-rolled steel.

Therefore, as shown in the above relational expression, the factor affecting the formation of the internal oxide layer has a greater effect on the temperature than the cooling rate, so a method of reducing the temperature of the manufacturing process may be considered. As a method for reducing internal oxidation during the hot rolling process, there is a method of lowering the furnace temperature, the finishing rolling temperature, or the coiling temperature. However, the reduction in the heating furnace temperature and finishing rolling temperature makes it difficult to control the sheetability and shape of the hot rolled coil in the case of high-strength high-carbon steel, and specifically, it may cause wave formation or crown failure, which reduces the yield during cold rolling operation. can lower it Accordingly, it can be said that lowering the coiling temperature as a method to reduce internal oxidation is the most realistic method. However, in the case of high carbon steel, the edge region may be broken by low-temperature structure such as bainite, Selection is important.

Therefore, the technical idea of the present invention is to select an appropriate winding temperature at which cracks in the edge region do not occur when manufacturing a high-carbon steel hot-rolled coil containing 0.7% to 0.8% carbon. It relates to a method of inhibiting formation.

One aspect of the present invention, the hot-rolled coil steel material in which the internal oxide formation is suppressed is, by weight, carbon (C): 0.70% to 0.80%, silicon (Si): more than 0% to 0.40%, manganese (Mn): 0 % to 0.50%, aluminum (Al): more than 0% to 0.02%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.02%. The remainder consists of iron (Fe) and impurities that are unavoidably contained in the steelmaking process.

The hot-rolled coil steel may include an internal oxide layer having a depth of, for example, greater than 0 μm and less than or equal to 8 μm in the middle region of the entire width, for example, at a position half of the total width. Preferably, the inner oxide layer may have a depth of, for example, 4 μm to 8 μm. In addition, the hot-rolled coil steel may include an internal oxide layer having a depth of more than 0 μm to 3 μm in the edge region of the full width. In addition, in the hot-rolled coil steel, an internal oxide layer may not be formed in the edge region.

The hot-rolled coil steel may have a pearlite structure as a whole.

Hereinafter, the role of each component included in the slab and hot-rolled coil steel and the content thereof will be described as follows. At this time, the content of the component elements all mean wt%.

carbon (C)

Carbon is included for the purpose of increasing the strength of steel. When the carbon content is less than 0.70%, it may be difficult to secure strength. When the carbon content exceeds 0.80%, workability and weldability may be deteriorated. Accordingly, carbon is preferably added in an amount of 0.70% to 0.80% of the total weight of the steel.

Silicon (Si)

Silicon is added as a deoxidizer for removing oxygen in the steel, and also serves to further enhance the solid solution strengthening effect. When the content of silicon exceeds 0.40%, oxides are formed on the surface of the steel to deteriorate weldability, plating properties, toughness, and toughness of the heat-affected zone of the steel. Therefore, silicon is preferably added in an amount of more than 0% to 0.40% of the total weight of the steel.

Manganese (Mn)

Manganese has a strength-enhancing effect, and by combining with sulfur (S) to form MnS, it prevents red hot brittleness and improves machinability. When the content of manganese exceeds 0.5%, the toughness is lowered, and the manufacturing cost of the steel material can be greatly increased. Therefore, manganese is preferably added in an amount of more than 0% to 0.50% of the total weight of the steel.

Aluminum (Al)

Aluminum is used as a deoxidizer and, like silicon, suppresses cementite precipitation, stabilizes austenite, and improves strength. When the content of aluminum exceeds 0.02%, there is a problem in that hot brittleness occurs due to aluminum oxide or the like during casting, thereby causing cracks and lowering of ductility. Therefore, aluminum is preferably added in an amount of more than 0% to 0.02% of the total weight of the steel.

Phosphorus (P)

Phosphorus is included to improve machinability, and may partially contribute to strength improvement. When the phosphorus content exceeds 0.02%, the toughness and fatigue resistance of the weld may be reduced. Therefore, phosphorus is preferably limited to more than 0% ~ 0.02% of the total weight of the steel.

Sulfur (S)

Sulfur combines with manganese to form MnS, thereby contributing to improved machinability. If the content of sulfur exceeds 0.02%, it may cause room temperature and low temperature brittleness of the steel material and impair impact toughness. Therefore, sulfur is preferably limited to more than 0% ~ 0.02% of the total weight of the steel.

The remaining component of the present invention is iron (Fe). However, since unintended impurities from raw materials or the surrounding environment may inevitably be mixed in the normal manufacturing process, this cannot be excluded. Since these impurities are known to any person skilled in the art of manufacturing processes, all details thereof are not specifically mentioned in the present specification.

Another aspect of the present invention provides a method for manufacturing a hot-rolled coil steel. According to this, the step of reheating the slab made of the above-mentioned alloy composition at a temperature of 1,180 ℃ ~ 1,220 ℃; hot rolling the reheated slab to a finishing temperature of 860°C to 900°C; winding the hot-rolled slab at a winding temperature of 590° C. to 630° C. to form a hot-rolled coil steel; and cooling the wound hot-rolled coil steel material.

The hot-rolled coil steel material, in the middle region of the total width, for example, having a depth of greater than 0 μm to 8 μm or less, for example, having a depth of 4 μm to 8 μm, may include an internal oxide layer. In addition, the hot-rolled coil steel may include an internal oxide layer having a depth of more than 0 μm to 3 μm in the edge region of the full width.

Hereinafter, a method for manufacturing a hot-rolled coil steel material in which the formation of internal oxides is suppressed according to the present invention will be described with reference to the accompanying drawings.

Manufacturing method of hot rolled coil steel

1 is a process flowchart schematically illustrating a method of manufacturing a hot-rolled coil steel material according to an embodiment of the present invention.

In the method for manufacturing a hot-rolled coil steel material according to the present invention, the semi-finished product to be subjected to the hot-rolling process may be, for example, a slab. The semi-finished slab can be obtained through the continuous casting process after obtaining molten steel of a predetermined composition through the steelmaking process.

Referring to FIG. 1 , the method for manufacturing a hot-rolled coil steel according to an embodiment of the present invention includes a slab reheating step (S10), a hot rolling step (S20), a winding step (S30), and a cooling step (S40). do.

The slab is, by weight, carbon (C): 0.70% to 0.80%, silicon (Si): more than 0% to 0.40%, manganese (Mn): more than 0% to 0.50%, aluminum (Al): 0% Exceeds to 0.02%, phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.02%, and the balance may contain iron (Fe) and unavoidable impurities.

Slab reheating step (S10)

In the slab reheating step (S10), the slab of the alloy composition is reheated to a reheating temperature (SRT) of 1180 ° C to 1220 ° C. If the reheating temperature is less than 1180 °C, the precipitation-type elements cannot be sufficiently dissolved in the slab. When the reheating temperature exceeds 1220° C., the grains may be coarsened to increase the grain size. Therefore, by reheating the slab to a temperature of 1180° C. to 1220° C., the slab can have a uniform structure by controlling the grain size.

Hot rolling step (S20)

In the hot rolling step (S20), the reheated slab is hot rolled to a finishing temperature (FDT) of 860 ° C to 900 ° C. The hot rolling may be configured to perform, for example, wide rolling, rough rolling, and finishing rolling. When the finishing temperature is less than 860 ° C., the pro-eutectoid ferrite or pro-eutectoid cementite is rolled in a partially precipitated state, and the steel sheet becomes non-uniform, so that the uniformity of properties in the steel sheet may be reduced, and the low-temperature toughness may be greatly reduced. When the finishing temperature is higher than 900° C., the surface quality of the steel sheet may be deteriorated due to the generation of solid surface scale.

Winding step (S30)

In the winding step (S30), the hot-rolled slab is wound at a coiling temperature (CT) of 590° C. to 630° C. to form a hot-rolled coil steel. In the winding step (S30), the hot-rolled slab may be cooled to the winding temperature to be wound. When winding at the above temperature, excessive crystal grain growth is inhibited, ductility and moldability are excellent, and excellent strength can be secured. The coiling temperature may be preferably 600 °C to 620 °C, and more preferably 605 °C to 615 °C.

When the coiling temperature is higher than 630° C., an internal oxidation reaction by oxygen diffusing to the inside through the surface at a high temperature may occur, which may adversely affect wear resistance and fatigue resistance. When the coiling temperature is less than 590° C., a low-temperature transformation phase such as bainite or martensite is generated, and the steel sheet is excessively hardened to aggravate the winding phenomenon and cause a significant decrease in workability. Although there is an advantage that cementite is easily dispersed finely after annealing in the structure of the low-temperature transformation phase, high-carbon steel containing 0.70 wt % or more of carbon has high hardness in the low-temperature transformation phase due to the high carbon content, and thus the manufacturability and workability of the steel sheet can't allow for a drop in

Cooling step (S40)

In the cooling step (S40), the wound hot-rolled coil steel material is cooled. For the cooling, air cooling performed in a natural cooling method may be used, or water cooling performed in a rapid cooling method may be used.

Experimental example

Hereinafter, preferred experimental examples are presented to help the understanding of the present invention. However, the following experimental examples are only for helping understanding of the present invention, and the present invention is not limited by the following experimental examples.

Hot-rolled coil steels having the composition shown in Table 1 below were prepared. The remainder consists of iron (Fe) and impurities that are unavoidably contained in the steelmaking process. The compositions of Comparative Examples and Examples were the same.

ingredient C Si Mn Al P S content (wt%) 0.75 0.20 0.40 0.25 0.01 0.01

Table 2 shows the process conditions for forming the hot-rolled coil steels of Comparative Examples and Examples.

division reheat temperature
(℃) Rolling end temperature
(℃) winding temperature
(℃) Comparative Example 1 1200 880 730 Example 2 1200 880 670 Example 1200 880 610

Referring to Table 2, the comparative example is a hot-rolled coil steel manufactured according to the conventional process conditions, and by performing the winding temperature at 730°C and 670°C, it was performed at a temperature exceeding 630°C, the upper limit of the range of the present invention. . On the other hand, the Example according to the present invention was carried out at a temperature of 610 ℃. This is to suppress the formation of an internal oxide layer occurring in the entire coil by reducing the coiling temperature, thereby increasing the cooling rate of the steel material during winding after the completion of hot rolling compared to the conventional case.

2 is a scanning electron microscope photograph showing the depth of the internal oxide layer of Example and Comparative Example manufactured by the method for manufacturing a hot-rolled coil steel material according to an embodiment of the present invention.

Referring to FIG. 2 , the internal oxide layer is shown in 1/2 of the total width of the wound hot-rolled coil steel material (ie, the central region of the width). Here, 1/2 of the total width means a position advanced in the width direction of the hot-rolled coil steel. In Comparative Example 1 having the highest coiling temperature of 730°C, an internal oxide layer having a depth of 15.3 μm was formed, and in Comparative Example 2 having a coiling temperature of 670°C, an internal oxide layer having a depth of 9.6 μm was formed. On the other hand, in the embodiment having the lowest coiling temperature of 610° C., an internal oxide layer having a depth of 7.2 μm was formed. Accordingly, it can be seen that the depth of the inner oxide layer decreases as the coiling temperature decreases.

Also, in all cases, it was confirmed that the inner oxide layer of the edge region had a smaller depth than that of the central region. This is analyzed because the cooling rate of the edge region is faster than that of the central region.

Also, in the case of the examples, no cracking of the edge region was observed. It is analyzed that the coiling temperature is higher than the production temperature of bainite, for example, 530 ° C. to 550 ° C., so that the cracking phenomenon is prevented.

In articles such as bearings, it is required that the inner oxide layer be controlled in the range of depths of 6 μm to 8 μm. In the case of Comparative Examples, an internal oxide layer having a depth of 15.3 μm or 9.6 μm is formed in a region of 1/2 of the total width, which exceeds the above criteria. In this case, the usable portion of the hot-rolled coil steel is limited, and thus the yield may be reduced. On the other hand, in the embodiment, considering that an internal oxide layer having a depth of 7.2 μm is formed in a region of 1/2 of the total width, and the depth of the internal oxide layer in a region of 1/2 of the total width is the maximum value, the criterion can be satisfied. can Therefore, in the embodiment, the entire area of the hot-rolled coil steel can be used without limitation.

According to the above-mentioned results, it can be seen that the internal oxidation of the hot-rolled coil steel is closely related to the cooling rate, and the cooling rate can be controlled by the winding temperature. The hot-rolled coil is cooled more slowly toward the inside of the hot-rolled coil in the width direction from the edge of the hot-rolled steel material, and the formation of the internal oxide layer is promoted.

The technical spirit of the present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it is the technical spirit of the present invention that various substitutions, modifications and changes are possible within the scope without departing from the technical spirit of the present invention. It will be apparent to those of ordinary skill in the art to which this belongs.

Claims (5)

By weight, carbon (C): 0.70% to 0.80%, silicon (Si): greater than 0% to 0.40%, manganese (Mn): greater than 0% to 0.50%, aluminum (Al): greater than 0% to 0.02% , phosphorus (P): more than 0% to 0.02%, sulfur (S): more than 0% to 0.02%, and the balance contains iron (Fe) and unavoidable impurities,
comprising an inner oxide layer having a depth greater than 0 μm and less than or equal to 8 μm in the middle region of its full width,
Hot rolled coil steel. The method of claim 1,
The hot-rolled coil steel includes an internal oxide layer having a depth of 4 μm to 8 μm in the middle region of the entire width,
Hot rolled coil steel. The method of claim 1,
The hot-rolled coil steel includes an internal oxide layer having a depth of more than 0 μm to 3 μm in the edge region of the full width,
Hot rolled coil steel. By weight, carbon (C): 0.70% to 0.80%, silicon (Si): greater than 0% to 0.40%, manganese (Mn): greater than 0% to 0.50%, aluminum (Al): greater than 0% to 0.02% , phosphorus (P): more than 0% ~ 0.02%, sulfur (S): more than 0% ~ 0.02%, and the balance is to reheat the slab containing iron (Fe) and unavoidable impurities at a temperature of 1,180 ° C. to 1,220 ° C. step;
hot rolling the reheated slab to a finishing temperature of 860°C to 900°C;
winding the hot-rolled slab at a winding temperature of 590° C. to 630° C. to form a hot-rolled coil steel; and
Including; cooling the wound hot-rolled coil steel material;
Manufacturing method of hot rolled coil steel. 5. The method of claim 4,
The hot-rolled coil steel includes an internal oxide layer having a depth of greater than 0 μm and less than or equal to 8 μm in the middle region of the total width,
Manufacturing method of hot rolled coil steel.

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