KR102031462B1 - Method of manufacturing sub-peritectic steel - Google Patents

Abstract

The present invention relates to a method for producing apocrystal steel.
In one embodiment of the present invention by weight, C: 0.07 to 0.17%, Mn: 1.5 to 2.5%, Ti: 0.015 to 0.05%, continuous casting of molten steel containing the remaining Fe and other unavoidable impurities to obtain a thin slab step; Rough rolling the thin slab to obtain a bar; Finishing rolling the bar to obtain a hot rolled steel sheet; And winding the hot-rolled steel sheet, wherein each step is performed continuously, and the mold flux introduced into the mold during the continuous casting is 45-60% of crystalline steel and has a carbon content of 10% by weight or more. It provides a manufacturing method.

Description

METHOD OF MANUFACTURING SUB-PERITECTIC STEEL}

The present invention relates to a method for producing apocrystal steel.

In general, apocrystal steels with a carbon content of 0.07 to 0.17% by weight easily form non-uniform coagulation shells during solidification shrinkage due to the crystallization reaction, and thus have a weak fracture strength locally due to the occurrence of an air gap at the interface with the mold contact surface. Thin solidified shells are formed and cracks are generated in these areas. Since the non-uniform solidification is intensified when the circumferential speed increases, conventional conventional mills usually reduce the casting speed by about 20% compared to general steel grades to produce aporous steels below 2mpm.

However, in the performance-rolling direct connection line, which is recently used to increase the efficiency of the process and reduce the quality variation of the steel, continuous casting can only be performed at a relatively high speed in order to match the continuous casting speed with the rolling speed. It was very difficult to manufacture in the direct connection line.

One aspect of the present invention is to provide a method for manufacturing apocrystalline steel that can be cast at a high speed, but does not generate cracks on the surface of the slab.

In one embodiment of the present invention by weight, C: 0.07 to 0.17%, Mn: 1.5 to 2.5%, Ti: 0.015 to 0.05%, continuous casting of molten steel containing the remaining Fe and other unavoidable impurities to obtain a thin slab step; Rough rolling the thin slab to obtain a bar; Finishing rolling the bar to obtain a hot rolled steel sheet; And winding the hot-rolled steel sheet, wherein each step is performed continuously, and the mold flux introduced into the mold during the continuous casting is 45-60% of crystalline steel and has a carbon content of 10% by weight or more. It provides a manufacturing method.

According to one aspect of the present invention, it is possible to cast at a high speed, but also to provide a method for producing apocrystal steel that can significantly improve the productivity compared to the conventional hot-dip steel by allowing stable production of apocrystal steel without cracks on the surface of the lubrication characteristics. can do.

Figure 1 shows the solidification cell shape according to the carbon content of the steel.
Figure 2 is a schematic diagram of the equipment for the play-rolling direct connection process applicable to the present invention.
It is a schematic diagram of the mold copper plate which concerns on one Embodiment of this invention.
4 is a photograph of an invention example and a comparative example, (a) is invention example 1, (b) is comparative example 1, and (c) is photograph of comparative example 2. FIG.

Figure 1 shows the solidification cell shape according to the carbon content of the steel. Typically, in the case of apocrystal steel, a thin solidification shell is formed around the meniscus during the initial solidification process, and the solidification shrinkage is large in this process, which causes an air gap between the solidification shell and the mold surface. Due to the air gap, a non-uniform coagulation shell is formed as in region (c) of FIG. 1.

If such a non-uniform solidification shell is formed, a portion having a low breaking strength occurs locally, which causes cracking during casting or, in severe cases, an operation accident due to leakage of molten steel such as breakout.

For this reason, in the existing process, the casting speed is reduced by about 20% compared to general steel, but there are still many quality issues due to cracks on the surface of cast steel.

Accordingly, the inventors of the present invention have a method for minimizing the formation of the non-uniform coagulation shell, and increase the crystalline rate of the mold flux, but in order to solve the problem of deterioration of lubricity caused by the increase of the crystalline rate of the mold flux. The present invention has been proposed in view of the fact that controlling the injected mold flux condition enables not only to stably perform high-speed casting in a performance rolling direct connection line, but also to produce high-strength apocrystalline steel without surface cracks.

To this end, an embodiment of the present invention is a method for producing apocrystal steel by a play-rolling direct connection process, in weight%, C: 0.07 to 0.17%, Mn: 1.5 to 2.5%, Ti: 0.015 to 0.05%, the rest Continuous casting of molten steel containing Fe and other unavoidable impurities to obtain a thin slab; Rough rolling the thin slab to obtain a bar; Finishing rolling the bar to obtain a hot rolled steel sheet; And winding the hot rolled steel sheet, and the mold flux introduced into the mold during the continuous casting provides a method of manufacturing apocrystal steel having a crystalline rate of 45 to 60% and a carbon content of 10% by weight or more.

Hereinafter, the present invention will be described.

In the present invention, the target material is aporous steel, and may contain, for example,% by weight, C: 0.07 to 0.17%, Mn: 1.5 to 2.5%, and Ti: 0.015 to 0.05%. In the case of the high-strength apocrystalline steel, there is a high risk of solidification non-uniformity due to the crystallization reaction, and the likelihood of cracking on the surface of the cast steel is increased. Therefore, there is a need for a method for reducing the coagulation unevenness of the apocrystal steel.

The remaining component of the apocrystal steel, which is the subject material of the present invention, is iron (Fe), and in the ordinary manufacturing process, impurities that are not intended from the raw material or the surrounding environment may be inevitably mixed, and thus cannot be excluded. Since these impurities are known to those skilled in the art, all of them are not specifically mentioned in the present specification.

Figure 2 is a schematic diagram of the equipment for the play-rolling direct connection process applicable to the present invention. As shown in FIG. 2, the performance-rolling direct connection equipment includes a continuous casting machine including a ladle 10, a mold 20, and a segment cooling table 30, a thin slab heater 40, a rough rolling mill 50. , A bar heater 60, a finish rolling mill 70, and a winding machine 80. In the performance-rolling direct connection facility, the mold 20 receives molten steel from the ladle 10 to accommodate the molten steel and discharge the segment cooling zone to perform continuous casting. The slab cooling slab (a) of the first thickness is produced through the segment cooling table 30, and the rough mill 50 rolls the slab into a bar (b) of a second thickness thinner than the first thickness. In addition, the finish rolling mill 70 is to roll the bar of the second thickness into a strip (c) of the third thickness, to produce the product by producing the strip in the form of a coil through the winding machine (90).

More specifically, molten steel is supplied from the ladle 10 into the mold 20, and solidification is started by heat exchange with cooling water flowing in the outer wall of the mold 20.

In this process, mold flux is prevented to insulate molten steel and reoxidize during casting, to capture inclusions in molten steel and to lubricate solidified slabs in the mold. The injected mold flux is important for the formation of the solidification shell in the mold because it forms a film to control the heat transfer between the mold and the molten steel.

On the other hand, it is preferable that superheat degree of the said molten steel is 15-25 degreeC. When the degree of superheat of the molten steel is less than 15 ° C., the casting may be stopped due to freezing due to a temperature drop during casting, and when the temperature exceeds 25 ° C., mold level hunting may occur and surface quality defects may occur.

In the present invention, it is preferable to control the crystalline rate of the mold flux introduced into the mold to 45 to 60%. When the crystalline rate of the mold flux is less than 45%, there is little effect of reducing heat transfer between the mold and the molten steel, thereby forming a non-uniform solidified shell, which may cause problems during casting. On the other hand, if it exceeds 60%, the solidification point is excessively high, there is a disadvantage that it is difficult to secure enough lubricity.

In addition, the mold flux preferably has a carbon content of 10% by weight or more. In the present invention, by increasing the carbon content of the mold flux as described above to ensure that the carbon sufficiently lubricity. If the carbon content is less than 10% by weight, it is difficult to secure the effect. In the present invention, when the carbon content in the mold flux is 10% or more, the lubrication action desired by the present invention can be sufficiently obtained, and therefore the upper limit thereof is not particularly limited. However, excessive carbon content in the flux may cause surface defects such as smut, so the carbon content is preferably 20% or less.

In addition, the mold flux may contain 10 to 50% of CaO and 10 to 50% of SiO ₂ in weight%, and in this case, the basicity expressed by CaO / SiO ₂ is preferably 1.3 to 1.8. . If the basicity is less than 1.3, the amount of heat transfer in the mold may increase, which may adversely affect the uniform solidification shell formation. If the basicity is higher than 1.8, the lubricity of the flux may be deteriorated, and a restrictive breakout may occur. Therefore, the basicity preferably has a range of 1.3 to 1.8, more preferably has a range of 1.4 to 1.8, still more preferably has a range of 1.5 to 1.8, and most preferably has a range of 1.6 to 1.8. desirable.

On the other hand, the mold flux may further comprise less than 2% by weight of MgO. The MgO may be included to promote crystallization of the mold flux film as an element affecting the crystallization of the mold flux film. However, when the MgO is more than 2% by weight, the crystallization rate may be insufficient, resulting in a slow cooling effect.

In addition, the mold flux of the present invention may further include Al ₂ O ₃ , Na ₂ O and the like.

The mold flux of the present invention provided as described above may increase the solidification temperature to 1200 ° C. or more, thereby reducing the meltability of the mold flux and degrading the lubricating ability.

Accordingly, in the present invention, in order to increase the lubricity, the mold flux may have a viscosity of 0.3 to 0.8 Pa · S at 1300 ° C.

It is also desirable to maintain the viscosity of the residual glassy in the mold flux film at 5 Pa.S or less at temperatures above 950 ° C. When the viscosity of the residual glass in the mold flux film at a temperature of 950 ° C. or more exceeds 5 Pa · S, the viscosity in the solid film is high, so that the lubricity is poor.

It is a schematic diagram of the mold copper plate which concerns on one Embodiment of this invention. Slow cooling in the mold is also influenced by the distance between the cooling water holes of the mold and the contact surface of the molten steel. Accordingly, in the present invention, it is preferable to control the distance between the end face of the copper plate shown in FIG. 3 and the cooling water hole of the mold copper plate, and to control the distance to 15 to 20 mm. If the distance between the end face of the copper plate and the coolant hole of the mold copper plate is in contact with the molten steel is less than 15mm, the effect of preventing heat increase is not sufficient, and if it exceeds 20mm, there is a risk of breakout due to the lack of solidification shell formation during high-speed casting. .

In addition, in the present invention, during the continuous casting, the heat transfer value in the mold is preferably 2.0 to 2.5MW / ㎡ on the basis of the long side. If the heat transfer value in the mold is less than 2.0 MW / m 2, the air gap in the mold may cause heat transfer due to poor contact with the copper plate, resulting in a restrictive breakout, and exceeding 2.5 MW / m 2. In this case, the risk of cracking may increase due to increased heat quantity.

In the present invention, a thin slab is obtained by continuously casting molten steel prepared as described above. At this time, the continuous casting is preferably carried out at a casting speed of 4.0 ~ 8.0mpm (m / min). The reason why the casting speed is more than 4.0mpm is because the high speed casting and the rolling process are connected, and a certain casting speed is required to secure a target rolling temperature. If the casting speed exceeds 8.0mpm, the operation success rate may be reduced due to instability of the molten steel. Therefore, the casting speed is preferably in the range of 4.0 to 8.0mpm, and has a range of 4.5 to 7.5mpm. It is more preferable, and it is still more preferable to have a range of 4.5-6.5mpm.

At this time, the thickness of the thin slab is preferably 80 ~ 120mm. When the thickness of the thin slab exceeds 120mm, not only the high speed casting is difficult, but also the rolling load increases during rough rolling, and when the thickness of the slab is less than 80mm, the temperature of the cast slab rapidly occurs, making it difficult to form a uniform structure. In order to solve this problem, it is possible to additionally install a heating device, but this is a factor to improve the production cost, it is desirable to exclude as possible. Therefore, the thickness of the thin slab is preferably controlled to 80 to 120 mm, more preferably 85 to 115 mm, even more preferably 90 to 110 mm or less.

Thereafter, the thin slab is roughly rolled to obtain a bar. The thickness of the bar is preferably 10 ~ 20mm. When the thickness of the bar exceeds 20mm, the rolling load may increase during finishing rolling, and when the thickness of the bar is less than 10 mm, the rolling deformation resistance may increase, causing difficulty in operation, and it may be difficult to secure the temperature during finishing rolling. .

Thereafter, the bar is finish rolled to obtain a hot rolled steel sheet. At this time, it is preferable that the said finish rolling temperature is 750 degreeC or more. When the finish rolling temperature is less than 750 ° C, the load of the roll during hot rolling increases greatly, thereby increasing energy consumption and slowing down the working speed.

Thereafter, the hot rolled steel sheet is wound up. At this time, the winding temperature is preferably 500 ~ 650 ℃. If the coiling temperature is less than 500 ℃ may have a problem that the strength is too high due to cold rolling and poor shape in the cold rolling, if the temperature exceeds 650 ℃ secondary surface may occur to reduce the surface quality Therefore, it is preferable to control the said winding temperature to 500-650 degreeC.

The apocrystalline steel of the present invention obtained as described above may have a thickness of 1.5 mm or less, and thus, in the present invention, it is possible to easily prepare the apocrystalline steel of a thin film at a high casting speed.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, it is necessary to note that the following examples are only for illustrating the present invention in more detail, and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by the matters described in the claims and the matters reasonably inferred therefrom.

(Example)

The molten steel having the alloy composition of Table 1 was subjected to a play-rolling direct connection process to prepare apocrystalline steel in the continuous continuous rolling mode, thereby continuously casting the molten steel to obtain a thin slab. At this time, the continuous casting using the mold flux having the conditions shown in Table 2, the mold flux film was collected through a flux collecting facility installed directly under the mold during casting, the crystallinity in the film analyzed by SEM, optical microscope analysis After measuring the amount of residual C, the residual glassy viscosity and the amount of heat monitored during the operation, whether the cracks and non-uniform coagulation shell generated in the slab produced during the operation, the results are shown in Table 3 below. On the other hand, in the case of the crystalline rate, the crystalline rate was calculated through the area ratio by image processing on data obtained through SEM analysis of the flux film collected during casting.

division
Alloy composition (% by weight) C Si Mn P S Al Ti N Comparative Example 1 0.096 1.0 2.8 0.019 0.001 0.025 0.015 0.008 Comparative Example 2 0.07 0.03 1.4 0.008 0.001 0.021 0.13 0.0064 Inventive Example 1 0.08 0.357 1.8 0.014 0.0008 0.022 0.022 0.0065

division SiO2
(weight%) CaO
(weight%) basicity MgO
(weight%) Al ₂ O ₃
(weight%) Na ₂ O
(weight%) Total C
(weight%) Solidification temperature
(℃) Viscosity
(@ 1300 ℃, Pa.S) Comparative Example 1 27.1 35.7 1.32 3.6 3.5 8.8 8.2 1151 0.52 Comparative Example 2 27.5 36.3 1.32 3.5 3.1 8 8.9 1183 0.71 Inventive Example 1 22.9 37.9 1.66 1.5 4.6 6.4 11.2 1211 0.56

division Average crystallinity
(%) Carbon
(weight%) Residual glass
Viscosity (@ 950 ° C, Pa.S) Cracks, non-uniform
Solidified shell
Occurrence Heat quantity
(MW / m2) Comparative Example 1 47 0.058 6 Occurrence (BO) 1.75 Comparative Example 2 47.5 0.068 8.2 Occurrence (BO) 2.75 Inventive Example 1 54.7 0.14 4.2 none 2.2

As can be seen through Tables 1 to 3, in the case of Inventive Example 1, which satisfies the crystallinity and carbon content of the mold flux proposed by the present invention and the residual glassy viscosity at 950 ° C., no cracks or uneven solidified shells are generated. It can be seen that good casting operation is possible but good product quality is secured.

However, in the case of Comparative Examples 1 and 2, as the carbon content of the mold flux of the present invention, the residual glassy viscosity conditions, etc. are not satisfied, cracks are generated on the surface of the cast steel and a non-uniform solidified shell is formed to break out during operation (BO). It can be seen that has occurred.

4 is a photograph of an invention example and a comparative example, (a) is invention example 1, (b) is comparative example 1, and (c) is photograph of comparative example 2. FIG. As can be seen from Figure 4, in the case of Inventive Example 1 it can be seen that the surface is good, but in the case of Comparative Example 1 a non-uniform coagulation shell was generated, in the case of Comparative Example 2 it can be seen that the surface crack occurred. .

a: slab b: bar
c: strip
10: ladle 20: mold
30: Segment cooling table 40: Pak slab burner
50: roughing mill 60: bar heater
70: finish rolling mill 80: winder

Claims (16)

Weight casting, continuously casting molten steel containing C: 0.07 to 0.17%, Mn: 1.5 to 2.5%, Ti: 0.015 to 0.05%, remaining Fe and other unavoidable impurities to obtain a thin slab;
Rough rolling the thin slab to obtain a bar;
Finishing rolling the bar to obtain a hot rolled steel sheet; And
Winding the hot rolled steel sheet;
Each of the above steps is carried out continuously,
The mold flux introduced into the mold during the continuous casting is 45 to 60% crystalline rate, 10 to 20% by weight of carbon content manufacturing method of apocrystalline steel.
The method according to claim 1,
The molten steel has a superheat degree of 15 ~ 25 ℃ a method for producing a steel.
The method according to claim 1,
The mold flux has a basicity (CaO / SiO ₂ ) of 1.3 to 1.8 method for producing apocrystalline steel.
The method according to claim 1,
The mold flux is a method for producing aporous steel further comprises 2% by weight or less (excluding 0%) of MgO.
The method according to claim 1,
The mold flux is a method for producing apotable steel having a solidification temperature of 1200 ℃ or more.
The method according to claim 1,
The mold flux is a method for producing apocrystalline steel having a viscosity of 0.3 ~ 0.8 Pa.S.
The method according to claim 1,
A method for producing apocrystalline steel, wherein the residual glassy viscosity of the mold flux is 5 Pa.S or less at a temperature of 950 ° C or higher.
delete The method according to claim 1,
A method for manufacturing apotable steel, wherein a distance between the end face of the copper plate in contact with the molten steel and the cooling water hole of the mold copper plate is 15-20 mm.
The method according to claim 1,
In the continuous casting, the heat transfer value in the mold is a method for producing apotable steel is 2.0 ~ 2.5MW / ㎡ based on the long side.
The method according to claim 1,
Casting method of the continuous casting the apocrystal steel is 4.0 ~ 8.0mpm.
The method according to claim 1,
The thickness of the thin slab is 80 ~ 120mm Apocrystal steel manufacturing method.
The method according to claim 1,
The thickness of the bar is 10 ~ 20mm A method of manufacturing apo steel.
The method according to claim 1,
The finish rolling temperature is a method for producing apocrystal steel is 750 ℃ or more.
The method according to claim 1,
Winding temperature is 500-650 degreeC The manufacturing method of the apo crystal steel.
The method according to claim 1,
The hot rolled steel sheet has a thickness of less than 1.5mm Apo steel.

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