METHOD FOR REDUCING EDGE SERRATION DEFECTS IN THIN SLAB

Abstract

Disclosed herein is a method for reducing edge serration defects in a thin slab, including: reheating a thin slab having an alloy composition including carbon (C), niobium (Nb), aluminum (Al), iron (Fe) and inevitable impurities to uniformalize the thin slab into an austenite structure; controlling precipitates that occur in the reheated thin slab by controlling an temperature of the thin slab discharged from a heat-treatment furnace at a temperature at which niobium carbide (NbC) start to precipitate; and hot-rolling the thin slab. The method is advantageous in that the temperature of a thin slab discharged from a heat-treatment furnace is controlled, so that the edge serration defects in the thin slab can be reduced, thereby improving the productivity of the thin slab.

Description

RELATED APPLICATION

This application is a continuation application under 35 U.S.C. §365(c) of International Application No. PCT/KR2009/007993, filed Dec. 30, 2009 designating the United States. International Application No. PCT/KR2009/007993 was published in English as WO2010/110529 A1 on Sep. 30, 2010. This application further claims the benefit of the earlier filing date under 35 U.S.C. §365(b) of Korean Patent Application No. 10-2009-0026030 filed Mar. 26, 2009. This application incorporates herein by reference the International Application No. PCT/KR2009/007993 including the International Publication No. WO2010/110529 A1 and the Korean Patent Application No. 10-2009-0026030 in their entirety.

TECHNICAL FIELD

The present disclosure relates to a method for reducing edge serration defects in a thin slab and, more particularly, to a method for reducing edge serration defects which are typical defects of a thin slab.

BACKGROUND

Among various slab casting methods, a thin slab casting process is used to fabricate a thin slab which has dimensions close to those of a final product. In the thin slab casting process, since a rough rolling process can be omitted, the process can be simplified.

In such a continuous thin slab casting process, compared to typical continuous casting processes, a thin slab is fabricated at a high speed. Liquid molten steel is completely solidified into a thin slab in a mold and a stand, and the thin slab has finer crystal grain than a typically produced slab.

However, since the slab is thin, it is rapidly cooled, and particularly, edges of the thin slab cools more rapidly than the center portion thereof; thus residual stress is formed at the edges thereof.

Therefore, as shown in FIG. 1, serration defects occur at the edges of a thin slab or a hot-rolled coil that have residual stress due to uneven cooling.

In other words, edges of a thin slab excessively cools, and the elongation of the center portions of the thin slab in a width direction is greater than that of the edges; thus the thin slab elongates in a thickness direction, and serration defects occur at the edges of the thin slab.

Particularly, steel including niobium (Nb) and 0.20 to 0.28 wt % of carbon and produced by a continuous thin slab casting process may have many serration defects. In this case, the steel is easily damaged even by a stress lower than the strength of a raw material.

Thus, since iron manufacturing companies need to do additional work for such hot-rolled coil having edge serration defects, productivity decreases.

The foregoing discussion of the background section is to provide general background information, and does not constitute an admission of prior art.

SUMMARY

An aspect of the present invention provides a method for reducing edge serration defects in a thin slab by controlling the temperature of the thin slab discharged from a heat-treatment furnace.

An aspect of the present invention provides a method for reducing edge serration defects in a thin slab, including: uniformizing a thin slab having an alloy composition including carbon (C), niobium (Nb), aluminum (Al), iron (Fe) and inevitable impurities into an austenite state by reheating in a heat-treatment furnace; controlling a temperature of the thin slab being discharged from a heat-treatment furnace to be higher than a precipitation temperature of niobium carbide (NbC); and hot-rolling the thin slab.

Here, the carbon (C) may be present in an amount of 0.20 to 0.28 wt %. Further, the temperature of the thin slab being discharged from the heat-treatment furnace may be controlled to be 1060 to 1100° C.

Further, the temperature of the thin slab being discharged from the heat-treatment furnace may be controlled to be 1080 to 1100° C.

According to embodiments of the present invention, edge serration defects in a thin slab can be reduced by controlling the temperature of the thin slab being discharged from a heat-treatment furnace. Therefore, the quality of a thin slab can be improved, and the yield of a product can be increased; thus the reliability and productivity of a product are improved.

DESCRIPTION OF DRAWINGS

FIG. 1 is a photograph showing edge serration defects in a hot-rolled coil;

FIG. 2 is a graph showing the state of steel including niobium (Nb) and carbon in an amount of 0.20 to 0.28 wt %;

FIG. 3 is a graph showing the serration defects of an edge of a coil depending on temperatures of steel discharged from a heat-treatment furnace; and

FIG. 4 shows electron microscope photographs for comparing microstructure of a thin slab according to embodiments of the present invention with that of a typical thin slab.

DETAILED DESCRIPTION OF EMBODIMENTS

Hereinafter, a method for reducing edge serration defects in a thin slab according to an embodiment of the present invention will be described in detail with reference to the attached drawings.

FIG. 2 is a graph showing the state of steel including niobium (Nb) and carbon in an amount of 0.20 to 0.28 wt %, FIG. 3 is a graph showing the serration defects of edges of a coil depending on the temperature of the steel being discharged from a heat-treatment furnace, and FIG. 4 shows electron microscope photographs comparing a microstructure of a thin slab according to embodiments of the present invention with that of a comparative thin slab.

An alloy composition used in embodiments of the present invention includes carbon (C) in an amount of 0.20 to 0.28 wt %, niobium (Nb), aluminum (Al), manganese (Mn), sulfur (S), nitrogen (N), iron (Fe), and inevitable impurities.

A thin slab fabricated using the alloy composition is reheated in a heat-treatment furnace to be uniformalized into an austenite structure, and is then hot rolled while controlling the temperature of the thin slab being discharged from the heat-treatment furnace at 1060 to 1100° C. to control the formation of precipitates.

More concretely, the temperature of the thin slab being discharged from the heat-treatment furnace is controlled to be at or to exceed a temperature at which niobium carbide (NbC) starts to precipitate, so that niobium carbide (NbC) does not precipitate during the reheating process but precipitates during a hot rolling process; thus edge serration defects in the thin slab are reduced.

The edge serration defects occurring in a thin slab are chiefly related to niobium carbide (NbC) precipitates. The niobium carbide (NbC) precipitates inhibit the growth of a grain boundary during hot rolling, so as to decrease the size of crystal grains.

However, the precipitates formed at grain boundaries act as sources of stress concentration during hot rolling to cause voids in the grain boundaries to be formed, and these voids grow into cracks; thus the grain boundaries is damaged. Particularly, the grain boundary embrittlement phenomenon of a thin slab is due to the precipitation of niobium carbide (NbC) that happens because edges of a thin slab cool rapidly because it is thin. This phenomenon is serious.

Therefore, the precipitation of niobium carbide (NbC) is intentionally avoided during a reheating process; thus serration defects from occurring at edges of hot-rolled coils are prevented. The reason is that a stress concentration phenomenon does not occur because electric potential freely moves even though the thin slab elongates in a thickness direction during a hot rolling process.

FIG. 2 shows the temperature at which niobium carbide (NbC) starts to precipitate in steel including niobium (Nb) and carbon in an amount of 0.20 to 0.28 wt %. As shown in FIG. 2, in the steel including 0.20 to 0.28 wt % of carbon and niobium (Nb), a large amount of niobium carbide (NbC) is precipitated at an austenite (γ) grain boundary when the temperature of a thin slab being discharged from a heat-treatment furnace is 1060 to 1080° C.

Therefore, the formation of precipitates is intentionally controlled by controlling the temperature of the thin slab being discharged from the heat-treatment furnace to range from 1060 to 1100° C., and then a hot rolling process is performed.

When the temperature of the thin slab being discharged from the heat-treatment furnace is lower than 1060° C., the effect of preventing the edge serration defects of the thin slab is slight due to the mass production of niobium carbide (NbC) precipitates, and, when the temperature thereof is higher than 1100° C., austenite crystal grains become rough and large, thus strength decreases.

Further, the formation of precipitates is intentionally controlled by controlling the temperature of the thin slab being discharged from the heat-treatment furnace to be 1080 to 1100° C., and then a hot rolling process may be performed. The reason is that the lower limit of the temperature of the thin slab being discharged from the heat-treatment furnace is variable and can be set to anywhere within the range of 1060 to 1080° C. depending on the content of carbon.

Meanwhile, the edge serration defects occurring in the thin slab are also related to aluminum nitride (AlN) precipitate and manganese sulfide (MnS) precipitates in addition to the niobium carbide (NbC) precipitates. However, the precipitation of niobium carbide (NbC) is mainly controlled because niobium carbide (NbC) precipitates greatly influence the edge serration defects, because the strength of steel deteriorates and the production cost thereof increases when the thin slab is excessively reheated in order to control the precipitation of aluminum nitride (AlN) and manganese sulfide (MnS). In this procedure, the precipitation of aluminum nitride (AlN) may also be controlled.

The alloy composition according to embodiments of the present invention will be described in brief as follows.

Carbon (C) is an element necessary for imparting high strength. In the case of steel, carbon is added in an amount of 0.20 to 0.28 wt % in order to impart high strength to the steel.

When a small amount of carbon is added, the strength of the steel becomes low, and the amount of niobium carbide precipitated therefrom decreases; thus solid-solution strengthening elements need to be added, and the production cost thereof increase. Further, when carbon is excessively added, the amount of niobium for solid-dispersing carbon increases; thus the production cost thereof increases. When carbon is excessively added, the amount of niobium carbide (NbC) precipitated therefrom also increases, and the growth of particles is inhibited; thus workability decrease.

Niobium (Nb) is added in order to precipitate carbon (C) and nitrogen (N) into niobium carbide (NbC) and niobium nitride (NbN). The niobium carbide (NbC) and niobium nitride (NbN) precipitates inhibit the growth of a grain boundary, and make the size of crystal grains small, thereby contributing to improving the strength. Niobium is added in an amount of 0.005 to 0.020 wt %. When the amount of niobium is less than 0.005 wt %, the amount of niobium carbide (NbC) and niobium nitride (NbN) precipitates is excessively small, and thus the effect of improving the strength due to precipitation hardening is not expected. Further, when the amount of niobium is more than 0.020 wt %, the strength is excessively increased so that flexibility deteriorates, and the appearance becomes bad.

Aluminum (Al) is an element serving as a deoxidant, and makes the dissolved oxygen in steel to be kept low. Further, aluminum which forms a carbide reacts with nitrogen in a solid-solution state to produce aluminum nitride (AlN) precipitates and remove nitrogen in the solid-solution state. Aluminum is added in an amount of 0.01 to 0.05 wt %. When the amount of aluminum is less than 0.01 wt %, the effect thereof is slight. Further, when the amount of aluminum is more than 0.05 wt %, workability deteriorates.

Here, the amount of the niobium and aluminum is given for the steel that includes carbon in an amount of 0.20 to 0.28 wt %. Further, in embodiments of the present invention, the temperature of a thin slab being discharged from a heat-treatment furnace is determined based on the state of steel including niobium (Nb) and carbon in an amount of 0.20 to 0.28 wt %.

Hereinafter, the method for reducing edge serration defects in a thin slab will be described in more detail with reference to the following Example.

EXAMPLE

A thin slab including niobium (Nb) and carbon in an amount of 0.20 to 0.28 wt % was cast and then reheated in a heat-treatment furnace so that the temperature of the thin slab being discharged from the heat-treatment furnace is 1040 to 1090° C., and then the thin slab was hot-rolled to fabricate a hot-rolled coil.

It can be seen from FIG. 3 that niobium carbide (NbC) started to precipitate at a temperature of 1060 to 1080° C. and that the edge serration defect index was rapidly reduced when the temperature of the thin slab being discharged from the heat-treatment furnace was equal to or higher than the precipitation temperature of niobium carbide (NbC).

Further, when the temperature of the thin slab discharged from the heat-treatment furnace was lower than 1060° C., niobium carbide (NbC) actively precipitated, and thus the edge serration defect index of the edge of hot-rolled coil increased.

It can be seen from FIG. 4 that, although the microstructure of the thin slab changes depending on the temperature of the thin slab discharged from the heat-treatment furnace, the amount of precipitates in the thin slab fabricated at a temperature of 1100° C. (refer to FIG. 4(b)) is smaller than the amount of precipitates in the thin slab fabricated at a temperature of 900° C. (refer to FIG. 4(a)). The reason is that niobium carbide (NbC) did not precipitate.

Although the embodiments of the present invention have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims.

Claims

1. A method of producing a steel sheet, the method comprising:

providing a steel slab of an alloy composition comprising iron (Fe), carbon (C), niobium (Nb) and aluminum (Al);

rolling the steel slab into a steel sheet; and

controlling temperature of the steel slab for rolling such that niobium carbide (NbC) precipitates during rolling while inhibiting precipitation of niobium carbide.

2. The method of claim 1, wherein controlling comprises:

heating the steel slab, before rolling, to a temperature higher than NbC precipitation temperature of the alloy composition; and

letting the steel slab cooled to a temperature lower than the NbC precipitation temperature as the steel slab enters and is rolled.

3. The method of claim 1, wherein providing the steel slab comprises continuously casting the alloy into the steel slab.

4. The method of claim 3, further comprising: continuously supplying the steel slab to a roller for rolling.

5. The method of claim 4, wherein controlling comprises heating the steel slab while being supplied for rolling to a temperature higher than NbC precipitation temperature of the alloy composition such that when the steel slab is rolled, the steel slab is cooled to a temperature lower than the NbC precipitation temperature.

6. The method of claim 1, wherein the thin slab is reheated to be in an austenite state.

7. The method of claim 1, wherein the carbon (C) is present in an amount of 0.20 to 0.28 wt %.

8. The method of claim 1, wherein the thin slab is reheated in a furnace and discharged from the furnace at a temperature of 1060 to 1100° C.

9. The method of claim 1, wherein the thin slab is reheated in a furnace and discharged from the furnace at a temperature of 1080 to 1100° C.