WO2010110529A1

WO2010110529A1 - Method for reducing edge serration defects in thin slabs

Info

Publication number: WO2010110529A1
Application number: PCT/KR2009/007993
Authority: WO
Inventors: 유석현; 문홍길; 장진수
Original assignee: 현대제철 주식회사
Priority date: 2009-03-26
Filing date: 2009-12-30
Publication date: 2010-09-30
Also published as: JP2012500895A; US20110132503A1; JP5392634B2; KR101129757B1; BRPI0917243A2; KR20100107770A; CN102137942A

Abstract

In the present invention, a thin slab having an alloy composition comprising carbon (C), niobium (Nb) and aluminium (Al) and a remainder of iron (Fe) and unavoidable impurities is reheated in a heat-treatment furnace where it is homogenised to an austenitic structure, and the production of precipitates is controlled by maintaining the heat-treatment-furnace extraction temperature of the thin slab at no lower than the NbC precipitation temperature and no higher than the AlN precipitation temperature, and then hot rolling is carried out. The present invention is advantageous in that it improves productivity since edge serration defects in thin slabs are reduced by controlling the heat-treatment-furnace extraction temperature of thin slabs.

Description

Slab edge saw blade defect reduction method

The present invention relates to a thin slab edge saw blade type defect reduction method, and more particularly to a thin slab edge saw blade type defect reduction method for reducing the edge saw blade type defect, which is one of the typical defects of the thin slab.

In the slab casting process, the thin slab is cast to a shape closer to the final product by making the thickness of the slab thinner, and the rough rolling process can be omitted in the hot rolling mill, and thus it is mainly applied to the process omission and simplification.

Unlike the general continuous casting process, the thin slab continuous casting process produces a thin slab at a high speed, and the solid molten steel is solidified into the thin slab in the mold and the stand part, so that the fine grains are finer than the general slab. Have

However, because the thin slab is fast cooled due to its thin thickness, the edge of the thin slab is quenched as compared with the middle portion, thereby generating residual stress.

Therefore, as shown in FIG. 1, there is a problem that a saw blade type defect occurs in a portion corresponding to an edge portion of the hot rolled coil in which the thin slab is wound, that is, a portion in which residual stress due to uneven cooling is formed.

This is because the degree of supercooling is large at the edge of the thin slab, and the degree of extension in the width direction from the center of the short side portion of the thin slab is much greater than that of the edge portion in the width direction during rolling. A saw blade defect occurs.

In particular, in the case of steel pipe steel with niobium added and having a carbon concentration of 0.20 to 0.28 wt% among the products produced through the thin slab continuous casting process, saw blade-type defects occur frequently at a lower edge than the strength of the raw material. There is a problem that breakage is likely to occur even under stress.

Therefore, the steelmaker manufacturing the thin slab hot rolled coil has a problem in that the productivity is reduced because the loss is processed by reprocessing and out of order grades.

The present invention is to solve the conventional problems as described above, an object of the present invention is to control the extraction temperature of the thin slab edge blade saw blade type defect reduction method for reducing the thin slab edge portion saw blade defects. To provide.

According to a feature of the present invention for achieving the above object, the present invention comprises carbon (C) and niobium (Nb) and aluminum (Al), the balance of the alloy composition of iron (Fe) and other unavoidable impurities The thin slabs having the thin slab are reheated in a heat treatment furnace to homogenize the austenite structure, while the extraction temperature of the thin slabs is maintained at an NbC precipitation temperature or more than an AlN precipitation temperature to control the formation of precipitates, followed by hot rolling.

The carbon is 0.20 ~ 0.28wt%, niobium is 0.005 ~ 0.020wt%, aluminum is 0.01 ~ 0.05wt% content range.

If the content of the carbon is 0.20wt% or more and less than 0.25wt%, the extraction temperature is maintained at 1060 ~ 1100 ℃ by heat treatment of the thin slab.

If the carbon content is 0.25wt% or more and 0.28wt% or less, the extraction temperature of the thin slab is maintained at 1080 to 1100 ° C.

The present invention controls the extraction temperature by the heat treatment of the thin slab to reduce the saw blade-type defects of the thin slab edge portion. Therefore, product reliability and productivity are improved by improving the quality of the thin slab and increasing the error rate of the product.

1 is a photograph showing the saw blade type defect of the hot rolled coil edge portion.

Figure 2 is a state diagram of the steel for Nb addition steel pipe having a carbon concentration of 0.20 ~ 0.28wt%.

3 is a graph showing the saw blade-type defect reduction of the coil edge according to the extraction temperature of the heat treatment furnace.

Figure 3 is an electron micrograph showing the microstructure of the thin slab prepared by the present invention in comparison with the prior art.

Hereinafter, a preferred embodiment of the thin slab edge saw blade type defect reduction method according to the present invention will be described in detail.

FIG. 2 is a state diagram of Nb-added steel pipe steel having a carbon concentration of 0.20 to 0.28 wt%, and FIG. 3 is a graph showing reduction of saw blade type defects in coil edges according to extraction temperature of a heat treatment furnace, and FIG. 3 is manufactured by the present invention. Electron micrograph showing the microstructure of the thin slab in comparison with the conventional.

The present invention is 0.20 to 0.28wt% of carbon (C), niobium (Nb), aluminum (Al), manganese (Mn), sulfur (S), nitrogen (N), the balance is iron (Fe) and other It has an alloy composition composed of unavoidable impurities.

The thin slab manufactured on the basis of such alloy composition is reheated in a heat treatment furnace to homogenize into austenite structure, while the extraction temperature is maintained at 1060 to 1100 ℃ by heat treatment of the thin slab to control the formation of precipitates, followed by hot rolling. do.

In more detail, by controlling the extraction temperature by the heat treatment of the thin slab above the NbC precipitation start temperature, the NbC precipitation occurs in the hot rolling process rather than the reheating process, thereby reducing edge saw blade defects.

Edge saw blade defects formed in thin slabs are mainly related to NbC precipitates. NbC precipitates prevent grain boundary growth during hot rolling, thereby miniaturizing grain size.

However, the precipitate precipitated at the grain boundary acts as a stress concentration source during hot rolling, and thus becomes a place of voids, and these voids eventually grow into cracks and cause grain boundary destruction. Particularly, thin slabs having rapid cooling at the edges due to the thin thickness have more severe grain boundary embrittlement due to NbC precipitation.

Therefore, NbC precipitation is intentionally avoided during reheating to prevent saw blade defects in the hot rolled coil edge portion. This is because even if the tension is applied in the thickness direction of the thin slab during hot rolling, the dislocation movement is free and stress concentration does not occur.

Preferably, the extraction temperature of the thin slab is controlled to be equal to or greater than the NbC precipitation temperature and less than or equal to the AlN precipitation temperature.

AlN is a precipitate that precipitates in the range of 1100 ~ 1120 ° C and causes grain boundary embrittlement, similar to NbC. Therefore, the extraction temperature is set in the heat treatment furnace to prevent AlN precipitation during reheating in order to prevent saw blade type defects in the hot rolled coil edge.

The starting temperature of NbC deposition of steel (C) 0.20 to 0.28 wt% and niobium (Nb) was confirmed in the state diagram of FIG. 2.

As shown in FIG. 2, the carbon (C) is 0.20 to 0.28 wt%, and niobium (Nb) is added to the austenitic grain boundary at an extraction temperature of 1060 to 1080 ° C. (A). A large amount of NbC precipitates are precipitated.

Therefore, the extraction temperature is maintained at 1060 ~ 1100 ℃ by heat treatment of the thin slab intentionally controlled to form the precipitate, and then hot rolling.

If the extraction temperature is lower than 1060 ℃ by heat treatment of the thin slab, the formation of a large amount of NbC precipitates do not have the effect of preventing the saw blade-type defects at the edge portion, and if it is higher than 1100 ℃, the austenite grains are coarsened, making it difficult to secure the strength. In addition, when the temperature is higher than 1100 ° C., defects may occur due to the formation of AlN precipitates.

The heat treatment of the thin slab may intentionally control the formation of the precipitate by maintaining the extraction temperature at 1080 ~ 1100 ℃, and then hot rolling. This is because the lower limit of the extraction temperature can vary in the range of 1060 ~ 1080 ℃ according to the carbon content by heat treatment of the thin slab.

More specifically, if the content of carbon is 0.20wt% or more and less than 0.25wt%, the extraction temperature is maintained at 1060 ~ 1100 ℃ by heat treatment of thin slabs. And, if the content of carbon is 0.25wt% or more and 0.28wt% or less, the extraction temperature is maintained at 1080 ~ 1100 ℃ by heat treatment of the thin slab.

As can be seen in the enlarged view of Figure 2, the higher the carbon content, the higher the precipitation temperature of NbC. The precipitation temperature of NbC is about 1080 ° C. in the range of 0.25 wt% or more and 0.28 wt% or less. Therefore, the extraction temperature is controlled by heat treatment of the thin slab 1060 ~ 1100 ℃ or 1080 ~ 1100 ℃.

On the other hand, the edge saw blade defects formed in the thin slab is related to AlN and MnS precipitates in addition to NbC precipitates.

However, NbC precipitates are the ones that have the greatest influence on edge saw blade defects, and excessively increasing the reheating temperature to control AlN and MnS precipitates can lead to a decrease in strength of steel and an increase in production cost. Basically, the precipitation of NbC is controlled. Of course, the precipitation of AlN in this process can be controlled by the heat treatment extraction temperature.

The present invention is more effective in the case of steel pipe steel having niobium added with a carbon concentration of 0.20 ~ 0.28wt% (heavy carbon steel) among the products produced through the thin slab continuous casting process.

The alloying element which becomes a basic component of this invention is demonstrated briefly.

Carbon (C) is an indispensable element for imparting high strength, and in the case of steel pipe, it is based on the addition of 0.20 to 0.28 wt% to secure the strength.

The addition of a small amount of carbon not only lowers the strength, but also reduces the amount of NbC precipitates, which increases the cost of manufacturing because it requires the addition of solid solution strengthening elements. In addition, when excessively added, the content of niobium for solid solution of carbon must be increased, which leads to an increase in manufacturing cost, thereby increasing the precipitation of fine NbC and inhibiting grain growth, thereby decreasing workability.

Niobium (Nb) is added to precipitate carbon and nitrogen present as solid solutions in steel in the form of precipitates of NbC and NbN. These precipitates prevent grain boundary growth during hot rolling and thus have a small grain size, thereby contributing to the improvement of strength.

Niobium is added in the range of 0.005-0.020 wt%. When niobium is added in an amount less than 0.005wt%, the precipitation amount is too small to expect the effect of improving the strength due to precipitation hardening, and when the addition amount is more than 0.020wt%, there is a problem that the ductility is reduced due to excessive strength and the tube defect is increased.

Aluminum (Al) is a component that functions as a deoxidizer and keeps the amount of dissolved oxygen in the steel sufficiently low. In addition, aluminum reacts with nitrogen, a solid solution element, as a carbide-forming element to form precipitates of AlN to remove solid solution elements.

Aluminum is added in the range of 0.01 to 0.05 wt%. If the amount is less than 0.01wt%, the effect is insignificant, and if it exceeds 0.05wt%, workability is inhibited.

Here, niobium and aluminum are the ranges normally contained in the steel pipe whose carbon is 0.20 to 0.28 wt%.

In addition, the present invention determines the extraction temperature by heat treatment of the thin slab on the basis of the state diagram of steel in which carbon is in the range of 0.20 to 0.28 wt% and Nb is added.

Hereinafter, the above-described thin slab edge saw blade type defect reduction method will be described in detail by way of examples.

EXAMPLE

The thin slab containing 0.20 to 0.28 wt% of carbon (C) and niobium (Nb) is reheated in a heat treatment furnace after casting, and extracted while changing the extraction temperature of the heat treatment furnace to 1040 ~ 1090 ℃, followed by hot rolling. It was prepared with a hot rolled coil.

According to Figure 3, NbC precipitation started in the range of 1060 ~ 1080 ℃, when the extraction temperature is more than the NbC precipitation temperature by the heat treatment of the thin slab edge index of the hot rolled coil drops sharply.

In addition, NbC precipitation was active when the extraction temperature was lower than 1060 ° C. by heat treatment of the thin slab, and the saw blade type defect index of the hot rolled coil edge was also increased.

According to Figure 4, there is a difference in the microstructure according to the extraction temperature of the heat treatment furnace, (b) in the case of the heat treatment furnace extraction temperature (1) tissue photographs, the amount of precipitate compared to the (a) texture photo of the heat treatment furnace extraction temperature 900 ℃ We can confirm that there is little. This is because NbC has not been precipitated.

Within the scope of the basic technical idea of the present invention, many other modifications are possible to those skilled in the art, and the scope of the present invention should be interpreted based on the appended claims. will be.

Claims

Thin slab containing carbon (C) and niobium (Nb) and aluminum (Al), the balance being alloy composition of iron (Fe) and other unavoidable impurities

Reheating in a heat treatment furnace to homogenize the austenite structure, while maintaining the extraction temperature of the thin slab at a temperature above NbC precipitation temperature or below AlN precipitation temperature to control the formation of precipitates, followed by hot rolling. Edge saw blade defect reduction method.
The method according to claim 1,

The carbon is 0.20 ~ 0.28wt%, niobium is 0.005 ~ 0.020wt%, aluminum is 0.01 ~ 0.05wt% content range of the thin slab edge saw blade type defect reduction method.
The method according to claim 1 or 2,

When the content of carbon is 0.20 wt% or more and less than 0.25 wt%,

Thin slab edge saw blade type defect reduction method characterized in that the extraction temperature of the thin slab is maintained at 1060 ~ 1100 ℃.
The method according to claim 1 or 2,

If the content of the carbon is 0.25wt% or more and 0.28wt% or less,

Thin slab edge saw blade type defect reduction method characterized in that the extraction temperature is maintained at 1080 ~ 1100 ℃ the heat treatment of the thin slab.