JP5392634B2 - Method for reducing edge sawtooth defects in thin slabs - Google Patents

Description

  The present invention relates to a method for reducing edge serrated defects of a thin slab, and more particularly, an edge serrated edge of a thin slab that reduces edge serrated defects, one of the typical defects of thin slabs. The present invention relates to a defect reduction method.

  Thin slabs are cast into a shape close to the final product by further reducing the thickness of the slab in the slab casting process. Since the rough rolling process can be omitted in a hot rolling plant, the process can be omitted and simplified. Mainly applied to

  In such a thin slab continuous casting process, unlike a general continuous casting process, a thin slab is cast at a high speed, and solidification of liquid molten steel into a thin slab is completely performed in the mold and the stand part. Therefore, the thin slab has fine crystal grains compared to the general slab.

  However, since the thin slab is cooled quickly due to the thin thickness, the edge portion of the thin slab is rapidly cooled as compared with the intermediate portion, and residual stress is generated.

  Therefore, as shown in FIG. 1, a problem that sawtooth defects occur in a portion corresponding to an edge portion of a hot-rolled coil wound with a thin slab, that is, a portion where residual stress is formed due to non-uniform cooling. There is a point.

  This is because the degree of supercooling is large at the edge of the thin slab, and the extent of extending in the width direction from the center of the short side of the thin slab during rolling is greater than the extent of extending the edge in the width direction. The slab is subjected to tension in the thickness direction, which causes sawtooth defects.

  Especially, in the case of niobium-added steel pipe steel having a carbon concentration of 0.20 to 0.28 wt% among products produced by the continuous casting process of thin slab, sawtooth defects frequently occur at the edge part. In this case, there is a problem that damage is likely to occur even with a stress lower than the strength of the raw material.

  For this reason, in the steel industry that manufactures a thin slab hot-rolled coil, loss processing is performed by performing reprocessing and out-of-order determination, and there is a problem that productivity is reduced.

  The present invention is to solve the above-mentioned conventional problems, and its purpose is to control the extraction temperature of the heat treatment furnace of the thin slab to reduce the sawtooth defects at the edge of the thin slab. An object of the present invention is to provide a method for reducing the sawtooth defects at the edge portion.

  In order to achieve the above object, according to one aspect of the present invention, an alloy composition containing carbon (C), niobium (N) and aluminum (Al), the balance being iron (Fe) and other inevitable impurities The thin slab having slab is reheated in a heat treatment furnace to homogenize the austenite structure. After controlling the formation of precipitates by maintaining the extraction temperature of the thin slab heat treatment furnace above the NbC precipitation temperature, Provided is a method of reducing edge sawtooth defects of a thin slab that is rolled.

  The carbon ranges from 0.20 to 0.28 wt%.

  The extraction temperature of the heat treatment furnace for the thin slab is maintained at 1060 to 1100 ° C.

  The extraction temperature of the heat treatment furnace for the thin slab is maintained at 1080 to 1100 ° C.

  The present invention controls the extraction temperature of a thin slab heat treatment furnace to reduce the sawtooth defects at the edge of the thin slab. Therefore, there is an effect that the reliability and productivity of the product are improved by improving the quality of the thin slab and increasing the actual yield of the product.

It is a photograph which shows the serrated defect of a hot rolled coil edge part. It is a phase diagram of the steel for Nb addition steel pipes which has a carbon concentration of 0.20-0.28 wt%. It is a graph which shows reduction of the serrated defect of a coil edge part by the extraction temperature of a heat treatment furnace. It is an electron micrograph which shows the fine structure of the thin slab manufactured by this invention compared with the thing by a prior art.

  Hereinafter, a method for reducing edge sawtooth defects of a thin slab according to a preferred embodiment of the present invention will be described in detail.

  FIG. 2 is a phase diagram of Nb-added steel pipe steel having a carbon concentration of 0.20 to 0.28 wt%, FIG. 3 is a graph showing the reduction of sawtooth defects at the coil edge portion by the extraction temperature of the heat treatment furnace, and FIG. It is an electron micrograph which shows the fine structure of the thin slab manufactured by this invention compared with the thing of a prior art.

  The present invention contains 0.20 to 0.28 wt% of carbon (C), niobium (Nb), aluminum (Al), manganese (Mn), sulfur (S), and nitrogen (N), with the balance being iron ( Fe) and an alloy composition consisting of other inevitable impurities.

  The thin slab produced on the basis of such an alloy composition is reheated in a heat treatment furnace and homogenized to an austenite structure, while the extraction temperature of the thin slab heat treatment furnace is maintained at 1060 to 1100 ° C. After controlling the production, hot rolling is performed.

  More specifically, by controlling the extraction temperature of the heat treatment furnace for thin slabs above the NbC precipitation start temperature, NbC precipitation occurs not in the reheating process but in the hot rolling process, thereby reducing edge sawtooth defects. To do.

  Edge serrated defects formed in thin slabs are mainly associated with NbC precipitates. NbC precipitates hinder grain boundary growth during hot rolling to refine the crystal grain size.

  However, the precipitates precipitated at the grain boundaries act as stress concentration sources during hot rolling and become void generation sites. Such voids eventually grow into cracks and cause grain boundary fracture. In particular, in a thin slab in which the edge portion is rapidly cooled due to a thin thickness, the grain boundary embrittlement phenomenon due to NbC precipitation occurs more severely.

  Therefore, NbC precipitation is intentionally avoided during reheating to prevent sawtooth defects at the hot rolled coil edge. This is because the potential can move freely and no stress concentration phenomenon occurs even if it is pulled in the thickness direction of the thin slab during hot rolling.

  The NbC precipitation start temperature of the niobium (Nb) -added steel having a carbon (C) concentration of 0.20 to 0.28 wt% is confirmed from the phase diagram of FIG.

  According to FIG. 2, in the case of niobium (Nb) -added steel having a carbon (C) concentration of 0.20 to 0.28 wt%, the austenite phase (γ) at the extraction temperature of the heat treatment furnace of 1060 to 1080 ° C. (A) A large amount of fine NbC precipitates are precipitated at the grain boundaries.

  Therefore, hot rolling is performed after intentionally controlling the formation of precipitates while maintaining the extraction temperature of the heat treatment furnace for thin slabs at 1060 to 1100 ° C.

  If the extraction temperature of the thin slab heat treatment furnace is lower than 1060 ° C., there is no effect of preventing sawtooth defects at the edge due to the large amount of NbC precipitates. If the extraction temperature of the thin slab heat treatment furnace is higher than 1100 ° C. It is difficult to ensure strength due to coarsening of the grains.

  Hot rolling can also be performed after maintaining the extraction temperature of the thin slab heat treatment furnace at 1080 to 1100 ° C. to intentionally control the formation of precipitates. This is because the lower limit of the heat treatment furnace extraction temperature of the thin slab can vary in the range of 1060 to 1080 ° C. depending on the carbon content.

  On the other hand, the edge serrated defect formed in the thin slab is related to AlN and MnS precipitates in addition to the NbC precipitates. However, NbC precipitates are the precipitates that have the greatest influence on edge serrated defects, and excessively increasing the reheating temperature to control AlN, MnS precipitates reduces steel strength and production. Basically, the precipitation of NbC is controlled such that there is a risk of increasing the cost. Of course, the precipitation of AlN in this process can be controlled.

  Next, the alloy element which is a basic component of the present invention will be briefly described.

  Carbon (C) is an indispensable element for imparting high strength. In the case of a steel pipe, carbon (C) is basically added in an amount of 0.20 to 0.28 wt% to ensure the strength.

  When carbon is added in a small amount, the strength is low, and the amount of NbC precipitates is reduced, so that a solid solution strengthening element has to be added, resulting in an increase in manufacturing cost. And if carbon is added in an excessive amount, the content of niobium for dissolving carbon must be increased, resulting in an increase in manufacturing cost, thereby increasing the precipitation of fine NbC and hindering particle growth. Processability is reduced

  Niobium (Nb) is added to precipitate carbon and nitrogen present as solid solution elements in steel in the form of NbC and NbN precipitates. Such precipitates prevent grain boundary growth during the hot rolling and reduce the crystal grain size, thereby contributing to an improvement in strength.

  Niobium is added in the range of 0.005 to 0.020 wt%. When niobium is added at less than 0.005 wt%, the amount of precipitation is so small that the effect of improving strength by precipitation hardening cannot be expected, and when added at an amount exceeding 0.020 wt%, the strength is increased. There is a problem that softness is reduced due to an excessive amount and pipe-forming defects are increased.

  Aluminum (Al) is a component that plays a role as a deoxidizer, and maintains the amount of dissolved oxygen in the steel sufficiently low. Aluminum is a carbide-forming element and reacts with nitrogen as a solid solution element to generate a precipitate of AlN to remove the solid solution element.

  Aluminum is added in the range of 0.01 to 0.05 wt%. When aluminum is added in an amount of less than 0.01 wt%, the effect is insignificant, and when it is added in an amount exceeding 0.05 wt%, the workability is impaired.

  Here, niobium and aluminum are in a range usually contained in a steel pipe having a carbon concentration of 0.20 to 0.28 wt%.

  In the present invention, the extraction temperature of the thin slab heat treatment furnace is determined based on the phase diagram of the Nb-added steel having a carbon concentration of 0.20 to 0.28 wt%.

  Hereinafter, the method for reducing the above-mentioned sawtooth defect on the edge portion of the thin slab will be described in detail with reference to examples.

[Example]
After casting a niobium (Nb) -added thin slab having a carbon (C) concentration of 0.20 to 0.28 wt%, it is reheated in a heat treatment furnace, but the extraction temperature of the heat treatment furnace varies in the range of 1040 to 1090 ° C. After being extracted, hot rolling was performed to produce a single-phase hot rolled coil.

  According to FIG. 3, NbC precipitation started in the range of 1060 to 1080 ° C. If the extraction temperature of the thin slab heat treatment furnace was equal to or higher than the NbC deposition temperature, the index of the sawtooth defects at the edge of the hot rolled coil decreased rapidly.

  When the extraction temperature of the thin slab heat treatment furnace was less than 1060 ° C., NbC precipitation was active. As a result, the sawtooth defect index of the hot rolled coil edge also increased.

  According to FIG. 4, although there is a difference in microstructure depending on the extraction temperature of the heat treatment furnace, the (b) structure photograph at the heat treatment furnace extraction temperature of 1100 ° C. is compared with the (a) structure photograph at the heat treatment furnace extraction temperature of 900 ° C. It can be confirmed that the amount of the precipitate is small. This is because NbC is not precipitated.

  Within the scope of the basic technical idea of the present invention, those skilled in the art can make various modifications, and the scope of rights of the present invention is attached. It should be construed based on the claims.

Claims (2)

Alloy composition containing carbon (C) 0.20 to 0.28 wt% , niobium (N b ) 0.005 to 0.02 wt% and aluminum (Al), the balance being iron (Fe) and other inevitable impurities a thin slab having, although uniform austenite structure, and then re-heated in a heat treatment furnace, after the NbC precipitates by maintaining the extraction temperature of the heat treatment furnace of the thin slab to 1060-1100 ° C. was controlled so as not to be generated A method for reducing edge sawtooth defects in a thin slab, characterized by performing hot rolling. 2. The method of reducing edge sawtooth defects of a thin slab according to claim 1, wherein an extraction temperature of the heat treatment furnace of the thin slab is maintained at 1080 to 1100C.

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