JP2005279735A - Method for reducing edge seam of ultra low carbon steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for reducing an edge seam of ultralow carbon steel by which the edge seam can be reduced even when the temperature is dropped below the Ar<SB>3</SB>transformation point when a slab corner part is at a first stage of the width press to the rough rolling, and a continuous casting step is not affected. <P>SOLUTION: In the method for reducing the edge seam of ultra low carbon steel, transformation of a slab corner part is started after starting the second stage of the rough rolling by increasing the Mn content of steel when performing the hot rolling of ultralow carbon steel. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a method for reducing edge seams of ultra-low carbon steel. Extremely low carbon steel refers to steel having a C content of 0.005% or less. In addition, the unit of component content is mass% and is abbreviated as%.

  In the edge seam, in the process from the slab width press in the hot rolling process to the pre-rough rolling stage, the temperature of the slab corner part is lower than that of the other part, and this part is transformed from γ to α, and the deformation resistance decreases rapidly. Stretched larger than the other parts by width reduction and horizontal rolling to form a large step on the surface or end face of the width end of the material to be rolled, and the stripes that fell into the surface in the subsequent horizontal rolling and extended in the rolling direction. It became a trap of the shape. This edge seam is particularly likely to occur in very low carbon steel.

In principle, the edge seam can be prevented if the slab corner is not transformed in the process from the width press to the pre-rough rolling stage. As a method for that purpose, 1) a place to which the descaling water of the rough rolling is applied. (Patent Document 1) Method of suppressing temperature drop of slab corner part in width press to rough rolling process by increasing transport pitch, increasing heating temperature, etc. 2) Ar ₃ transformation point of slab by changing component There is known a method of shifting the temperature to a lower temperature side than the operating temperature range of the width press to rough rolling process. Also, as a measure different from preventing transformation, in order to reduce the step at the boundary between the transformation α part and the untransformed γ part, 3) the width killing amount (width reduction amount) in the width press is reduced. Methods to regulate are known.
JP 2002-248516 A

However, in the method 1), when the Ar ₃ transformation point temperature of the steel is high, it is very difficult to suppress the temperature drop of the portion so that the slab corner portion does not fall below the Ar ₃ transformation point temperature. In addition, in the method 2), for example, when Ti, Nb, or B is added, the Ar ₃ transformation point temperature decreases. However, since these elements greatly increase the strength when added in a small amount, When Ti, Nb, and B are added as edge seam reduction means, there is a problem that the strength becomes too high. In the method 3), it may be necessary to separately prepare a slab having a width that does not match any of the predetermined slab widths integrated into a small number, and the load of the continuous casting process increases.

Therefore, the present invention provides an edge seam reduction method for ultra-low carbon steel that can reduce edge seam even if the slab corner portion falls below the Ar ₃ transformation point temperature in the width press to rough rolling pre-stage, and does not affect the continuous casting process. Objective.

  The inventor has intensively studied to achieve the above object, and as a result, has obtained the following knowledge.

First, it has been found from the results investigation that the incidence of edge seams in ultra-low carbon steel decreases as the Mn content increases. Therefore, in order to investigate this cause, a TTT diagram was obtained by a constant temperature transformation experiment for a sample with a changed Mn content. As shown in FIG. 1, for example, when the Mn content is increased, the Ar ₃ transformation point is almost changed. However, it was found that the FS (ferrite transformation start) curve shifted to the long time side.

  That is, if the Mn content is high, the start of transformation of the slab corner portion is delayed to the latter stage side of the rough rolling, and even if the slab corner portion is transformed thereafter, only horizontal rolling is performed, so that the step generated at the γ / α interface It has been found that the position at which the plate falls to the surface is easily limited to the range where the edge of the plate width is normally cut off (the cutting allowance by trimming), and as a result, the occurrence rate of edge seams is reduced.

  The present invention has been made based on the above findings.

  That is, the present invention is an edge seam reduction method for ultra-low carbon steel, characterized in that the transformation of the slab corner portion is started after the start of rough post-rolling in hot rolling of ultra-low carbon steel. In the present invention, it is preferable to start the transformation of the slab corner portion after the start of rough post-rolling by increasing the Mn content of the steel.

According to the present invention, since the transformation of the slab corner portion is started after the start of rough post-rolling, even if the slab corner portion is lower than the Ar ₃ transformation point temperature in the width press to the pre-rough rolling stage, the transformation is performed up to the ingress side after the rough rolling. Even if it is delayed and remains untransformed, even if it is transformed after that, only horizontal rolling is performed, so the level of the step that occurs at the γ / α interface is small, and the position where it falls to the surface is usually at the end of the plate width As a result, the occurrence rate of edge seams is reduced.

  Further, the increase in Mn is preferable because the influence on the strength is relatively small, and even if the amount is increased to such an extent that the start of transformation is effectively delayed, the strength does not become too high. In addition to the increase in Mn, the transformation may be delayed by, for example, keeping or heating the slab corner portion to suppress the temperature drop.

  The slab corner part remains untransformed during processing from the width press to the rough rolling pre-stage, so there is no need to regulate the width killing amount in the width press, so a slab with a width other than the predetermined width integrated into a small number Since it is not necessary to prepare, the continuous casting process is not affected.

  The present invention is applied to a hot rolling process for ultra-low carbon steel. The effect is unclear even when applied to other than ultra-low carbon steel. The Mn content of steel (Mn for short) is preferably in the range of 0.05 to 0.15%, and within this range, for example, the ferrite transformation start at 900 ° C. per 0.01% increment of the Mn amount. Delay about 20s.

  The range of 1100-1230 degreeC is suitable for slab heating temperature. The range of the slab corner may be regarded as an internal region of a triangular prism having two side surfaces, a surface portion having a width of about 30 mm from the corner of the slab width end and a width end surface portion having a width of about 30 mm from the same angle. The term “rough post-stage rolling start” as used in the present invention refers to the start of rolling at a stand where the entry side thickness of the same material to be rolled is 80% or less of the initial slab thickness for the first time in rough rolling.

  An ultra-low carbon steel slab (0.002% C-0.01% Si-0.08% Mn) heated to 1120 ° C in a heating furnace is reduced in width by a width press, and then a rough rolling mill having a 5-stand tandem arrangement configuration The first to third stands are rolled to a thickness of 35% of the initial slab thickness (rough pre-stage rolling), and then rolled (rough post-stage rolling) on the fourth and fifth stands of the roughing mill, None, cold rolled steel sheet made of hot-rolled steel sheet manufactured in Step A (comparative example), rolled (finish rolled) with a finishing mill with a 7-stand tandem arrangement, and wound up The rate was about 0.02%. According to the structure investigation of the sample taken from the material to be rolled before the start of rough post-rolling, in process A, about 30 s had elapsed from the start of the transformation of the slab corner portion before the start of rough post-rolling.

  Therefore, in order to delay the start of transformation of the material to be rolled more than the estimated elapsed time from the transformation start in step A, the amount of Mn of the slab is increased to 0.14% in step A, and the other conditions are the same as in step A. Step B (Example) was adopted. In the cold-rolled steel sheet made of the hot-rolled steel sheet manufactured in the process B, the rate of degradation due to edge seams was reduced to about 0.01%. According to the structure inspection of the sample taken from the material to be rolled before the start of the rough post-rolling, the slab corner portion remained untransformed at the time before the start of the rough post-rolling in the process B.

  The present invention can be used for a hot rolling process of ultra-low carbon steel.

It is a TTT figure which shows an example of the influence of Mn which has on the isothermal transformation behavior of an ultra-low carbon steel.

Explanation of symbols

1 0.002% C-0.01% Si-0.08% Mn steel Ar ₃ transformation temperature 2 0.002% C-0.01% Si-0.08% Mn steel FS (ferrite transformation start ) Curve 3 Ar ₃ transformation temperature of 0.002% C-0.01% Si-0.14% Mn steel 4 002% C-0.01% Si-0.14% Mn steel FS (ferrite Transformation start) curve

Claims (2)

  An edge seam reduction method for ultra-low carbon steel, characterized by starting transformation of a slab corner portion after the start of rough post-rolling in hot rolling of ultra-low carbon steel.   2. The edge seam reduction method according to claim 1, wherein the transformation of the slab corner portion is started after the start of rough post-rolling by increasing the Mn content of the steel.

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