JPH08243621A - Manufacture of hot rolled steel sheet minimized in seam flaw - Google Patents

Abstract

PURPOSE: To provide a rolling method that is free from seam flaw, to decrease a rate of incidence of repair and to improve yield. CONSTITUTION: In this rolling method, at the time of manufacturing a steel sheet by hot rolling, in rolling pass whose rolled shape ratio α is 0.6-0.8, the seam flaw is not generated by heating the ridge line parts of a base stock to be rolled within the temp. range of -20 deg.C+50 deg.C of the surface temp. in the middle part in the width direction. The rolled shape ratio a is expressed by the equation I. α=2R.(H1 -H2 )/(H1 -2 ) ...(I) Where, R: the radius of rolling roll, H1 : the thickness on the inlet side of rolling, H2 : the thickness on the outlet side of rolling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a cut plate or a steel plate with ears, and more particularly to a method for manufacturing a steel plate while preventing seam flaws from being generated at the corners of the four cross sections.

[0002]

2. Description of the Related Art When manufacturing thick plates, vertical cracks called seam flaws or side cracks are likely to occur on the front and back sides near the ridgeline along the longitudinal direction of the steel sheet. In order to prevent the occurrence of this vertical crack, various methods have been proposed so far. For example, in Japanese Examined Patent Publication No. 16-16928, in order to ensure the prevention of defects during the production of high-grade steel, the longitudinal direction of the ingot,
Disclosed is a method of performing corner cutting on the end in the end direction. However, this method was inferior in workability and yield, and it was difficult to apply it to mass-produced steel.

Japanese Patent Laid-Open No. 56-19908 discloses a method of performing shape reduction of a slab or light reduction rolling in the initial stage of tenter rolling in order to prevent the occurrence of flaws in the production of mass-produced steel. , Causing a drop in productivity. In the method disclosed in Japanese Examined Patent Publication No. 50-14632, the center of both end surfaces of the slab is dented by convex caliber rolls so that the flaw generation position is near the ridgeline of the steel plate so that the cutting range is at the end cutting. It is the one. However, it did not prevent the occurrence of seam flaws, but required cutting off the edges.

[0004] In Japanese Patent Laid-Open No. 1-232109, the widthwise end and the longitudinal end of the rough sheet bar are heated by using induction heating to prevent material defects at the side end and the front and rear ends. However, this is to compensate for the temperature drop at the side end and the front and rear ends, and is not intended to reduce seam flaws.

Further, Japanese Patent Application Laid-Open No. 4-123802 discloses a method of erasing seam flaws by heating and rolling the seam flaws with an end heating device before finish rolling. It is premised on erasing seam flaws and does not prevent the generation of seam flaws themselves.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a method for producing a hot-rolled steel sheet having few seam defects regardless of slab shaping during casting and slab shaping during rolling.

[0007]

Means for Solving the Problems The gist of the present invention is to manufacture a steel sheet by hot rolling in which the rolling shape ratio α in the next pass is set.
Is between 0.6 and 0.8 inclusive, the side edge ridge of the material to be rolled is placed in the widthwise central portion of the surface temperature −20 ° C. to +5.
It is a method for producing a hot-rolled steel sheet with few seam flaws, which is characterized by heating to a temperature range of 0 ° C. The rolled shape ratio α is expressed by the equation (1). α = 2R · (H ₁ −H ₂ ) / (H ₁ + H ₂ ) ... (1) where R: rolling roll radius, H ₁ : rolling entry side plate thickness, H
₂ : Rolling exit side plate thickness.

[0008]

[Function] The occurrence of seam flaws is closely related to the shape of the end face during rolling. In the conventional rolling method, as shown in FIGS. 1A, 1B and 1C, the upper and lower parts of the material cross section are first overhanged by rolling during thick plate rolling (A: double bulge), and the central part of the material cross section progresses as the rolling pass progresses. (B: transition from double bulge to single bulge → C: single bulge). The relationship between the rolling shape ratio α and the end surface shape at this time is shown in FIG. FIG. 2 shows that the end shape changes from the double bulge to the single bulge when the rolled shape ratio α is in the range of 0.8 <α ≦ 1.2, and a step is generated at the steel plate side end.

The occurrence of seam flaws is caused by the fact that the side surface of the steel sheet wraps around the upper and lower surfaces during rolling, and as shown in FIG. 3, a linear wraparound occurs at the boundary between the wrapping around the upper and lower surfaces and the old upper surface. Defects (called seam defects or four-round defects) occur. This entrainment occurs due to the step (deflection point) at the steel plate side end portion shown in FIG. 3A, and if this step is eliminated, seam flaws do not occur. The w in the figure shows the step near the ridge appearing in the transition process from the double bulge to the single bulge.

As a method of eliminating this step, heretofore, there has been a method of changing the rolling pass schedule so as to prevent the step from occurring (Japanese Patent Laid-Open No. 56-19908, etc.), or shaping into a shape that does not cause the step. How to put it
16928 and Japanese Patent Publication No. 50-14632) have been considered. However, these did not fundamentally eliminate the cause of the step. The present invention
The generation of this step is fundamentally prevented.

The main cause of the step difference is a decrease in the temperature of the ridge. The surface area per unit volume is large in the corners and near the ridges, and the temperature drop is larger than that in other areas due to radiation. For this reason, the deformation resistance is larger than that of the other portions, and the metal flow is less likely to occur, and as a result, a step is formed. That is, it was found that if the temperature decrease at the ridge portion is prevented, no step is generated and, as a result, a seam flaw is not generated.

The reasons for limitation will be described below. It is necessary to heat the ridgeline before the step is formed. Further, since a step occurs when the rolling shape ratio α is 0.8 <α ≦ 1.2,
It is necessary to perform heating before a step occurs, that is, during a rolling pass in which the rolling shape ratio α of the next pass is 0.8 or less. Further, when heating is performed at a rolling shape ratio of less than 0.6, the temperature difference becomes large again in the rolling pass where the step is formed, and the heating effect is lost. For the above reason, the rolling shape ratio α of the next pass when heating the ridge portion is limited to 0.6 or more and 0.8 or less.

The heating temperature of the ridge portion needs to be set so that the deformation resistance of the ridge portion does not become extremely higher than the deformation resistance of the center portion in the width direction. For this reason, the minimum value of the heating temperature is set to −20 ° C. of the surface temperature in the widthwise central portion. If the deformation resistance of the ridge portion is smaller than the deformation resistance of the center portion in the width direction, no step can be formed and seam flaws do not occur, but excessive heating causes an increase in energy cost. The surface temperature is + 50 ° C. For the above reasons, the heating temperature of the ridgeline portion of the rolled material is set to the surface temperature in the widthwise central portion of −20 ° C.
Limited to + 50 ° C.

[0014]

EXAMPLE A rolling pass schedule (a portion of 140 mm or less) when a slab of 240 mm × 1800 mm × 5000 mm was used is shown in Tables 1- (1) and (2). Table 1-
In (1), tenter rolling with a tenter ratio of 2 is performed when the thickness difference of the material to be rolled is 200 mm. In Table 1- (2), tenter rolling is not performed. Heating of the side edge ridge is A, B, C, D,
It was performed at the timings of E, F, and G.

The heating timing A is the rolling shape ratio α in the next pass.
Is less than 0.6, which is different from the condition of the present invention. Further, the heating timings D, E, G have rolling shape ratios α of more than 0.8, which are different from the conditions of the present invention. B meets the conditions of the present invention,
It is the heating timing of C and F. The heating was performed using high frequency induction heating.

Table 2 shows heating conditions and rolling results. In Table 2, the heating timing of the side edge ridge is different from that of the present invention.
The occurrence rate of seam flaws was high at 1, 6, 7, 8, 11, and 12. Moreover, the heating temperature is different from the condition of the present invention.
o.2 also has a high incidence of seam flaws. As is clear from Table 2, the present invention prevents the occurrence of seam flaws. As a result, the maintenance and repair rate and the rejection rate were significantly reduced, and as a result, the yield was significantly improved.

[0017]

[Table 1]

[0018]

[Table 2]

[0019]

According to the present invention, the seam is formed by adjusting the deformation resistance by heating the ridge to eliminate the step of the ridge that occurs in the transition process in which the end surface shape of the material to be rolled changes from the double bulge to the single bulge. It virtually eliminates the occurrence of defects. Therefore, the yield in thick plate manufacturing can be improved and the maintenance rate can be reduced. Further, when manufacturing a steel plate such as a grooved steel plate or a less-trimmed steel plate in which the shape of the end surface is a problem, the quality of the end surface of the steel sheet can be guaranteed. Improved quality is obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a plate thickness during rolling and a rolling shape ratio α and a change in end face shape during a rolling process.

FIG. 2 is a schematic diagram of a relationship between a rolling shape ratio α and an end surface shape.

3 (a) to 3 (d) are schematic views of a seam flaw formation process accompanying rolling progress.

Claims (1)

[Claims] 1. When manufacturing a steel sheet by hot rolling,
Between the rolling passes where the rolling shape ratio α (shown by the equation (1)) of the next pass is 0.6 or more and 0.8 or less, the side edge ridge portion of the material to be rolled is placed in the widthwise central surface temperature −20 ° C A method for producing a hot-rolled steel sheet with less seam flaws, which comprises heating to a temperature range of + 50 ° C. α = 2R · (H ₁ −H ₂ ) / (H ₁ + H ₂ ) ... (1) where R: rolling roll radius, H ₁ : rolling entry side plate thickness, H
₂ : Rolling exit side plate thickness.