JPH0745041B2 - Overlap flaw prevention method in hot rolling of metal slabs - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a hot width rolling method, and more specifically to a hot width rolling method for supplying a slab of ultra-low carbon steel from a continuous casting process to a steel plate rolling process. In a process, the present invention relates to a method for preventing overlap flaws which occur at a position corresponding to a skid of a heating furnace.

(Prior art) In order to fully utilize the low-cost mass production capacity, which is a feature of continuous casting machines, an energy-saving process has been established by integrating the slab size and directly connecting with the rolling process. Various methods have been proposed as a method of supplying the continuous cast slab to the steel plate rolling process.

Regarding slab size integration, there is a method of increasing the frequency of continuous casting mold replacement according to the type of width of rolled products, but this is not practical because it reduces the operating rate of continuous casting equipment. On the other hand, there is a method of rolling a metal slab having a width of 1000 to 2000 mm and a thickness of 200 to 340 mm in the width direction and changing the slab width to a width of 600 to 1800 mm.

This method is, for example, as disclosed in JP-A-55-117501, a metal slab is subjected to a single pass by a vertical roll having a hole shape, or after a plurality of passes or more in the width direction rolling, and then by the width rolling. Width rolling of a metal slab is carried out by rolling only the locally increased portion of the sheet thickness that is rolled down (hereinafter simply referred to as horizontal rolling down), making it approximately the same as the original slab thickness, and then performing width rolling again using a hole-type roll. Is the way.

Like this rolling method, the characteristic of the width reduction rolling method in the case of manufacturing a narrow slab from a wide slab is that the ratio of the roll contact arc length during rolling to the average plate width, that is, the roll gap, compared to conventional rolling. The shape ratio is as small as 0.1 to 0.2, and the repetitive vertical roll width reduction amount per one time is as large as 50 to 300 mm.

Further, the heating furnace in this width rolling process is disclosed in
As presented in JP-A-55-19465, based on the heat quantity possessed by the continuous casting slab, the temperature distribution of the entire slab is made uniform and, if necessary, a heat of 200 to 300 ° C. It is equipped with a furnace that has the function of raising the temperature in about 40 minutes.

(Problems to be solved by the invention) Recently, the demand for workability of thin plate products has dramatically increased, and in order to obtain excellent deep drawability, the mechanical properties of the steel sheet are the yield strength (YS) and high elongation. High ductility and high Rank Ford value are required.

For this reason, ultra-low carbon steel with a chemical composition of C 0.005% or less by weight is used. A continuous cast slab of 0.005% or less of ultra-low carbon steel (hereinafter simply referred to as a slab) is produced in the slab width direction portion supported by the skid S in the heating furnace during rolling after low temperature heating as shown in FIG. Low temperature part
When the surface temperature of C ₀ (hereinafter referred to as the skid portion equivalent position) during rolling approaches the transformation temperature, the deformation resistance value decreases by 5 to 10 kg / mm ² compared to the other regardless of the content of other components. .

As is well known, the slab is presented in the prior art,
When performing width direction rolling with a width reduction rolling machine having a vertical roll with a caliber, since the width reduction force does not propagate to the width center part,
Although locally increased sheet thickness occurs at both ends of the sheet width, especially at the skid part, which is the low temperature part, at the transformation temperature, it rises abnormally compared to other locally increased sheet thickness parts. A locally thickened portion is generated.

When this abnormally thickened portion is horizontally rolled to the original thickness of the slab or to less than the original thickness, as will be described in detail later, when the abnormally raised locally thickened portion is rolled in the thickness direction, the width outside Since a plastic flow occurs on the side (that is, the unconstrained side) in a flawed manner and a local width expansion phenomenon occurs, and the local increased thickness portion also precedes the plastic flow on the inner side of the sheet width (that is, the restrained side). Fraying occurs and abnormally deep overlapping flaws occur. Therefore, width rolling in the energy saving process has been difficult.

The present invention provides an effective hot width rolling method for a slab that solves the above problems.

(Means for Solving the Problem) The present invention has been made in order to advantageously prevent the occurrence of the above-mentioned overlapping defects under an energy-saving process, and the characteristic feature thereof is that the C content is 0.005% or less continuously. Cast metal slabs are width-rolled by a width reduction mill having vertical rolls with caliber through a heating furnace, and then horizontally reduced to a thickness before rolling or a thickness less than the original thickness by a thickness reduction mill having horizontal rolls. Repeat the above to obtain the target metal slab thickness and width.In rolling, the average surface temperature of the low temperature part generated in the width direction part of the slab supported by the skid in the heating furnace was 900 ° C during rolling.
A method for preventing overlap flaws during hot rolling of a metal slab is characterized in that the above rolling is carried out at a temperature not lower than or above 870 ° C.

In the present invention, the target continuously cast slab is
The content of C is 0.005% in which the components are overlapped and markedly flawed.
Below, there is no limitation on the content of other components.

The original size of the flab is 200 to 400 mm thick and 1000 to 2000 mm wide, and the target slab after hot rolling obtained by width rolling of 50 to 300 mm per pass and thickness of the dock bone part generated by width rolling or more is The conditions generally known in this field may be 600 to 1800 mm width and 150 to 400 mm thickness.

Further, the hole shape of the vertical roll with caliber is described in JP-A-56-67.
It may be one known in the publication No. 06. An example of this is shown in FIG. In the figure, C: the axis of rotation of the vertical roll with gulliver,
θ: Inclination angle of upper and lower brim 14 °, R ₁ : Curvature 40-60mm, R ₂ :
The curvature is 20 to 30 mm, L: the protruding length of the brim is 180 mm, K: the length of the bottom of the hole die is 280 mm (same as the slab thickness of 280 mm).

(Operation) Hereinafter, the operation of the present invention will be described with reference to the drawings.

FIG. 2 (a) shows an example of a stand configuration of the width reduction rolling machines V ₁ and V ₂ , in which a horizontal roll 3 is arranged between the vertical rolls 2 and 4 with calibers. Slab 1 is roll 2, 3, 4
Reversibly rolled between.

2 (b), (c), and (d) show slab cross-sectional shapes during the rolling process.

That is, a continuously cast slab having a rectangular cross-sectional shape with an original width W ₀ and an original thickness h ₀ shown in FIG. 2 (b) is taken by vertical rolls 2 and 4 with calibers as shown in FIG. 2 (c). Further, when the reduction amount ΔW ₁ is rolled in the plate width direction, local plate thickness increasing portions 5, so-called dog bones, are generated at both widthwise end portions of the slab.

When the locally increased thickness portion 5 becomes large, surface defects are generated due to the biting of the vertical roll from the hole type, and the rolling power is increased due to the widthwise rolling.

Since these factors regulate the width rollable amount, as shown in FIG. 2 (d), horizontal rolling is performed by the horizontal rolling mill H to the original thickness of the slab or to a thickness less than the original thickness, and the plate thickness h is reduced. ₀ + h ₊ ,
The plate thickness is reduced to the same plate thickness h ₀ or less as the other plate thickness, and width rolling in the plate width direction is performed again to reduce only the plate width or the plate width and the plate thickness. The rolling process shown in FIG.

For example, when only the dog bone portion is horizontally rolled, as shown in FIG. 2 (d), the metal slab has a sectional shape in which the center portion of the plate width is h ^- thinner than the end portions of the plate width. In addition, after rolling until the width rolling target dimension W _n is obtained, it is rolled by the horizontal rolling mill to the target thickness dimension h _{n as} shown in FIG. 2 (e).

Figures 3 (a) and (b) show C0.005% in this rolling process.
It is the figure which showed the back surface of the material to be rolled of the following ultra-low carbon steels with a back surface as an upper surface.

FIG. 3 (a) shows that the rolling amount ΔW ₁ is rolled in the sheet width direction, and the locally increased sheet thickness portion 5 at the skid equivalent position W of the heating furnace is abnormally raised as shown in the figure.

FIG. 3 (b) shows a state in which the slab is rolled to the original thickness by a horizontal rolling mill, and FIG. 3 (c) shows a state in which the slab is rolled to a thickness less than the original thickness. In the locally thickened thickness portion 5 where the thigh is abnormally raised, the portion inside the width of the width is sagging, and at a position of 150 to 300 mm from the end of the width of the plate, a maximum of 5 to 10 mm to several 100 mm in length. Overlapping defects 8 with a depth of 6 mm occur.

Further, since the outside of the plate width is a non-restraining surface, the shape 9 has an abnormally wide width. Further, in the case of FIG. 3 (b), the central portion of the plate width has a recessed shape 6 as compared with other portions. The width reduction amount is 50 mm
In the following cases, the height h ₊ of the locally increased sheet thickness shown in FIG. 2 (c) is small, and the position corresponding to the skid portion has a slight width expansion even after horizontal rolling, but there is an overlapping shape on the inner side of the sheet width. No flaws occurred and there was no problem.

The present inventors conducted various investigations on the rolling behavior of the ultra low carbon steel described above. As a result, in the surface layer of the skid part and the non-skid part of the heating furnace, the ultra low carbon steel is 850-9
Transformation occurs at a relatively high temperature of 00 ° C and changes from the austenite phase to the ferrite phase.

In this case, the ferrite phase with a lower temperature has a deformation resistance of 5 to 10 kg / mm ² less during rolling, and the relationship between temperature and deformation resistance becomes discontinuous. We obtained the knowledge that it occurs.

Fig. 1 shows the target dimensions of 600-180 for the original dimensions of 1000-2000mm width and 200-400mm thickness for ultra-low carbon steel slabs with C0.005% or less.
It shows the temperature and deformation resistance during rolling when rolling into a 0 mm width, 150 to 400 mm thick slab, and the presence or absence of overlapping flaws in the locally increased sheet thickness.

As can be seen from the figure, the surface average temperature of at least the skid part of the heating furnace of the slab is 900 ° C or higher during rolling, or 8
If widthwise rolling and horizontal reduction rolling are carried out at 70 ° C or less, there is no significant change in deformation resistance and no abnormally raised locally thickened portion is generated compared to other portions.

For example, when the surface average temperature at the skid equivalent position is 870 ° C, even if the surface temperature of the part located between the skid equivalent position and the steady part exceeds 870 ° C and less than 900 ° C, The overlap flaw does not occur in the portion after thickness rolling.

As a result, the ultra low carbon steel can be rolled in the width direction without the occurrence of overlapping flaws or partial width expansion.

(Example) An ultra-low carbon steel having C 0.005% or less shown in Table 1 was manufactured by a continuous casting machine, the metal slab was charged into a heating furnace, heated, and then width-direction rolled.

The rolling conditions are as shown in Table 2, and the continuous casting slab width is 1800 mm with the 3-stand reverse rolling method of V ₁ -HV ₂ .
A target slab dimension width of 860 mm and a thickness of 250 mm were manufactured from the thickness of 280 mm based on the rolling schedule of Table 3.

Next, Examples are shown in Table 4 together with Comparative Examples.

Depending on the average cross-section charging temperature of the metal slab manufactured by the continuous casting machine and charged into the heating furnace, the heating zone of the heating furnace, the set temperature of the soaking zone and the level of the in-service time are set, and the skid part of the rolled material is set. The rolling temperature at the corresponding position was changed.

In Comparative Example, in order to maximize the furnace capacity of the slabs with an average charging temperature of 900 to 970 ° C, the heating zone and soaking temperature are set
The temperature was set to 1120 ℃ and 1200 ℃, and the in-furnace time was set to 25 to 35 minutes, which is the shortest time to match the performance of the front and rear processes.

As a result, the rolling temperature of the skid part was higher than 870 ℃ and lower than 900 ℃, and overlapping defects occurred.

On the other hand, in the example, the average charging temperature of the slab cross section is 900 ° C.
The heating furnace set temperature is set to
1050, 1100 ℃, the example is a slab cross-section average charging temperature is 980 ℃ or more, and the heating zone of the heating furnace, the soaking temperature is 1120 ℃, 1200 ℃, The shortest time is 25 to 35 minutes.

As a result, overlapping flaws did not occur at all. Further, in the example, the slabs having a cross-section average charging temperature of 900 ° C. or higher and lower than 970 ° C. are extended in the in-furnace time, and the rolling temperature at the skid portion equivalent position is set to 90 °
The temperature was 0 ° C or higher and lower than 930 ° C, and no overlapping flaws occurred.

That is, regardless of the charging temperature of the cross section of the slab, if the rolling temperature at the position corresponding to the skid portion is set to 870 ° C. or lower and 900 ° C. or higher, the overlap flaw does not occur.

(Effect of the invention) The conventional rolling method in the width direction rolling of the metal slab of the ultra-low carbon steel under the process of directly connecting the continuous casting process and the hot rolling process is compared with other parts at the position corresponding to the skid part of the heating furnace. An abnormally raised local plate thickness increase occurred, and there were many quality defects resulting from this, and the slab shape was also unstable.

On the other hand, according to the method of the present invention, abnormal deformation of the width and thickness of the heating furnace skid part during rolling is eliminated, so that the quality and shape are good, and the width can be integrated by rolling under the direct coupling process, which is very useful industrially. The effect is brought.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagram showing the relationship between temperature and deformation resistance during rolling, and FIG. 2 (a) is an explanatory view of an example of the structure of a stand of a width reduction rolling mill for carrying out the method of the present invention. , FIG. 2 (b), (c),
(D) and (e) are explanatory views showing changes in the cross section of the slab in the width rolling process, and FIGS. 3 (a), (b), and (c) show the position corresponding to the heating furnace skid part from the lower surface of the metal slab. FIG. 4 is an explanatory view of a slab shape in the rolling process, FIG. 4 is an explanatory view of a vertical roll hole type, and FIG. 5 is a perspective view showing a transportation state of a slab skid in a heating furnace. 1: Slab, 2, 4: Vertical roll 3: Horizontal roll, 8: Overlap flaw 9: Widened part

Claims (1)

[Claims] 1. A continuously cast metal slab having a C content of 0.005% or less is width-rolled by a width-reducing mill having vertical rolls with caliber through a heating furnace, and then by a thickness-reducing mill having horizontal rolls. In the rolling to obtain the target metal slab thickness and width by repeating horizontal reduction to the original thickness or thickness less than the original thickness, the low temperature generated in the width direction part of the slab supported by the skid in the heating furnace during rolling. Average surface temperature of 90 parts
A method for preventing overlap flaws in hot rolling of a metal slab, which comprises rolling at 0 ° C. or higher or 870 ° C. or lower.