JPH06315702A - Manufacture of thin steel sheet excellent in surface property and manufacturing equipment line therefor - Google Patents

Abstract

PURPOSE:To remove a surface defect caused by fine defects by heating a slab than hot-scarfing it before hot rolling it, hot-rolling it within a specific time and setting each time between rolling passes within a specific time. CONSTITUTION:After the steel slab is heated by a heating furnace 1, the upper and lower surfaces of the steel slab and both longitudinal side surfaces are hot-scarfed, oxygen of high purity is jetted from a nozzle to oxidize, melt and cut the iron. In this way, fine flaws are removed. Rough rolling is performed by a rough rolling mill 5 within one minute after scarfing. When one minute passes after scarfing, the grain field may be oxidized, corroded again to generate fine cracks and to bring a surface defect in succeeding stages. Descaling devices 4 are provided after and before the roughing mill 5. The steel slab is rolled by a roughing mills 6, finishing mills 7 and coiled by a coiler 10. The times between passes of these rolling mills 5, 6, 7, too, are set within one minute.

Description

Detailed Description of the Invention

[0001]

FIELD OF THE INVENTION The present invention relates to a thin steel sheet having excellent surface properties, in which fine flaws or patterns and traces caused by hot rolling of hot rolled steel sheet, cold rolled steel sheet and surface treated steel sheet are radically eliminated. It is about manufacturing.

[0002]

2. Description of the Related Art In the hot rolling process of thin steel sheets, the scale layer produced in the heating process is removed by descaling with high pressure water before finish rolling.

However, particularly in recent years, the use of ultra-low carbon steel added with Ti or Nb or a combination thereof has been dramatically increased, and various surface-treated steel sheets as well as various hot-rolled and cold-rolled steel sheets have appeared. As a result, surface defects such as surface flaws and stains that cannot be eliminated by the measures taken so far have become frequent. Therefore, various measures such as descaling have been made. For example, a surface crack of a steel slab is prevented by specifying the steel composition and rolling conditions (see Japanese Patent Laid-Open No. 3-294001), a descaling method for improving the surface properties of Si-containing steel, and high pressure for descaling. A technique for increasing the water pressure according to the composition in the steel (see Japanese Patent Laid-Open No. 4-187317) or a technique for maintaining good surface properties by avoiding rolling in a specific temperature range (Japanese Patent Laid-Open No. 4-247829). See). Furthermore, a coating agent is applied to the surface of the steel material before heating, and descaling is performed after heating the steel material (JP-A-4-2593).
No. 90).

However, although these conventional techniques bring about a considerable improvement effect, they have not reached the fundamental solution of minute defects.

[0005]

The hot-rolled steel sheet has very small ridge-shaped flaws or mere patterns without flaws, and the cold-rolled steel sheet has a stretched shape. Those are the object of defects to be solved in the present invention. In most cases, hot-rolled steel sheets and cold-rolled steel sheets are overlooked by ordinary inspections or left as defects below the standard. When such galvanized steel sheets are made into such minute defects, whether they are hot-dip galvanized or electrogalvanized, they have a great influence on the growth of the plating phase and become serious defects that cannot be overlooked. .

Although not observed in the inspection of the original plate, when it is galvanized, a bamboo leaf-shaped or tree leaf-shaped pattern appears to appear. It is an object of the present invention to fundamentally eliminate these microscopic defects, which are difficult to recognize in the original plate, but are clearly visible when galvanized or the like.

[0007]

The inventors have found that these defects are caused by fine grain boundary oxidation occurring in the heating stage of hot rolling, and that this oxidation is an extremely fast reaction involving the liquid phase. , Once such an oxidation state occurs,
It has been found that it is difficult to completely remove this by conventional descaling with high-pressure water or descaling combined with processing. Based on this knowledge, the present invention has completed a drastic solution in which these defects do not appear at all.

The present invention is characterized by the following items. (1) When hot-rolling a thin steel sheet, hot scarves are applied to the upper and lower surfaces of the steel slab and both sides of the long sides after heating and before hot rolling is started, and hot rolling is started within 1 minute, and hot rolling is performed. A method for producing a thin steel sheet having an excellent surface property, characterized in that the time between each pass is set to within 1 minute. (2) A hot scarfer is arranged in front of the edger / rough rolling mill. 1) Sheet steel manufacturing equipment line for manufacturing the sheet steel described in 1)

The details of the present invention will be described below. First,
This will be described with reference to the equipment sequence shown in FIG. The most important point of the present invention is that the hot rolling of the billet leaving the heating furnace 1 is performed by the hot scarf 2 at the start of hot rolling, and then hot rolling is performed as quickly as possible.
The scarfer 2 is not particularly limited, and may be one that is usually installed after continuous casting. Using fuel such as coke oven gas, high-purity oxygen is ejected from the nozzle,
The iron is oxidized and melted. In this way, the minute flaws on the surface layer that cannot be avoided by heating are removed. These microscopic flaws are usually extremely small, within 2 crystal grains at most, and the depth of fusing is only 0.3 mm per surface.
1.0 mm is sufficient.

After the scarf, hot rolling is carried out immediately. Rolling is started within 1 minute on each surface. When the rough rolling machine 5 performs the first rough rolling after rolling by the edger 3, since the upper and lower surfaces must be rolled within this time, this rough rolling is the first rolling for the upper and lower surfaces. If it exceeds 1 minute, the grain boundaries are again oxidized and eroded to form microcracks, which appear as surface defects in a later step. It should preferably be within 30 seconds. A descaling device 4 is usually provided before and after the rough rolling.

For the same reason, the time between each pass of hot rolling is 1
It must be within minutes. The temperature decreases and the oxidation and erosion decrease in the latter pass, while the surface shear deformation due to hot rolling increases in the latter stage, and the risk of micro-defects is as great as in the former stage. Although this condition is naturally satisfied in finish rolling, it is an important condition in rough rolling. However, regarding the side surface, since the wraparound to the upper and lower surfaces due to the metal flow due to rolling becomes smaller in the latter stage, the time regulation between each pass of the hot rolling is applied to the upper and lower surfaces, that is, ordinary rolling by the upper and lower rolls.

The hot rolling is not particularly limited to the above. The roll material is not specified such as a normal cast iron roll or high speed steel roll. However, since high-speed rolls have high hardness and good roll surface, they are suitable for surface flaws of hot-rolled steel sheets, and in this sense, high-speed rolls are preferable.

There is no particular limitation on the rolling temperature or the like. The IF steel which is a particular problem in the present invention is also an extremely low carbon steel, and the exit temperature of the finish rolling mill 7 is usually 900
Finish at ~ 930 ° C. Then runout table 8
After cooling with, it is wound by the winder 10. In terms of material, the winding temperature is 680 to 680 in addition to the usual case of around 600 ° C.
The temperature may be as high as 770 ° C. In the case of high temperature winding, a scale that is difficult to remove is generated, which is not preferable from the viewpoint of surface defects, but the present invention is effective even under such severe winding temperature conditions.

The steel strip hot rolled and wound by the method of the present invention is
After that, it is pickled and shipped as hot-rolled steel sheet as it is, cold-rolled and annealed to form a cold-rolled steel sheet, or hot-dip galvanized by processing in a continuous hot-dip galvanizing line that combines annealing and hot-dip galvanizing. There are various types such as a steel plate, or a galvanized steel plate obtained by further electrogalvanizing the cold rolled and annealed plate. In either case, the method of the present invention makes the surface beautiful, but has a great effect on surface defects such as patterns or stains that occur particularly when galvanizing.

It is considered that such an effect is obtained for the following reason. In the case of zinc crystal growth or galvannealing, the behavior at the time of alloying may differ depending on a slight difference in the surface of the original plate, and the slight difference may cause a clear surface defect. Since the slight difference in surface texture is derived from minute surface defects in the hot rolling heating stage, the cause is eliminated by removing the minute surface defects in the present invention,
It is thought that this will radically solve the problem.

Of course, there are various kinds of surface flaws other than the flaws of the present invention, and it is necessary to take measures against them. In steelmaking, it is necessary to reduce inclusions and minimize the inclusion of powder etc. during continuous casting. In hot rolling, attention must be paid to roll flaws or scratches after winding. Even after pickling, mechanical flaws such as handling flaws must be prevented from entering.

[0017]

EXAMPLES The present invention will be further described below based on examples. 0.0023% C-0.15 which is a typical IF steel
% Mn-0.009% P-0.004% S-0.032
% Al-0.043% Ti steel was refined in a converter, RH vacuum degassed to give ultra-low carbon, and continuously cast into a slab. Slab is 300mm thick x 1520mm width x 12.0
The size was set to 00 mm.

This steel slab was hot rolled under the conditions shown in Table 1. In Table 1, the time between passes represents the time at the slab equivalent position, and is the average value of the entire slab length. Steels with steel symbols D to H were subjected to hot scarves after heating and before rolling. The amount of hot scarf was 0.5 mm on each side. Descaling refers to the presence or absence of after heating and before rolling, and the descaling before the rolling mill after coarse No. 2 was performed as usual.

[0019]

[Table 1]

After forming the hot-rolled steel strip in this manner, ordinary pickling was carried out and the surface of the coil was observed. The result is
It shows in Table 2. In the conventional method, surface defects were conspicuous in steel C that was not subjected to descaling.
The surface defects of steels B and B remained mild.

Subsequently, the coil was passed through the next step shown in Table 2 and the surface was observed. The results are also shown in Table 2.
Shown in.

[0022]

[Table 2]

The surface quality of the conventional method or the equipment row of the present invention having different conditions is extremely poor, whereas the surface quality of the coil of the method according to the present invention of the equipment row of the present invention is very good.

[0024]

EFFECTS OF THE INVENTION As a result of the development of various surface-treated steel sheets according to the present invention or an increase in the demand for surface quality, surface defects caused by micro defects in hot rolling, which are becoming a serious problem, are fundamental Therefore, it is not only a dramatic improvement in economic efficiency, but also has a great effect from the viewpoint of effective use of resources.

[Brief description of drawings]

FIG. 1 is an explanatory view showing an example of a thin steel sheet manufacturing equipment line of the present invention.

[Explanation of symbols]

 1 Heating Furnace 2 Hot Scarfer 3 Edger 4 Descaling Device 5,6 Rough Rolling Machine 7 Finishing Rolling Machine 8 Runout Table 9 Pinch Roll 10 Winding Machine

Claims (2)

[Claims] 1. Hot rolling of a thin steel sheet is performed by hot squeezing the upper and lower surfaces of the steel slab and both sides of the long side after hot rolling before hot rolling is started, and hot rolling is started within 1 minute, and A method for producing a thin steel sheet having extremely excellent surface properties, characterized in that the time between each pass of hot rolling is set to within 1 minute. 2. A thin steel plate manufacturing equipment line for manufacturing a thin steel plate according to claim 1, wherein a hot scarf is arranged in front of the edger / rough rolling mill.