JPS5842778B2 - Continuous casting method for slabs for cold-rolled steel sheets - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for obtaining an inexpensive composition that does not cause surface roughness when press-forming cold-rolled steel sheets, and for continuously casting this weakly deoxidized steel without forming pinholes. be.

Up until now, many methods have been proposed for casting weakly deoxidized steel equivalent to rimmed steel by continuous casting. In order to reduce the amount of free oxygen in molten steel to 70p by adding a deoxidizing agent or performing vacuum degassing treatment.
Must be kept below pm.

However, the former method of adding deoxidizing agents, especially Al and Si, causes the rimmed steel to lose its characteristic of good surface customization, and the latter method of vacuum degassing treatment or the combination of a small amount of deoxidizing agent is recommended. In this method, when box annealing is performed after cold rolling, the effect of suppressing crystal grain growth due to precipitates is lost, and the outer periphery of the coil is particularly hot, so abnormally coarse grains become elongated in this area, causing the pressing There was a drawback that rough skin occurred during molding.

The present invention provides a weakly deoxidized steel composition that eliminates the above-mentioned difficulties, and also enables continuous casting of this weakly deoxidized steel.

The weakly deoxidized steel composition in this invention that does not cause surface roughness during press forming of a cold rolled steel sheet contains 0.02 to 0.15% C, P
ζ0.002%, si<, o, o't% and klζ0.
0.05% (a small amount for deoxidation), the balance is Fe and unavoidable impurities.
．． 20%, with free oxygen adjusted to 70-250 ppm.

The cold-rolled steel sheet obtained from the molten steel whose composition has been adjusted in this way is
It has the characteristic that abnormal coarse grain growth is suppressed during recrystallization annealing, but when the free oxygen content is 70pp
m or more (up to 250 ppm), when continuous casting is performed, pinholes are generated just below the surface as described above, and the slab cannot be used for the next process as a sound slab.

Therefore, in this invention, as a result of intensive study on continuous casting without the formation of pinholes, we succeeded by applying electromagnetic flow to the solidification interface of molten steel being cast in a continuous casting machine.

The electromagnetic flow referred to in this invention is the result of investigation by the present inventors. 1) Nucleation of pinholes that later grow into pinholes is caused by:
2) Pinhole nuclei are generated from the solidification initiation point, that is, the solidification interface at the molten metal level in the casting. 3) The element concentration in molten steel is determined by the concentration of elements at the solidification interface. Focusing on the fact that the molten steel concentrates significantly at the surface of the molten steel in the mold, the element concentration at the solidification interface of the surface of the molten steel was set to be below the generation limit of pinhole nuclei, and at the same time, the flow rate was set at a level that did not disturb the powder on the surface of the molten steel. Point.

In other words, it is applied to the entire surroundings of the solidification interface at the surface of the mold.
It refers to a film-like flow of molten steel that is effective in suppressing the formation of pinhole nuclei, and as described later, it refers to electromagnetic flow installed in a mold or applied by a near motor, and is different from the so-called electromagnetic stirring that is conventionally performed. They are distinct.

In this way, by applying electromagnetic flow of molten steel to the entire circumference of the solidification interface at the surface of the molten metal during the casting process, the concentration of component elements at the solidification interface is suppressed, resulting in a slab with no pinholes around the entire surface layer. A healthy solidified layer is formed, and since this flow is gentle as described below and is applied to a portion close to the solidified wall, it does not disturb the surface of the hot water (powder).

Quantitatively, the flow velocity of the electromagnetic flow mentioned above is 0.1 to 1.0 m.
/see, which simultaneously satisfies both the suppression of pinhole nucleation and the prevention of disturbing the powder on the surface of the hot water.

If the area covered by this electromagnetic flow is too wide, it will be affected by the immersion nozzle for pouring molten steel and the surface powder will be disturbed, and normal powder casting will not be possible due to powder entrainment. It is preferable to generate it as close to the solidification wall as possible.

At this time, electromagnetic flow can be used to float and remove the bubbles generated by installing a device directly below the mold, but this inevitably creates stagnation of the molten steel flow at the mold and inner hot water surface, so it is better to use the electromagnetic flow inside the mold. Also, an electromagnetic flow generator, i.e., a linear motor, is installed inside the upper part of the mold, and this creates a film-like flow along the mold wall just below the surface of the steel inside the mold, causing the bubbles to disappear at the stage of forming nuclei. It is more preferable to do so.

The component composition of this invention will be explained in detail below.

First, it is known that Mn combines with S and O to precipitate as MnS and Mn0.

Depending on the size and amount, these precipitates suppress grain growth, but if the suppression is incomplete, abnormal grain growth occurs when annealed at high temperatures.

These inventors have determined that in continuously cast steel that is a weakly deoxidized steel that is not deoxidized with Al or Si, Mnζ0.20% and We found free oxygen ≧70 ppm, preferably ≧1100 ppm.

In this case, the lower limit of Mn is 0.06%, and if the value is lower than this, slow recrystallization will occur during box annealing, leading to problems such as increasing the annealing temperature or taking more time. occurs.

On the other hand, if the upper limit of free oxygen exceeds 250 ppm, pinhole formation cannot be prevented even if electromagnetic flow is applied during continuous casting, and bumping or breakout may occur during casting, causing operational problems. This will cause problems for those above.

The above is the reason for limiting Mn and free oxygen.

C is preferably 0.10% or less because cold-rolled steel sheets used for press forming need to be soft, but when used as tin plate blanks, it is necessary to maintain hardness. Therefore, the upper limit was set at 0.15%.

The lower the lower limit, the better from the point of view of formability, but in ordinary converter melting, the lower limit should be 0.02% from an economical point of view.

Since Si and Al are not actively used for deoxidation in this invention, their unavoidable contents, that is, Si<0.02%,
AAζ was set at 0.005%.

Note that P is preferable for low seed materials, and S is set to P<;: 0.025% and Sζ0.020%, respectively, in order to prevent deterioration of the material.

Molten steel adjusted to such a composition is cast using electromagnetic flow using a well-known continuous casting device to produce a weakly deoxidized steel slab without pinholes, which is then hot-rolled and cold-rolled according to conventional methods. Then, recrystallization annealing is performed at a temperature of 500° C. or higher to obtain a cold-rolled steel sheet with uniform grains of appropriate size.

Next, embodiments of the present invention will be described.

Twelve types of steel with varying Mn and free oxygen levels were melted and continuously cast while applying electromagnetic flow to the pro-solidification interface of the mold to create slabs.

For electromagnetic flow, linear motors are installed along both long sides of the inside of the slab mold, and the directions of the thrusts applied are opposite to each other, and O14 to 1.0 m is applied to the molten steel at the solidification interface from the surface of the molten steel in the mold to 100 mm below. A film-like rotational flow along the inner wall of the mold is applied.

Among these slabs, healthy slabs without pinholes are heated in a heating furnace at 1100 to 1280°C to a finishing temperature of 87°C.
After hot rolling at 0°C to 890°C to a plate thickness of 2.5 mm, it was rolled up at 5700°C to 650°C, and then 0.8 mm thick.
The material was cold rolled to 1.0 mm, then box annealed at a temperature higher than the recrystallization temperature, and subjected to 1.0% skin pass rolling.

Slabs in which the amount of free oxygen in the molten steel was 250 ppm or more were not sent to subsequent processes because pinholes were generated and the slabs had no value as commercial products.

As a method of determining the presence or absence of surface roughness during press forming of the cold-rolled steel sheet produced as described above, a stretch test using an Erichsen tester was conducted.

This is a method in which the surface of the steel plate is visually observed during the elongation process to check for roughness, and it can be determined whether or not roughness has occurred.

The chemical compositions of the invention steel and comparative steel are shown in Table 1, and the manufacturing conditions and results of the overhang test are shown in Table 2.

Further, FIG. 1 shows the relationship between the range of Mn and free oxygen and the occurrence of rough skin.

As is clear from Table 2, all of the comparative steels with composition ranges outside the range of the steel of the present invention had rough skin.

This is due to the occurrence of abnormally coarse grains as shown in Figure 2.

On the other hand, A and B satisfy the composition range of the steel of the present invention.

Both steels C and D had a healthy structure as shown in FIG. 2a, and no roughening occurred.

From the above, it can be seen that the steel sheet of the present invention is a cold-rolled steel sheet that does not cause surface roughness during press forming, which corresponds to rimmed steel manufactured by continuous casting.

*1) In all box annealing cycles 710-6, 10, and 12, the temperature at the outer periphery of the coil is approximately 750°C.

*2) Slow recrystallization occurred in the comparison steel, and poor quenching was observed.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the relationship between the range of Mn and free oxygen and the occurrence of rough skin, and Fig. 2a is a microscopic photograph showing the structure and weave of the steel of the present invention in an example. The same is a set of comparative examples

Claims (1)

[Claims] I C0.02~0.15%, P≦0.025%, S
<0.020%, a small amount of Si<0 to deoxidize
．． 01% and Al<, 0.005%, the balance is Fe and unavoidable impurities.
6 to 0.20% and a free oxygen content of 70 to 250 ppm, continuous casting is performed while applying electromagnetic flow to the solidification interface of the molten steel, and a sound slab without roughness during press forming and without pinholes is obtained. A continuous casting method for slabs for cold-rolled steel sheets, characterized by: