JPS582263B2 - Thin steel sheet with excellent workability and surface quality - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a thin steel sheet with excellent workability and surface quality.

In recent years, the continuous casting method for steel has made remarkable progress, and various attempts have been made to manufacture soft thin steel sheets by continuous casting, but there is still no thin steel sheet comparable to rimmed steel or capped steel produced by the conventional steel ingot method. The current situation is that it has not been completed.

Rimmed steel for thin steel sheets manufactured by the steel ingot method exhibits extremely excellent material properties such as surface quality and workability, and is used in a wide range of applications, including hot-rolled steel sheets for processing, cold-rolled steel sheets, and surface-treated steel sheets such as tinplate and galvanized iron sheets. It has been used in

However, it is difficult to manufacture this rimmed steel by continuous casting due to the intense rimming action, and although some test examples have been reported in the Soviet Union and elsewhere, it has not been put to practical use.

Therefore, killed steel is generally selected for continuous casting due to process constraints.

Al stabilized steel, which has been commonly used in the conventional steel ingot method, is common among killed steels, but in continuous casting of this steel type, Al203 is produced as a deoxidation product,
The nozzle of the ladle or intermediate ladle may become clogged, making continuous casting difficult, and it also causes surface flaws due to Al203 inclusions, which are problems with conventional Al-killed steel, and So
l. The high Al content imposes restrictions on annealing conditions, making it difficult to satisfy all of the wide range of uses of conventional rimmed steel.

In addition, as a method to compensate for these drawbacks of Al-killed steel, US Pat.
A continuous casting method for An-Si composite deoxidized killed steel, such as No. 2781, has been proposed.

This method aims to reduce the constraints on annealing conditions comparable to those of conventional rimmed steel by reducing the amount of Al used to avoid nozzle clogging due to the formation of Al203, and by suppressing the residual amount of So4Al. The lack of deoxidizing power is compensated for by Si deoxidizing, and continuous casting is possible with good workability, and the surface quality is equivalent to that of rimmed steel.

However, even with this method, it is difficult to withstand a wide range of applications that are completely equivalent to the rimmed steel in the conventional steel ingot method.

In other words, in the deoxidation method of this method, the nonmetallic inclusions generated are a relatively low melting point composition of the Al2O3-Si02-MnO system, and the workability when used as a hot-rolled thin steel sheet is not necessarily suitable for general use. I'm not in a good condition.

Moreover, even when this is used as a cold-rolled thin steel sheet, its cold-rollability is inferior to that of conventional rimmed steel ingots in that it is harder, and its workability cannot be said to be perfect.

The present invention takes into account the shortcomings of these conventional methods and conducts various experiments and
It was developed after repeated research, and it improves casting workability in continuous casting, surface texture, and workability in hot-rolled thin steel sheets.
The present invention provides a thin steel sheet that is comparable to or even better than rimmed steel or capped steel in the conventional steel ingot process in all respects such as cold rollability and workability in cold rolled thin steel sheets.

That is, the thin steel sheet of the present invention is produced by continuously casting the molten steel made into killed steel by deoxidizing low carbon steel melted in a converter, open hearth, or electric furnace with A7, Si, Mn, and Cr. Manufactured by rolling, cold rolling, or surface treatment.

The composition system of the thin steel sheet of the present invention is as follows from the viewpoint of deoxidation and quality.
C=0.03~0.08%, Si=O, 03~0.15
%, Mn = 0.10 ~ 0.3 0%, hl
<0.0 15%, Cr = 0.02 ~ 0.2
0%, and C r > 0.17 (0.40=[
%Mn])%.

FIG. 1 shows the influence of Mn content on cold rollability.

M n < 0. It was found that the rolling power of cold rolling showed a significant decrease in the range of 30%, and properties comparable to those of rimmed steel obtained by the conventional steel rolling method were obtained.

Therefore, in continuous casting of killed steel, a soft material equivalent to conventional rimmed steel with a Mn content of 0.30% or less gives very good results.

However, this reduction in Mn content, which gives very good results in terms of processability, on the other hand, causes a new problem of original plate staining, and cannot be used for a wide range of applications.

As a result of various studies on this point, it was found that the cause of the original plate staining was due to the precipitation of graphite carbon during annealing of the steel plate.As a result of further research, as shown in Figure 2, it was found that the cause of the original plate staining was due to the decrease in the Mn content. It has been discovered that the addition of Cr is very effective in preventing staining of the original plate caused by this.

In this case, it was determined from the results of experiments that the amount of Cr added should be Cr>0.17 (0.40-(Mn%))% as shown in FIG.

Furthermore, as will be described later, in order to eliminate various disadvantages of continuous casting of Al-killed steel, when some of the Al is replaced with Si and the Al content is limited, the nonmetallic inclusions that are generally generated are Al2O3. Mn-Silicat containing
e is the main component, resulting in impaired workability as a hot-rolled thin steel sheet.

On the other hand, in the present invention, the addition of Cr shows the effect of changing the form of these nonmetallic inclusions, and the addition of Cr oxide creates dispersed inclusions that are relatively easy to break apart from extended inclusions. The processability of hot-rolled steel sheets is improved.

According to experiments, the range of Cr addition that shows this effect is Cr>
The range is 0.02%.

On the other hand, other component systems are determined based on various operational factors when manufacturing the thin steel sheet of the present invention, and for the purpose of continuous casting of killed steel for thin steel sheets for processing, carbon content, carbon content, etc. Although it is preferable that the silicon content is small, the carbon content is in the range of 0.03 to 0.08% due to the constraints on melting in general converter furnaces, open hearth furnaces, electric furnaces, etc. The amount is necessary in order to replace a part of Al deoxidation in conventional Al-killed steel with Si, but considering the influence on workability, it is necessary to suppress it to 0.03% or more and 0.15% or less.

As already mentioned, when Al content is high, Al2
This may cause problems such as nozzle clogging due to O3 inclusions or surface flaws due to Al2O3, so the lower the value, the better.

In terms of material, Sol. If the Al content is high, there are restrictions on the annealing conditions, and in order to avoid the annealing conditions comparable to Al-killed steel,
It is necessary to suppress it to 0.015% or less, preferably 0.015% or less.
It is preferable to set it to 0.010% or less.

To further explain the manufacturing process of the thin steel sheet of the present invention,
C10.0 3-0 melted in a converter, open hearth, electric furnace, etc.
．． 08% molten steel, A4 Fe-Si, Fe-M
Deoxidation adjustment is carried out in a ladle during or after tapping with n, Si-Mn, Fe-Cr, etc., and then A
Stirring is performed using r gas or the like to promote homogenization of components and floating separation of nonmetallic inclusions generated by deoxidation.

In this case, it is important to add the deoxidizing agent according to the oxygen content of the molten steel.In particular, the amount of Al added should be determined by accurately understanding the yield at the time of addition, the amount of reduction during Ar stirring, etc. A predetermined amount of residual Al must be ensured.

There is no problem with the method and order of addition of the deoxidizing agent as long as it is kept within the component range of the present invention.

Desirably, if Al is added before other deoxidizers are added, it is possible to make the generated nonmetallic inclusions into a dispersed type that does not extend during the hot rolling process, which brings about good results.

However, as already explained, in the present invention, C
Since r-deoxidation performs this action, there is no particular need to be particular about the order in which the deoxidizers are added.

The molten steel that has been deoxidized in this way can be cast in a commonly used continuous casting machine, and if the composition is within the range of the present invention, blowholes and pinholes that will impair the surface quality of the finished product will be sufficiently prevented. It can be suppressed.

The subsequent processes are to obtain hot-rolled thin steel sheets, cold-rolled thin steel sheets, and surface-treated steel sheets, respectively, through the usual hot rolling, pickling, cold rolling, annealing, and surface treatment steps. be.

In this way, according to the present invention, a material with good surface properties that has good workability in hot-rolled thin steel sheets, excellent cold-rollability, and excellent workability in cold-rolled thin steel sheets can be stably produced by continuous casting. This makes it possible to manufacture a material comparable to conventional rimmed steel by continuous casting, making it an extremely valuable contribution to steelmaking technology.

Furthermore, FIG. 4 shows the influence of nitrogen content in steel on the hardness of cold-rolled thin steel sheets.

It becomes significantly softer when the nitrogen content is less than 30 ppm.

Therefore, if the nitrogen content of the thin steel sheet according to the first aspect of the invention is further reduced to 30 ppm or less, a material with better workability than conventional rimmed steel can be obtained.

This is the second invention of the present invention.

In this case as well, the manufacturing process is such that low-nitrogen molten steel is melted in a converter, open hearth, electric furnace, etc., and then continuously cast according to the method described above.

Furthermore, in order to minimize the absorption of nitrogen during tapping and injection, it is effective to not deoxidize at the time of tapping but to perform deoxidation after tapping, or to cut off the injection flow. , yields good results.

Hereinafter, the present invention will be explained with reference to examples.

Example 1 Low carbon steel is melted in a 50 ton converter, and Al and F are melted at the time of tapping.
Required amounts of e-Si, Fe-Mn and Fe-Cr were added to prepare killed steel having the composition shown in Table 1 for deoxidation.

A to g in Table 1 are the component systems of the present invention.

This was cast using a vertical continuous casting machine with a cross section of 220 mm x
Six slabs of 930 mm were manufactured from each charge, and the finishing temperature of hot rolling was 820 to 850°C, and the winding temperature was 550°C.
It was hot-rolled at ~600°C to form a 2.0×930 hot-rolled coil.

Among these hot-rolled coils, the casting number of each charge is 1,
No. 2 was used as a hot-rolled cut plate as it was, and a part of it was sampled and mechanical tests were conducted.Cast No. 3 and No. 4 had 0.0% after pickling. 2
It was rolled into a cold rolled coil for a zinc original sheet of 7 mm x 915 zm, and the required rolling power (HHT) was tested.

The remaining two coils, casting numbers 5 and 6, were pickled, cold-rolled to a thickness of 0.27 mm, annealed, and tin-plated to produce tinplate. During this process, the original plate was inspected for stains.

Figure 3 shows the results of the hole expansion test among the mechanical test results for hot rolled thin steel sheets.

The test was conducted using a conical punch with an apex angle of 60°.
The effect of the addition is visible.

Furthermore, the standard of the conventional rimmed steel TOP piece is a hole expansion ratio of 2,
It was about 0±0.1, and about 1.7±0.1 for the bottom piece, and results sufficiently comparable to these were obtained.

Figure 1 shows the required cold-rolling power (HHT) when cold-rolling the hot-rolled coil to 0.27 mm in relation to the Mn content. It is clear.

In particular, the cold rollability rapidly improves when the Mn content reaches around 0.30%.

Furthermore, the effect of adding C'r in the present invention has a favorable effect on staining of the tinplate blank, and as shown in FIG. 2, prevents staining of the blank that occurs due to a decrease in Mn content.

In this figure, the ○ mark indicates a charge that did not cause the original plate stain, and the × mark indicates a charge that caused the original plate stain.

Example 2 12 charges of low carbon steel in a 50 ton converter (Table 2 A-
L) was dissolved.

Of these, 6 charges A-F undergo low nitrogen blowing and the other 6 charges
The charge was melted using normal operations.

In addition, 6 charges of A-F have Al in the ladle after tapping.
, Fe-Si, Fe-Mn, Fe-Cr, and Ar
Continuous casting was performed after gas stirring.

For the other 6 charges, Al, Fe-Si
, Fe-Mn, Fe-Cr added, Ar
After stirring the gas, continuous casting was performed.

Table 2 shows the check analysis values of the slabs thus produced.

These slabs were hot rolled at a finishing temperature of 820-8500C and a rolling temperature of 550-600C.2. O×9
Twenty hot rolled coils were manufactured.

FIG. 4 shows the hardness of hot-rolled steel sheets sampled from the hot-rolled coils manufactured in this manner.

A soft material, which was previously unobtainable with A-F charge hot rolled steel sheets with a nitrogen content of 30 ppm or less, has been obtained.
This shows that the present invention can provide a thin steel plate of superior quality to conventional rimmed steel in continuous casting.

[Brief explanation of the drawings] Figure 1 is a chart showing the influence of Mn content on cold rollability, and Figure 2 is a chart showing the influence of Mn content on cold rolled steel sheet original plate contamination.
Figure 3 is a diagram showing the effect of Cr content on the hole expandability of hot-rolled steel sheets; Figure 4 is a diagram showing the influence of N content on the hardness of hot-rolled steel sheets. It is.

Claims (1)

[Claims] I C = 0.03 to 0.08%, S i = 0.0
3-0.15%, Mn=0. 1 0 ~ 0.
30%, Cr=o. 0 2-0.20% and C
r>0.1 7 (0.4 0-[%Mn'])%
, A thin steel sheet with excellent workability and surface texture, consisting of Al<0.015% with the remainder being iron and accompanying impurities. 2C=0.03-0.08%, S i =0.
03-0.15%, Mn=0. 1 0-0.3
0%, Cr=0.02 to 0.20% and Cr>0.17 (0.40-(%Mn))
%, Al < 0.015%, N < 30 ppm, with the remainder being iron and accompanying impurities. A thin steel sheet with excellent workability and surface texture.