JPS5849620B2 - Method for producing cold-rolled steel sheets with excellent workability by applying continuous casting method - Google Patents

Description

[Detailed Description of the Invention] The present invention applies a continuous casting method to provide recrystallization properties, material and surface properties comparable to those of rimmed and capped steel produced by ordinary agglomeration methods, and is superior to rimmed and capped steel. The present invention relates to a method for manufacturing a cold rolled steel sheet that exhibits non-aging properties.

Currently, the continuous casting method is widely used in the production of steel slabs, and continuous slabs for cold rolling are produced as main-staple Al-killed steel or vacuum-degassed steel.

The reason why it is manufactured as killed steel in both cases is because when continuously casting steel, if oxygen is contained in the steel, blowholes will occur, which is not desirable for the casting process or for the product. This is because it is necessary to deoxidize the molten steel in advance.

As a method for performing this type of AA deoxidation, there is, for example, Japanese Patent Application Laid-Open No. 47-31828, and as a Si deoxidation method, there is, for example, Japanese Patent Application Laid-Open No. 50-78522. There is a Special Publication No. 47-47209.

However, the steel produced by the above method has the following problems.

■ Cold-rolled steel sheet Al-killed steel Normally, killed steel contains Al from 0.025 to 0.025 o. ioo
%, and therefore has better workability than rimmed and capped steels produced by ordinary ingot-forming methods, but is more expensive.Also, the recrystallization temperature after cold rolling is lower than that of capped steels, as shown in Figure 1. The temperature is about 50 to 60°C higher than that of steel, and therefore it is common knowledge that the annealing temperature is at least 600°C or higher, preferably 620°C or higher, which also causes high costs.

Furthermore, regarding the surface properties, it is well known that due to the large amount of A, the solderability or paintability is inferior to that of capped steel.

Furthermore, since the steel contains alumina, etc., there are many surface defects, which lowers the yield and increases the cost.

■Si-killed steel Although it can be produced at a lower cost than Al-killed steel from a cost perspective, the presence of Si-based inclusions greatly deteriorates the hole expandability, which is one of the properties required for cold-rolled steel sheets. Also, due to the high Si content, the material is harder than capped steel.
The number of stores that have fallen is high and the growth rate is low.

Furthermore, the surface treatment properties are also inferior due to the presence of Si.

This is because a thin oxide film based on Si is formed on the surface of the steel sheet during annealing.

■ Degassing treated steel Because of the degassing treatment, it is disadvantageous in terms of cost compared to rimmed and capped steel, and the regulation (C) becomes stricter in terms of aging.

That is, when the C content is in the range of 0.005 to 0.01%, the amount of aging in the finished product increases compared to capped steel, yield point elongation becomes a problem as stretcher strain in press working, and material deterioration due to aging occurs. It's also big.

Currently, slow-aging steel plates are used as capped steel plates to prevent stretcher strain in automobile outer panels, so degassing steel sheets may not be suitable for automobile outer panels depending on their C content.

As is clear from the detailed description above, the above steel types are
As a substitute for rimmed and capped steels, continuous casting materials cannot satisfy all of the material, surface characteristics, and cost aspects, and there has been a strong desire for continuous casting materials with such characteristics.

The present invention was made as a result of intensive studies to meet the above-mentioned needs, and the gist thereof is as follows.

That is, it contains the balance Fe and unavoidable impurities, and 4VX1+
3.2 (Mn 2XS)≦1, Mn-1. 7 X
S deficiency 0. Molten steel satisfying 0.5 is continuously cast to form a slab, then hot-rolled and coiled at 500-650°C, followed by cold rolling at 35-90% with a reduction wheel, and then 5
By recrystallizing and annealing at a temperature of 50°C or higher, it has material and surface properties comparable to or better than rimmed and capped steel produced by ordinary agglomeration methods, and is comparable in cost to the above-mentioned rimmed and capped steel. This method is characterized by obtaining a cold-rolled steel plate.

The contents of the present invention will be explained in detail below.

The present inventors have developed continuous cast materials of AA, Mn, S,
As a result of various investigations into the effects of N on the recrystallization degree and slab cracking, we found that by limiting the range of A#, Mn, S, and N, the recrystallization temperature could be rimmed, compared to Al-K steel. It has been discovered that it is possible to reduce the temperature to a level comparable to that of capped steel, and to produce steel plates without cracking of cast pieces.

First, Figure 1 shows an example of a recrystallization line (heating rate 2
0°C/h1 cold rolling rate of 70%).

Incidentally, Table 1 shows the components of the investigation sample shown in FIG.

, solA# 0.050% AA-K (■ in the figure)
Compared to capped steel (■ in the figure), the completion of recrystallization is 50%.
It shifts to the high temperature side by about ℃.

A6-K steel for conventional cold-rolled steel sheets actually contains about 0.03 to 0.08% of so#Al, and after cold rolling,
It is well known that in the early stage of annealing, this l combines with N and controls the texture of recrystallized grains so as to improve the Lankford value (γ value), which governs the deep drawability of steel sheets. In terms of recrystallization temperature, recrystallization softening is delayed due to the KlH precipitated at this early stage, and box annealing is 40 to 50℃ compared to rimmed and capped steels.
It is common knowledge to set a high annealing temperature.

This leads to a decrease in annealing T/I{ and an increase in energy consumption, resulting in a significant cost increase compared to rimmed and capped steels.

On the other hand, the present inventors have found that AA, Mn
, N, and S ranges are set appropriately, it has been found that the recrystallization temperature is on the same level as capped steel.

That is, solAl: 0.02 5%, Mn: 0.06
~0.02% and 4JK7+3.2(Mn-2X
S)≧1, N<40 A6, M to fill the throat
If a certain range of n, N, and S is set, due to the combined effect of these elements, box annealing shows the same recrystallization behavior as capped steel (■, ■, ■ in the figure), and low-temperature annealing (550℃ or higher) ) was sufficiently softened, and it was discovered that inexpensive continuous casting materials (■ to ■ in the figure) can be produced as an alternative to rimmed and capped steel.

Figure 2 shows these suitable component ranges.

The area surrounded by the straight lines ■ and O in the figure is the composition range of the capped steel and the steel of the present invention, which exhibits a recrystallization temperature at the garden level.

(However, S is 0.025% or less. The reason will be explained later.)
In addition, the straight line 0 shown in FIG. 2 indicates the range of ingredients for avoiding slab cracking, which will be described later, and the range of the present invention is as follows:
This is the diagonal line surrounded by ■ and ■.

The appropriate component range will be explained below.

First, the upper limit of Al was set to 0.025%.

If it exceeds this, even if N<40pIIIl, precipitation of A7N during box annealing increases the raw LA crystal temperature before recrystallization, so the upper limit was set (the lower limit of Al will be described later).

Next, when Mn exceeds 0.20%, the recrystallization temperature becomes high and the material becomes hard, so the upper limit was set.

It is well known that when the temperature becomes too low, slow recrystallization occurs, but sollAlO. 0 0 5-0.02
In the range of 5%, it was found that this phenomenon occurs when Mn<0.06%, and 0.06% was set as the lower limit for Mn.

Furthermore, when soAA7<0.0 25%, if the range of N is set to 40m or less, during box annealing, before the completion of recrystallization (540℃
We found that the recrystallization temperature is the same as that of capped steel because it does not precipitate as AlN in the case of 40 p
F was taken as the upper limit of N.

Furthermore, since klN precipitates after recrystallization, the steel sheet of the present invention exhibits non-aging properties.

satisfies the independent certain range of k/l p Mn mentioned above, and 4Jτ#+3.2(Mn-2XS)51(N×4
As a result of investigating materials with various composition ranges, it was discovered that by simultaneously satisfying 0 ppm), it is possible to secure a recrystallization temperature at the same level as capped steel.

(However, the range of S was set to 0.025% or less. The reason will be explained later.

) The above is the composition range of the steel of the present invention from the viewpoint of recrystallization.

In the present invention, the lower limit of the Al content is defined as <0.005% as described above.

In the case of deoxidation of Al alone, the present inventors found that *
This is based on the finding that if s o l A6 ''' O.O O 5%, no blowholes would occur and a sound surface quality could be obtained.

In order to avoid the slow recrystallization mentioned above, the lower limit of Mn is set as Mn≧
0.06%, and at the same time, Mn-1.7

This is a condition for the steel of the present invention to avoid embrittlement due to S that is not fixed as MnS.

As shown in the figure from the investigation results, the present inventors found that the embrittlement due to S was most noticeable in the vertical cracks of the slab in the steel of the present invention, and found that M n − 1.7 X S>0.0
5 (however, the reason for setting S to 0.025% will be explained later) was set as the avoidance condition.

The present invention described above can be summarized as follows.

That is, in continuous cast material A [: 0.005 to 0.02
5%. , Mn: 0.06 to 0.20%, and 4 J'K'T+3.2 (Mn-2 XS)
<1 and N and 40pIm, the recrystallization temperature will be at the same level as rimmed and capped steels, and compared to A#-K steel, low-temperature annealing will be possible, making it possible to produce inexpensive cold-rolled steel sheets by continuous casting. Manufacture is possible.

Moreover, in the steel of the present invention, if Al≧0.005% is satisfied, no blowholes will occur, and Mn-1.7X8≧0.0
If the Mn and S components are set so as to satisfy %S, brittleness caused by %S can also be avoided.

Next, the reasons for limiting other components will be described.

C was limited to 0.10% or less since cold-rolled steel sheets used for press forming are required to be soft.

From the point of view of formability, lower C is preferable, but in normal converter melting, from an economic point of view, the lower limit of C is 0.02%, and below this the molten steel becomes overoxidized and deoxidized. A used for
Since the amount of C also increases and the yield of Al is not stable, the lower limit of C is set to 0.02% in the present invention.

S satisfies Mn-1.7XS20.0 5, and at the same time,
The upper limit was set to 0.025% to prevent material deterioration.

Regarding Si, Q. Since 0.3% is the upper limit of unavoidable content, S i <0. 0.3%.

The lower P is, the better the material is, so the upper limit should be set. It was set to 0.025%.

After hot-rolling the continuous cast slab with the above composition range according to the usual method, it is coiled at 500°C to 650°C, followed by cold rolling with a reduction wheel of 35 to 90%, and then at 550°C.
Recrystallization annealing is performed at the above temperature.

The higher the winding temperature, the larger the crystal grains.
Shows a tendency to soften.

Therefore, a lower limit is necessary.

However, if the coiling temperature exceeds 650° C., AlN will precipitate at this stage, making it impossible to achieve a recrystallization temperature comparable to that of capped steel, which is the aim of the present invention.

The higher the winding temperature, the worse the scale removal in the next pickling process.

Therefore, an upper limit is required for the winding temperature.

A suitable temperature range for this purpose is 500-650°C.

If the cold rolling rate is too low, the progress of recrystallization in the annealing step will be extremely slow. Therefore, in order to prevent this, a lower limit value is required, which is set at 35% or more.

Generally, the higher the cold rolling rate, the higher the γ value, but if it is too high, the γ value decreases.

Therefore, the upper limit is set to 90%.

In this case, as for the upper limit of the recrystallization annealing temperature, since one of the objects of the present invention is to lower the annealing temperature, it is of course preferable that the upper limit temperature is also lower. However, there is no reason to actively limit it, and the upper limit of the recrystallization annealing temperature of ordinary rimmed and capped cold rolled steel materials may be used as a guideline for the upper limit of the annealing temperature in the present invention. .

Further, although box annealing is mainly used as an annealing method, continuous annealing methods can also be applied.

Examples will be described below.

EXAMPLE Eight types of steel with different levels of Al, Mn, N, and C were melted, and 7 types of steel were made by continuous casting, and 4 types of IW were made into slabs by ingot making.

The steel slab was heated at 1200°C in a heating furnace, and the finishing temperature was 8°C.
After hot rolling at 70℃ and making the plate thickness 2.5mm,
It was wound up at ~650°C, then cold rolled to 0.8°C, and then box annealed at 530°C ~ 650°C for 1 hour to provide 1% temper rolling.

The chemical compositions of the invention steel and comparative steel are shown in Table 2, the manufacturing conditions are shown in Table 3, and the mechanical property values and surface properties of the product sheets are shown in Table 4.

As is clear from Table 3, comparative steel D does not satisfy the range of N, steel E does not satisfy the range of solAl, N, and Mn, and steel F satisfies the composition range of each element, but 4V 'X
”?+3.2 (Mn-2XS)<1 is not satisfied, or G steel does not satisfy the formula Mn-1.7 It is capped steel.

It can be seen from this that the steel sheet of the present invention has good low-temperature recrystallization properties, material properties, and surface properties like the capped steel, and is also a non-aging continuously cast cold-rolled steel sheet.

In addition, when the annealing cycle is 530°C x 1 hour, the mA of the present invention
, B, and C, the elongation is less than 40%, which does not satisfy the purpose of the present invention.

[Brief explanation of drawings]

Figure 1 is a chart showing the progress of recrystallization as the temperature rises during annealing, and Figure 2 is a diagram showing the progress of recrystallization as the temperature rises during annealing.
Figure 3 is a diagram showing the range of S and the influence of Mn and S on slab cracking and hot selvage cracking.

Claims (1)

[Claims] IC: 0.02 to 0.10% Mn: 0.0
6~0.20% Al: 0.0 0 5~0.0
2 5% N<0.0040%, 8<0.0 2
5%, Si<0.0 3%. P<0.0 2 5%
, balance iron and unavoidable impurities, and 4 v'T
7 + 3. 2 (Mn-2XS)
<IMn-1.7
Winding at 500-650℃, followed by rolling reduction of 35-90%
1. A method for producing a cold-rolled steel sheet with excellent workability by applying a continuous casting method, which is characterized by cold rolling and then recrystallization annealing at a temperature of 550° C. or higher.