JPS6045691B2 - Method for producing thin steel sheets with good drawability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for manufacturing a thin steel sheet with good drawability.

In recent years, in order to achieve labor and energy savings, continuous development or process abbreviation of steel plate manufacturing processes has been actively promoted.

This invention provides a method for manufacturing thin steel sheets with advantageous drawability by a low-temperature direct rolling method in which a thin cast steel strip continuously cast using a strip caster is directly rolled at a temperature below the recrystallization temperature, instead of the conventional continuous slab casting method. The purpose is to disclose the development results for.

Conventionally, cold-rolled thin steel sheets that are soft and have good workability have been produced exclusively through the following steps: slab-hot rolling, cold rolling, and recrystallization annealing.

On the other hand, a method has been considered in which these steps are omitted as much as possible and thin steel strip products are directly continuously cast from molten steel through a rolling process, but the thin steel strips obtained by this method have a poor surface shape and are difficult to process. Because it is inferior to conventional methods, it cannot be used as is for applications that require processability comparable to conventional methods.

Therefore, rolling and recrystallization steps are required to improve the drawability of such continuously cast thin steel strips.

However, applying two rolling steps (hot rolling and cold rolling) results in a large energy loss, so being able to provide good drawability with just one rolling process is not a great industrial advantage. it is obvious.

Therefore, we researched and developed a method for producing thin steel sheets with good drawability by rolling thin-walled cast steel strips continuously cast from molten steel under conditions where the rolled structure remains, and then recrystallizing and annealing them.

In other words, the purpose of the present invention is to convert thin steel sheets with good drawability, which were manufactured through the conventional slab hot rolling, cold rolling, and recrystallization annealing process, to recrystallization monocrystallization through the continuous casting → cold rolling → recrystallization annealing process. The object of the present invention is to propose a manufacturing method that can omit the rolling process at a temperature higher than that temperature.

In the conventional manufacturing process from hot rolling to cold rolling, it is possible to improve the drawability of the steel sheet after cold rolling and annealing by appropriately selecting the conditions in the hot rolling process! did it.

On the other hand, in the case where the cast steel strip after solidification is directly cold rolled, as is the object of the present invention, other methods must be considered in place of hot rolling.

As a result of extensive research into the above points, the inventors have found that controlling the cooling rate of the cast steel strip after solidification in continuous casting is first of all advantageous for improving the drawability of the finished product. I found it.

On the other hand, in order to promote energy saving and process saving, it is desirable to roll during cooling because it reduces the rolling load and makes it possible to omit the coil winding and tasseling processes before rolling. If the rolling temperature is too high, recrystallization will proceed during rolling, and even if such a steel sheet is annealed, it will no longer be possible to improve drawability4.

As a result of repeated research on this point, the inventors found that, as described later, rolling at a rolling temperature lower than the recrystallization temperature and above a certain reduction rate determined by the combination of the cooling rate of the cast steel sheet improves drawability. I found out what I can do.

This invention is derived from the above knowledge.

In other words, the present invention provides continuous casting of a thin cast steel strip, during cooling or after cooling to room temperature, rolling at a temperature below the recrystallization temperature, and recrystallization annealing. Average cooling rate VCC at
/Mln), rolling start temperature TCC) and rolling reduction ratio R (%), the following formula, R≧(56-210g(10
×■)) ×10g (95/(8.5-T/100)) The conventional problem was solved by performing rolling.

The following steel types are particularly advantageously applicable to the present invention.

(1) C: 0.0030% by weight (hereinafter simply expressed as %) or less, Mn: 0.15-0.25%, SOI. Al: 0.
02-0.06% and Nb: 0.008-0.03%
A composition containing

(2) C: 0.0035%, Si: 0.02-0.60
%, Mn: 0.2-0.4%, P: 0.060-0.1
10%, SOL. Al: 0.02-0.06% and N
b: A composition containing 0.008 to 0.03%.

(3) C: 0.02-0.06%, Mn: 0.2-0.
4% and SOL. A composition containing 0.02 to 0.07% of AI.

(4) C: 0.05~0.09% and Mn: 0.2~
A composition containing 0.4%.

(5) C: 0.04-0.09%, Mn: 0.2-0.
9%, P:0.08-0% and GsOl. Al: 0
．． 02 to 0.06%.

(6) C: 0.06-0.10%, Mn: 0.2-0.
9% and SOL. A composition containing Al: 0.02 to 0.06%.

The background of the experiment that formed the basis of this invention will be explained below.

First, C: 0.001~0.10%, AI: 0.001~
0.07%, Mn: 0.15-0.90%, Si: 0.
02-0.6%, P:0.008-0% and Nb:
The composition contains 0.03% or less, and the plate thickness is 3 to 30%.
A cast steel strip of TEn is cast, and this cast steel strip is heated from 900℃ to 7
Average cooling rate from 60°C to 00°C per hour to 2 per second
The temperature was controlled over a range of 0°C. During this cooling, after rolling at a temperature T below the recrystallization temperature, recrystallization annealing was performed at 850° C. for 1 minute, and the drawability of the obtained product was examined. Note that the drawability was evaluated by the plastic anisotropy ratio (hereinafter referred to as γ value). Here, the cooling rate (■) of the cast steel strip after casting is regulated in the temperature range from 900'C to 700'C.
The cooling rate in the temperature range exceeding 00℃ or less than 700℃ does not affect the drawability of the product, and
This is because the cooling rate (2) to °C determines the drawability of the product.

The reason for this is not certain at this time, but 0.
For steel containing 1% or less C, the cooling process from 900'C to 700°C is the temperature range that passes through the A3 transformation point, and the faster the cooling rate here, the faster the transformation rate becomes and the grains become finer. It is presumed that this is because when a copper plate with such a fine crystal structure is rolled and annealed, strain energy is likely to be accumulated during rolling, and a texture favorable to drawability is formed during the annealing process. For these reasons, it is difficult to control the cooling rate to improve the drawability of thin product sheets.
It was decided to carry out the experiment in the range of 0'C to 700'C. Next, the experimental results are shown. The temperature of the cast steel plate is ■=10'C
Figure 1 shows the experimental results when rolling was carried out under the condition of T = 20'C, and the relationship between the rolling reduction ratio R and the r value, when the temperature was lowered to room temperature through the above cooling process of /Mjn and then rolled. Indicated by

It can be seen from FIG. 1 that the higher the rolling reduction rate, the higher the r value.

Hereinafter, the lower limit reduction rate at which such a high r value can be obtained will be referred to as the limit reduction rate RO. For example, in the case shown in Fig. 1, RO=
It is 55 (%). Next, we will discuss the results of rolling at various temperatures during cooling of the steel plate after casting. FIG. 2 shows the relationship between the rolling start temperature (TCC) and the critical reduction rate RO (%) when the cooling rate (2) from 900'C to 7000C is 5°C/Mln.

It can be seen from the figure that a higher rolling reduction ratio is required when rolling at a high temperature.

Furthermore, we also investigated the relationship with RO (5V) when rolling was performed with various cooling rates V and rolling start temperatures T, and as an example, R when the rolling start temperature T was 300°C.

．． The relationship with l5V is shown in Figure 3. As is clear from the figure, by increasing ■, a high r
It turns out that the value can be obtained.

In general, it is virtually impossible to perform strong reduction rolling with a mill that has a small rolling capacity, but even when such a mill is used, good drawability can be achieved even at a low reduction rate by increasing ■. Another major advantage of this invention is that it allows the production of steel plates.

When the above experimental results were summarized and the relationships among RO, ■, and T were organized, it was found that the relationship between the three was expressed by the following equation.

Therefore, a thin steel sheet with excellent drawability can be obtained by rolling at a reduction rate equal to or higher than this critical reduction rate RO. Next, embodiments of this invention will be described. Molten steel with various compositions shown in Table 1 is poured into molds that can be adjusted to various thicknesses by temperature control to produce 3TWL to 30TW.
A thin-walled cast steel strip with a thickness of
℃ by changing the cooling rate in the range of 0.2 to 900℃/Min, rolling at various temperatures and various rolling reductions from the middle of cooling to room temperature,
Thereafter, it was annealed at 850°C for 1 minute or at 700°C for 1 minute to obtain a thin steel plate.

The results of examining the drawability of each of the obtained thin steel sheets are also shown in Table 1 in terms of r values. C As is clear from Table 1, all the thin steel sheets obtained according to the method of the present invention showed a high r value, whereas as in the comparative example, the r value was When rolling was performed at a reduction rate lower than the critical reduction rate, only a low r value could be obtained.

Next, the mechanical properties of the thin steel sheet obtained according to this invention method, indicated by Steel 3 in Table 1, were investigated and the results were compared with those of a cold-rolled steel sheet of the same composition manufactured by the conventional method. Table 2
Shown below.

The mechanical properties of the thin steel sheet obtained according to the method of this invention were not inferior to those of the cold-rolled copper sheet according to the conventional method.

As described above, according to the method of this invention, even from a thin cast steel strip cast with a strip caster, a thin steel sheet with excellent drawability comparable to conventional cold-rolled steel sheet can be produced without the hot rolling process. It is highly effective in saving energy and process.

[Brief explanation of the drawing]

Figure 1 is a graph showing the relationship between rolling reduction R and r value when rolling is performed at V=10, and Figure 2 is a graph showing the relationship between rolling start temperature T and critical rolling reduction R when ■ = 5°C/Min. .

Claims (1)

[Claims] 1. When a thin cast steel strip is continuously cast, and during cooling or after cooling to room temperature, the cast steel strip is rolled at a temperature below the recrystallization temperature and then recrystallized annealed.
For the average cooling rate V (°C/min) to 0°C, rolling start temperature T (°C) and rolling reduction rate R (%),
A method for manufacturing a thin steel sheet with good drawability, which comprises rolling that satisfies the following formula: R≧{56-2log(10×V)}×log{95/(8.5-T/100)). 2. The thin plate-shaped steel piece contains C: 0.0030% by weight or less,
Mn: 0.15-0.25% by weight, Sol. Al: 0.
02-0.06% by weight and Nb: 0.008-0.0
The method according to 1, wherein the composition contains 3% by weight. 3. The thin plate-shaped steel piece contains C: 0.0035% by weight or less,
Si: 0.02-0.60% by weight, Mn: 0.2-0.
4-fold %, P: 0.060-0.110% by weight, Sol
．． Al: 0.02 to 0.06% by weight, and Nb: 0.
2. The method according to claim 1, wherein the composition contains from 0.008 to 0.03% by weight. 4 Thin plate-shaped steel piece contains C: 0.02 to 0.06% by weight
, Mn: 0.2 to 0.4% by weight and Sol. Al: 0
．． 2. The method according to claim 1, wherein the composition contains 02 to 0.07% by weight. 5 Thin plate-shaped steel piece contains C: 0.05 to 0.09% by weight
, and Mn: 0.2 to 0.4% by weight. 6 Thin plate-shaped steel piece contains C: 0.04 to 0.09% by weight
, Mn: 0.2-0.9% by weight, P: 0.08-0.1
1% by weight and Sol. 1. The method according to 1, wherein the composition contains Al: 0.02 to 0.06% by weight. 7 Thin plate-shaped steel piece contains C: 0.06 to 0.10% by weight
, Mn: 0.2 to 0.9% by weight and Sol. Al: 0
．． 2. The method according to claim 1, wherein the composition contains 0.02 to 0.08% by weight.