JPS59113121A - Production of low carbon hot rolled steel sheet - Google Patents

Abstract

PURPOSE:To produce economically a low carbon hot rolled steel sheet in the stage of rolling directly a continuous casting slab as it is or with heating of only the end part thereof after continuous casting by rolling the slab to a final thickness at a specific temp. and allowing the sheet to cool or annealing the sheet after coiling the rolled sheet. CONSTITUTION:A low carbon steel contg. 0.005-0.15% C, <0.03% Si, 0.10- 0.60% Mn is continuously cost and the continuous casting slab thereof is directly hot rolled as it is at a temp. of 900-1,150 deg.C or with heating only the end part tending to cool earlier, whereby a hot rolled steel sheet is produced. The steel sheet is hot rolled to a final thickness at 20-60% draft in a temp. range of the Ar3 transformation point temp. -600 deg.C and after the sheet is coiled at 500- 700 deg.C, the coil is allowed to cool or is annealed at 700-550 deg.C, whereby a hot rolled steel sheet for general fabrication is produced. Since the heat retained by the continuous casting slab is effectively utilized, the low carbon hot rolled steel sheet is produced at a low cost with good thermal efficiency.

Description

[Detailed Description of the Invention] The present invention relates to a method for manufacturing a low carbon hot rolled steel sheet, and its purpose is to research a method for manufacturing a low carbon hot rolled steel sheet that has no defects in quality for general processing such as bending, drawing, and stretching. A low carbon steel slab consisting of O 0.15X or less, Si 0.01X or less, Mn 0.60% or less, and the balance Fe and unavoidable impurities was made into a 90%
After heating to 0 to 1150°C, hot rolling is performed,
Human r3 transformation point temperature ~ 20 ~ in the temperature range of 600℃
We have developed a manufacturing method for low-carbon hot-rolled steel sheets that is characterized by rolling to the final thickness at a finishing reduction rate of 60%, then winding into coils, and then letting it cool. However, they succeeded in establishing a manufacturing method that did not cause any problems.

This was related to a new method of manufacturing by heating at a low temperature of 900 to 1150°C, instead of the conventional high-temperature heating of 1200 to 1300°C, but the present inventors have developed an even lower carbon As a result of research into a highly energy-saving manufacturing method for hot-rolled steel sheets, we have developed a new method for immediately rolling a high-temperature slab that has been continuously cast, that is, the method of the present invention.

As is well known, the direct rolling method, in which continuously cast high-temperature slabs are rolled directly into products, has been proposed in various ways in the industry due to its energy-saving effect and high productivity. 'Difficulty in securing the top rolling temperature tends to cause problems with the material, so no manufacturing method has been proposed that does not cause quality problems.

Now, with the continuous casting method, there is a problem of breakout, and it is industrially impossible to obtain a slab at a high temperature of 1,200 to 1,300 degrees Celsius with the current state of technology. Thanks to the development of cooling technology, temperatures of 1150℃ (
The average temperature of the entire slab cross section) is the highest.

Further, continuous casting equipment and hot rolling equipment are usually installed at a distance, and as a result, heat is lost during transportation, resulting in a significant temperature drop.

The present inventors have conducted research with the aim of overcoming the temperature constraints as mentioned above and directly rolling manufacturing hot-rolled steel sheets for general processing with no quality problems from slabs with a cast structure, and have developed the present application. 0 Summary of the Invention L 00.005 to 0.15%, SIo, 03% or less, Mn
0.10~0.60 9.kr2 transformation point temperature ~60
Rolling to the final thickness at a finishing reduction of 20-60% in a temperature range of 0°C, followed by a winding source ['700°C-500°C]
After winding into a coil at ℃, leave it to cool or heat from 700℃ to 55℃.
A method for producing a low carbon hot rolled steel sheet characterized by annealing in a temperature range of 0° C. will be described in more detail below.

The reason why the lower limit of 0 is limited to 0.005% in the present invention is that if it is less than 0.005%, the Ar3 transformation point is too high, and as a result, the desired material cannot be obtained no matter how the rolling conditions are devised. This is also because the cost of steel manufacturing technology is high and practical benefits cannot be obtained. The reason why the upper limit of 0 is set to 0.15 N is that if it exceeds 015X, the material becomes hard and is not suitable for general processing purposes. The upper limit for Si is set at 0.03N because if it exceeds 0.03N, the Si material becomes hard, which tends to cause problems during plating and surface treatment during use, and the purpose cannot be achieved.Also, the lower limit of Mn The reason why is set to 0.10% is that if it is less than 0.10%, the 1cAr3 transformation point will be high as in the case of O, making it impossible to obtain the desired material quality, and sulfide cracking will likely occur. The upper limit is set to 0.60 N because if it exceeds 0.6 N, the material becomes too hard and becomes a steel plate unsuitable for general processing.

The lower limit of the temperature range of continuously cast high-temperature slabs is 900℃.
The reason for this is that if the slab temperature is lower than this, the temperature at the end of the slab will be difficult to recover even by light heating during lowering due to the inevitable loss of heat that accompanies the transfer, and the probability of cracking due to embrittlement is high.

Furthermore, setting the upper limit of the temperature to 1150°C is the highest temperature that can be achieved with the current state of the art in continuous casting without worrying about breakouts, and it also reduces the specific water content in casting.
This is the temperature that can be achieved by techniques such as high speed drawing and slow cooling with air and water in a secondary cooling zone, which is why the upper limit is set at 1150°C. Next, the finishing rolling reduction ratio will be explained.

Figure 1 is a graph plotting the results of rolling a low carbon hot-rolled steel sheet with a finishing thickness of 2.0 ml at various finishing rolling temperatures and finishing rolling reductions, where the horizontal axis is the finishing rolling reduction and the vertical axis is the elongation value. JI85 sampled from 1/4 position in the width direction of the coil.
This figure shows the elongation value in the direction perpendicular to rolling using a No. 1 tensile test piece.

The finishing rolling temperature refers to the temperature of the steel sheet immediately after leaving the final finishing roll, and the finishing rolling reduction ratio refers to the temperature at which the finishing rolling temperature is Ar3.
If the temperature is above the transformation point, the rolling reduction rate at the final roll stun P,
If the finish rolling temperature is below the krB transformation point, the steel plate temperature is A.
It refers to the sum of the rolling reductions in the roll stands of each stage from the time when the temperature reaches or below the r3 transformation point, and in the present invention, the finishing rolling reduction ratio is abbreviated as the finishing rolling reduction ratio.

In the present invention, the elongation in the direction perpendicular to rolling was selected as an index of the steel sheet material quality because when a low carbon hot rolled steel sheet is rolled below the Ar3 transformation point and deteriorates in quality, this value best represents the degree of deterioration. .

In Fig. 1, the normal reheat rolling method (reheat temperature 125
0°C) finish rolling temperature 850°C, finishing reduction rate 18X
Those rolled with a low finish reduction car above the Ar3 transformation point show an average elongation of 45%. On the other hand, 800℃, 700℃
For those directly rolled below the Ar3 transformation point of 600°C, elongation is significantly inferior to conventional materials at less than 20% as the finishing reduction ratio changes, but at 20% or more it shows good elongation. This sets the lower limit of the finishing reduction rate to 20 in the present invention.
This is the reason why it is set as %.

It shows a very good value until the finishing reduction rate is 6ON, and 60
When X is exceeded, the elongation tends to decrease.

Therefore, in the present invention, the upper limit is set to 6ON. In this way, the present invention shows a relatively good elongation value equivalent to that of ordinary reheat-rolled material, but the most desirable finish rolling rate in the present invention is 3.
It is 0-50%.

As explained above, the key point of the present invention is to set the finishing reduction rate to 20 to 6ON in the temperature range from A, r3 transformation point to 600°C, and at temperatures below 600°C, rolling falls into the cold rolling range. Therefore, the texture of the obtained steel sheet has many K(111) planes parallel to the rolling surface, similar to cold rolled steel sheets, and the OCv value, which is an index of deep drawability such as the 〒 value, is extremely excellent. However, on the other hand, extremely high rolling power is required for rolling at temperatures below 600°C.
Energy-saving manufacturing becomes impossible.

Furthermore, in direct rolling, it is extremely difficult to maintain the high-temperature slab above the KAr3 transformation point until it is rolled in rough and finish rolling, even if heat is retained during transfer.
The higher Ar3 transformation point in direct rolling of the piece ~ 6
It is characterized by the discovery of new rolling conditions that can produce hot-rolled steel sheets for general processing with excellent f' in the temperature range up to 00°C, and in addition, the target sheet thickness is 1.
2 to 2.8111, which is a particularly advantageous manufacturing method. Therefore, it is sometimes very effective to compensate for the temperature of the end portion of the high-temperature slab, where the temperature drop is significant, by using an induction heating method or the like.

Next, the reason why the winding temperature is limited to 700 to 500℃ is 70.
This is because if it exceeds 0°C, crystal grains and carbides become coarse, making it impossible to obtain a good material, and at the same time, the pickling properties of the scale deteriorate, which significantly reduces the production efficiency of the pickling line.

Further, if the temperature is lower than 500°C, recrystallization softening will not occur, and the material will become hard, making it impossible to obtain the desired steel plate. In addition, in the present invention, after winding, annealing at a temperature range of 700'' to 550°C is used instead of the cooling means K, but the reason for the temperature limitation is shown in Table 1, which will be described later. Comparative method (comparative method 1), in which a steel ingot is bloom-rolled, charged into a heating furnace, reheated, and then hot-rolled (comparative method 1); and a method in which continuously cast high-temperature slabs are directly rolled under conditions other than those of the present invention (comparative method). This is a table showing the actual values of 2). Low carbon steel slabs with the components shown in Table 2 are rolled under the respective rolling conditions, and the properties of the products are determined by the tensile test values in the direction perpendicular to rolling using a JI No. 85 tensile test piece. The transformation point of the steel was 820°C.

Further, the finishing rolling reduction ratio in Table 1 indicates the rolling rolling ratio at the final finishing roll stamp P.

Now, the growth of Comparative Method 2 in Table 1 is about 10% compared to Comparative Method IK.
In contrast, the method of the present invention shows a value equivalent to that of Comparative Method 1, and the value of After winding 6
In this example, annealing was performed at 50°C for 1 hour, but due to the low coiling temperature of 500°C, some processed structure remained in the crystal structure of the steel sheet, so annealing at 650°C for 1 hour was performed to completely recover. When carrying out the method of the present invention for crystallized materials, if the conditions are such that processed structures remain, it is desirable to ensure quality by annealing.

In this case, the annealing will not be sufficiently recrystallized unless the temperature is 550°C or higher, and if it exceeds 700°C, crystal grains tend to become coarser, and this is also disadvantageous from a thermoeconomic standpoint. This is the reason why the annealing temperature is limited to 700 to 550°C.

As explained above, the method of the present invention provides a more economical method for producing hot-rolled steel sheets for general processing with excellent quality by direct rolling, which directly connects the continuous casting process and the rolling process.

[Brief explanation of the drawing]

FIG. 1 is a graph showing the correlation between the finishing rolling reduction and the elongation in the direction perpendicular to rolling. Agent: Patent attorney Masamitsu Akizawa and 2 others

Claims (1)

[Claims] (1) OO,005~0.15X, Si0
．． 03% or less, Mn 0.10-0.60%, balance F
When a high-temperature slab of 900 to 1150 °C obtained by continuous casting of steel containing e and unavoidable impurities is directly rolled in the rough and finish rolling processes as it is or by heating the edges.
, 20 in the temperature range from hrs transformation point temperature to 600°C
Rolling to the final plate thickness at a finishing reduction of ~60%, followed by winding into a coil at a winding temperature of 700°C to 500°C, and then cooling or annealing at a temperature range of 700°C to 550°C. A method for producing low carbon hot rolled steel sheets characterized by: