JPS6012121B2 - Manufacturing method of low carbon hot rolled steel sheet - Google Patents

Description

[Detailed Description of the Invention] The present invention relates to a method for manufacturing low carbon hot rolled steel sheets, particularly thin steel sheets, and its purpose is to provide a method for manufacturing low carbon hot rolled steel sheets that consumes less heat and has no defects in quality. It's about doing.

As is well known, in order to produce low-carbon hot-rolled steel sheets, a method has been adopted in which low-carbon steel slabs are heated to a high temperature of 1,200 to 1,300 degrees and then hot rolled. In addition to the purpose of reducing rolling power by taking advantage of the low deformation resistance of The goal is to have 3 or more metamorphosis points.

In other words, as is well known, when the finishing temperature of a hot-rolled steel sheet falls below the Ar3 transformation point, the quality of the steel deteriorates.
00 to 130,000, and the finish rolling temperature is set to the Ar3 transformation point or higher to randomize the crystal orientation of the rolled steel sheet to obtain a good material quality, as is widely known. Therefore, when rolling is started with the slab heating temperature set to a low temperature such as 900 to 1150°C, ultra-high speed rolling or high reduction rolling or mid-rolling in which the slab is rolled in a small number of roll stands, for example, 1 to 3 roll stands, or mid-rolling is performed. It is common knowledge in the field of rolling technology that unless measures such as reheating are taken, the finish rolling temperature will be below the Ar3 transformation point, resulting in coarsening of crystal grains and a very poor quality of the steel sheet material. Met. By the way, the present inventors have found that, contrary to the common knowledge, the above-mentioned A
We have found a method that is comparable to steel sheet materials rolled at a temperature higher than the r3 transformation point. The means is Fune 3 metamorphosis point ~ 600q
Another method is to roll the sheet to the final thickness at a finishing reduction rate of 20 to 60% in a temperature range of 20 to 60%, then wind it into a coil and leave it to cool. This is a method of rolling to the final thickness at a finishing reduction of ~60%, followed by annealing at a temperature range of 550~700°C.

Thus, if it is possible to roll in the temperature range from the Ar3 transformation point to 60 and 0, the heating temperature of the low carbon steel slab before rolling can be significantly lowered.

The fact that the heating temperature can be lower means that less thermal energy is consumed, which is extremely advantageous economically. Now, in order to heat a low carbon steel slab to a high temperature, a continuous heating furnace generally uses a heat amount of 300,000 to 600,000 kcal/ton. The above heat value is 1200~i30 for a low carbon steel slab at room temperature.
This is the amount of heat required to raise the temperature to 0.000, but if the temperature can be lowered even a little, the amount of heat required can be reduced accordingly. The heating temperature of the low carbon steel slab according to the present invention is 100 to 4
One of the key points is that hot rolling is carried out at a low heating temperature, and therefore it has an economically advantageous feature of low heat consumption. In the experience of the present inventors, the conventional method
When comparing the case where hot rolling was started at 250oo and the start of hot rolling at 1050oo according to the method of the present invention, it was found that the difference in the amount of heat required up to that point reached 80,000 kcal/ton. Therefore, the deformation resistance of the low carbon steel slab increases as the heating temperature decreases, and the rolling power during rough rolling and finish rolling increases, but the increased energy amount is approximately 10,000 kcal/ton in calorific value, In the case of the above example, the amount of heat saved is 80,000 kcal/ton and the amount of added heat is ca.
The difference in l/ton, 70,000 kcal, is the benefit of the present invention. Now, the reason why the heating temperature of the low carbon steel slab is limited to 900 to 115,000 in the present invention is that
If it is less than 1150 qo, the heat saving effect is remarkable, but as a result of the sharp increase in the deformation resistance of the slab, the rolling power becomes too large and the economic effect is lost.If it exceeds 1150 qo, the heat saving effect disappears and the present invention This is because one of the purposes of this is not fulfilled.

Furthermore, in the present invention, CO. 15% or less, Sio. 01
% or less "The reason for limiting Mno. to 60% or less is that all of these elements harden the steel, so if added in excess of the above value, the material quality of the steel plate aimed at in the manufacturing method of the present invention will deteriorate. This is because it becomes unusable for processing such as press working.

Also, the reason why the thickness of the rolled steel plate is limited to 12 mm or less is 1.
At the same time, it becomes difficult to wind the rolled steel sheet into a coil when the second side is used, and at the same time, the technology of the present invention is applied to thin hot-rolled steel sheets, such as those with a wall thickness of 1.2 to 2.3, where the finishing temperature tends to drop. This is because only then can we expect significant economic effects. Next, the present invention will be explained in more detail with reference to the drawings.

Figure 1 shows the cold forming performance (stretchability, deep drawability) conical cup test values (hereinafter abbreviated as CCV) of rolled steel sheets, with finish rolling temperature plotted on the machine axis and finish rolling reduction rate plotted on the vertical axis.
This is an illustration. Here, the final exit temperature refers to the temperature of the steel sheet immediately after leaving the final finishing roll, and the finishing reduction rate refers to the rolling reduction rate at the final roll stand. In Fig. 1, the steel plate located at the finish rolling temperature of 900°C is the conventional A steel plate that was rolled at a temperature higher than the transformation point, and the CCV is 59.7 to 59.8, which is a good value. .

In the figure, the ◎ mark indicates that the CCV is on the S59.6 side, and the 0 mark indicates that the CCV is on the S59.6 side.
7-59. Artery, △ mark indicates 60.0-60.3 armpit, × mark indicates 60.4 rib. The next highest rolling temperature is 650-85
In the temperature range of 000, which corresponds to the two-phase coexistence of ferrite + austenite, CC at a finishing reduction rate of around 10%
V is very bad. This corresponds to the case where the finish rolling temperature is below the Ar side transformation point in rolling in the secondary method. On the other hand, when the finish rolling temperature is 650 to 850°C and the finish rolling reduction is 20 to 55%, the CCV shows a good value. In this case, a finishing reduction of 30-50% gives the best results. To explain this point in detail, in the case of secondary hot rolling, the rolling reduction ratio in each stand of the Rentsugi finishing mill is such that the rolling reduction ratio is large in the earlier stand and gradually decreases in the later stages. When using 6 machines to achieve a finished thickness of 2.3 ribs, "No. 1 stand approx. 50%, No. 2 stand 40%, No. 3 stand 37%, No. 4 stand 2
8%, no. 5 stands 26%, No. 6 stands 15
%, and the rolling reduction at the final stand is usually in the range of 10 to 18%.

On the other hand, in the present invention, by setting the finishing reduction rate to 20 to 60% (the best range is 30 to 50%), the material of the final cold rolled steel sheet can be heated to a temperature of 700% below the Ar3 transformation point.
It was developed based on the new knowledge that it is good even at around 0.00. That is, as shown in FIG. 1, the steel plate produced by the method of the present invention is comparable to the rolled steel plate produced by the conventional method.

According to research conducted by the present inventors, it has been confirmed that when the finishing reduction rate is set to 60% or more, the CCV deteriorates, although the reason is unknown. This sets the upper limit of the finishing reduction rate to 60 in the present invention.
This is the reason why it is set as %. Next, at the finish rolling temperature 600
In the region below oo, rolling falls into the range of cold rolling, and the texture of the obtained steel sheet is parallel to the rolling plane (1
11) A texture with many planes is obtained, and the CCV is very good.

However, if the finish rolling temperature is less than 600 oo, the power required for rolling increases rapidly, which is economically disadvantageous and does not meet the purpose of the present invention. Also, when comparing the cases where the finish rolling temperature is 80,000 and 70,000, it is 800℃.
shows better results with a lower finishing reduction ratio.

As detailed above, the present invention has a remarkable feature in that the heating temperature of a low carbon steel slab is lowered by about 100 to 400 qo compared to conventional methods, and it has the following advantages. .

'a} The total amount of energy required to manufacture product steel sheets can be significantly reduced.

{b' Since the heating temperature is low, advantages such as reduction in equipment costs and repair costs for the heating furnace, reduction in the amount of scale formed on the slab, and reduction in the amount of wear of various rolls can be obtained.

[c- MnS
It has a favorable effect on metallurgical conditions such as the precipitation state of AIN and the distribution state of inclusions, and is advantageous in terms of material quality.

Therefore, in so-called thin low carbon hot rolled steel sheets having a finish thickness of 1.2 to 2.3 legs, the temperature decreases significantly during rolling and the finish rolling temperature tends to drop below the Ar3 transformation point.

In such cases, the present invention can have particularly significant effects. Next, Table 1 and Table 2 show a comparison of the rolling conditions and heating unit (heat amount) when steel plates are manufactured from low carbon rimmed slabs by the method of the present invention and the method of the subordinate method, and in Table 3, A comparison of the mechanical properties of the obtained steel sheets is shown below.

However, the steel composition in the examples shown in Table 1 is CO. 05%,
Sio. 01% "Mno.32% and finished plate thickness is 1.8
It's the skin.

Further, the examples shown in Table 2 have a steel composition of 0.11% and Sio. 0
1%, Mno. At 45%, the finished board thickness is 2.3 willow.

Note that the finishing rolling reduction in Tables 1 and 2 indicates the rolling reduction at the final finishing roll stand. According to the method of the present invention shown in Table 1, after winding at 530°C,
65,000 for 1 hour, and the increase in heat amount at this time was 20,000 kcal/t.

Therefore, in the final heat calculation, it is necessary to add the above-mentioned increment of 20,000 kcal/ton. Now, 10,000 kcal of heat is considered to be equivalent to about 6 kWh when calculating electric power.
From the results shown in Tables 1 and 2, it is clear that the method of the present invention can save about 60,000 kcal/ton of heat compared to the conventional method.

The steel sheets manufactured by the method of the present invention have slightly larger grains and are less expensive than those manufactured by the conventional method, but because the finishing reduction is large, there is not much difference in tensile test value, CCV, and elongation, and it can be used for similar purposes. It may be used for.

Recently, in addition to the method of blooming a steel ingot, cooling it, reheating it, and then hot rolling it, there are also methods in which hot rolling is performed without cooling and reheating after blooming, and in continuous casting, hot rolling is performed directly after the lotus gun. A method of manufacturing steel plates by rolling came to be adopted.

Even in this case, the method of the present invention can be easily applied because finish rolling can be carried out at low temperatures, and great economic benefits can be obtained. Therefore, the competitive anchor according to Example 2 is
Since the winding temperature is as low as 530°C, some processed structure remains in the crystal structure of the steel sheet, so the steel plate was subjected to moth dulling at 650 qo x lhr to completely recover and recrystallize, and the method of the present invention was carried out. In the case of conditions where processed structures remain, it is desirable to ensure quality by slowing down, for example, if the winding temperature is lowered.

In this case, the hammer will not recrystallize sufficiently unless the temperature is 550° C. or higher, and if it is 700° C. or higher, crystal grains tend to become coarser, which is also disadvantageous from a thermoeconomic standpoint. This is the reason why the competitive anchor temperature is limited to 550 to 700 degrees. Next, Table 3 shows a comparison of the mechanical properties of the steel plates obtained in the examples shown in Tables 1 and 2 as described above.
In Example 2, the value is 37 type.

As is clear from Table 3, "The mechanical properties of the steel plate obtained by the method of the present invention are comparable to those obtained by the conventional method.

As detailed above, the present invention provides an extremely economically advantageous means for producing low carbon hot rolled steel sheets.

[Brief explanation of drawings]

FIG. 1 is a graph showing the CCV of steel plates manufactured by changing the finishing rolling temperature and finishing rolling reduction ratio. O diagram

Claims (1)

[Claims] 1 C 0.15% or less, Si 0.01% or less, Mn 0.
After heating a low carbon steel slab of 60% or less to 900 to 1150°C, the final exit temperature Ar is applied when rolling it in a continuous finishing mill.
_3 12 m so that the final roll stand finishing reduction rate is 20 to 60% in the temperature range from transformation point temperature to 600℃
After rolling to a final thickness of less than m, and then winding the coil,
A method for producing a low carbon hot rolled steel sheet, which comprises allowing the steel sheet to cool or annealing at a temperature of 550 to 700°C after rolling.