JPH02415B2 - - Google Patents

Description

[Detailed description of the invention]

This invention relates to a method for producing cold rolled steel sheets with good drawability by continuous annealing. Cold-rolled steel sheets with good drawability have conventionally been produced mainly from low-carbon aluminum killed steel by box annealing. However, with the box annealing method, not only does the process take several days, but since the coil is heat treated, the heating and cooling rates vary in the radial direction of the coil, making it difficult to obtain a uniform material throughout the coil. It was hot. On the other hand, if a continuous annealing method is used, it is possible to eliminate the above-mentioned drawbacks caused by the box annealing method. However, continuous annealing involves rapid heating and rapid cooling, which results in poor crystal grain growth, and the precipitation of C dissolved in solid solution in the steel does not progress, resulting in hard steel with poor drawability and aging resistance. . In order to eliminate these drawbacks of the continuous annealing method, as disclosed in Japanese Patent Publication No. 1341/1983, grain growth in the direction advantageous for drawability is promoted by winding at a high temperature during hot rolling, and A method has been proposed in which after rapid cooling during continuous annealing, overaging treatment is performed at 300°C to 500°C for several seconds to several minutes to promote precipitation of unprecipitated solid solution C and improve aging resistance. However, high-temperature winding during hot rolling not only causes a decrease in pickling properties, but also the drawability of cold-rolled steel sheets produced by such a method is inferior to that of the end box annealed material. On the other hand, since the main cause of deterioration of the aging resistance of continuously annealed materials depends on solid solute C, the C content is
A method has also been proposed to improve aging resistance using ultra-low carbon steel material with carbon content reduced to 0.0050% or less. By the way, when manufacturing steel sheets for drawing,
High rankford value (value = (r ₀ + 2r ₄₅ +
In order to obtain r ₉₀ )/4), it is essential that the hot rolling finish be completed at the Ar ₃ transformation point or higher. This is because, regardless of whether the box annealing method or the continuous annealing method is used, when hot rolling is completed below the Ar ₃ transformation point, that is, in the two-phase region of α (ferrite) + γ (austenite) or in the α region, the drawability will deteriorate. unfavorable (110),
This is because the (100) orientation develops parallel to the sheet surface, which is thought to prevent the (111) orientation, which is advantageous for drawability, from developing parallel to the sheet surface during annealing. As mentioned above, extremely low C content is advantageous in terms of aging resistance, but it also increases the Ar ₃ transformation point, so during hot rolling finishing, γ
In order to finish finish rolling in the (austenite) region, it is necessary to increase the slab heating temperature and change the hot rolling reduction schedule, which is a big disadvantage in terms of energy saving. Moreover, as mentioned above, in order to improve aging resistance, drawability, and ductility by reducing solid solution C, hot-rolled coils are wound at high temperatures and the amount of C is reduced.
Many proposals have been made in the past regarding reducing C to an extremely low range of 0.01% or less, and furthermore, on appropriate combinations of annealing conditions after cold rolling. (r ₀ −2r ₄₅ +r ₉₀ )/2) became large, and both methods were still insufficient as methods for producing steel with good drawability. As a result of repeated research, the inventors found that steel with a high Al content can be manufactured with a hot-rolled finish that has drawability equal to or higher than the Ar ₃ transformation point, even if the hot-rolled finishing temperature is below the Ar ₃ transformation point. We have established a method that can advantageously produce steel plates with a material quality comparable to or higher than that of existing steel plates. In other words, when ultra-low carbon steel with a large amount of Al added is used as a material, when the slab heating temperature and hot rolling conditions are within a certain range, good drawability can be achieved even with a hot rolled finish lower than the Ar ₃ transformation point. They discovered that steel plates could be obtained. This invention is based on Sol.
A steel slab containing Al: 0.10 to 0.20%, with the remainder consisting essentially of iron and unavoidable impurities.
After heating to a temperature below 1150℃, finishing temperature 700℃
By performing hot rolling under the conditions of ~850℃ and a coiling temperature of 600℃ or less, then cold rolling according to a conventional method, and then continuous annealing, the above specific composition and grain growth properties during annealing of the steel sheet material are determined. Furthermore, the hot rolling conditions were adapted to be effective in developing the (111) texture, which affects drawability. The development history of this invention will be explained in detail below in accordance with the experimental results. Experiment () C: 0.0024%, N: 0.0039%, Mn: 0.15%,
Contains Sol.Al: 0.12%, P: 0.003%, S:
Slab heating temperature 1000 ~ using 0.005% small steel ingot
The hot-rolled finishing temperature was varied in the range of 580 to 960°C at 1300°C or lower, and the coiling equivalent temperature was processed at 700°C or lower, followed by cold rolling and continuous annealing in the laboratory, and the material properties were investigated. . Continuous annealing is annealing in a continuous annealing simulator, and the heat cycle is 30
After rapid heating at ℃/s and holding at 800℃ for 40 seconds,
It rapidly cools at 30°C/s. The value, Δr value, was used as a measure of drawability. The results are shown in FIGS. 1, 2 and 3. First, when the slab heating temperature is 1150°C or lower and the hot rolling finishing temperature is lower than 850°C, which is below the Ar ₃ transformation point, the value is highest as shown by the ○ and △ marks in Figures 1 and 3, and the Δr value is extremely high. Smaller, better characteristics,
A material with low anisotropy was obtained. Next, according to Figure 3, hot rolling finishing is carried out at 600°C to 850°C.
It can be seen that when the hot rolling is completed in the temperature range of , both the Δr values are good, and the material quality is particularly good when the hot rolling finishing temperature is 700 to 850°C. The reason for this is not clear, but only a portion of the processing strain introduced by hot finishing at a temperature lower than the _Ar3 transformation point and AlN
It is thought that this precipitation had some effect on the material, resulting in a better quality of the material. Hot rolling finishing temperature is 600~850
℃, especially in the range of 700 to 850℃, the quality of the material when wound at a high temperature of about 700℃ is worse than that when wound at a low temperature, and there is no improvement in material quality due to high temperature winding. Thus, high-temperature web material was accompanied by an increase in descaling cost during pickling, whereas this invention
By performing hot rolling finishing at a low temperature below the Ar ₃ transformation point, it is possible to obtain a material that far exceeds that of high-temperature rolled material even when rolled at a low temperature, which is particularly advantageous in terms of reducing pickling costs. . In this invention, the upper limit of the winding temperature during hot rolling is 600°C in order to prevent cost increases during pickling. Experiment () Sol.Al was varied in the range of 0.025 to 0.24% based on the component composition containing C: 0.0021%, N: 0.0037%, Mn: 0.16%, P: 0.003%, S: 0.005%. The steel was manufactured in the laboratory, and based on the results of the experiment (), the slab heating temperature was 1100℃, and the hot rolling finishing temperature was
Hot rolling was completed under conditions of 800°C and a coiling equivalent temperature of 520°C, and the plate thickness was 3.2 mm. After that, it was cold-rolled to a thickness of 0.8 mm and continuously annealed, and the effect of Al on the value was investigated. The results are shown in Figure 4. According to the diagram, Sol.
Materials with good deep drawability were obtained within the range of 0.10 to 0.20% Al. Next, the reason why the composition of steel is limited in the method of this invention will be explained. The upper limit of the C component range is set at 0.0030%, since if it exceeds 0.0030%, grain growth and deep drawability will be reduced, as well as aging properties. The reason why the upper limit of N was set at 0.0050% is that adding more than this will significantly suppress the growth of crystal grains during annealing and reduce drawability.In order to further improve drawability, 0.0050% The following restrictions are required. Mn is effective during hot rolling because it turns S, which causes red-hot brittleness, into inclusions, but the presence of more than 0.5% increases hardness and has a negative effect on deep drawability. It was set at 0.5%. Regarding the limitation of Sol.Al, under conditions where the heating temperature of the steel slab is low, and the hot rolling finishing temperature and coiling temperature are also low, as the amount of Al increases, the AlN
It was found that due to the synergistic effect of the coarsening of aluminum and the increase of solid solution Al, the grain size of the annealed plate becomes larger and the material becomes softer. However, if the amount of Sol.Al increases too much, not only will the addition cost increase, but the material will become hard, so the upper limit is set at 0.20%. On the other hand, if it is less than 0.10%, AlN will precipitate finely, grain growth will deteriorate, and the texture {111}, which is advantageous for drawability due to slab low-temperature heating and low-temperature finish rolling, as expected in the present invention, will not develop. So the Sol.Al range is 0.10~0.20%
shall be. Further, P and S contained as unavoidable impurities must be reduced as much as possible because they deteriorate drawing properties, but are not particularly stipulated. As described above, in this invention, a steel slab having the above composition is made into a hot-rolled plate by low-temperature heating, hot-rolling and low-temperature finishing, and low-temperature winding, followed by pickling, cold rolling, continuous annealing, and cold rolling according to a conventional method. Obtain rolled steel plate. Note that there is no need to specify any particular conditions for continuous annealing, and as long as the highest temperature of the steel plate is equal to or higher than the recrystallization temperature, there are no particular restrictions on the heating rate, cooling rate, presence or absence of overaging, etc. Examples of this invention will be listed below, and the effects will be verified by comparing them with comparative examples. After tapping in the converter, RH degassing was performed for 20 minutes to tap 14 types of steel with different compositions, which were then continuously cast into slabs with a thickness of 200 mm. These slabs are heated to 1010° to 1240°C in a heating furnace, held for 60 minutes, and then heated to 700°C to 930°C by hot rolling.
Finish rolling is completed in the temperature range of 520℃, and
It was made into a 3.2mm hot rolled coil. Table 1 shows the steel composition, slab heating temperature, and hot rolling finishing temperature.

【table】

[Table] After pickling, the coil was cold rolled to a thickness of 0.8 mm and subsequently subjected to continuous annealing. The conditions for continuous annealing are a heating rate of approximately 15°C/sec, soaking at 800°C for 25 seconds, and a cooling rate of approximately 45°C/sec. Thereafter, it was subjected to about 0.8% temper rolling. Table 2 shows the materials of the steel sheets manufactured in this manner.

[Table] As is clear from this table, the cold-rolled steel sheets produced according to the present invention have excellent drawability (and Δr value). As described in detail above, this invention comprises C,
Using a steel slab with a specific composition of N and Al, the heating before hot rolling is kept at 1150℃ or less, and the finishing rolling temperature is
A method for producing cold rolled steel sheets, which comprises hot rolling at 700 to 850°C and a coiling temperature of 600°C or less, followed by continuous annealing of rapid heating and rapid cooling after cold rolling. This makes it possible to produce cold-rolled steel sheets with excellent drawability.

[Brief explanation of drawings]

Fig. 1 is a chart showing the effect of slab heating temperature on yield stress and value, Fig. 2 is a chart showing the effect of hot rolling finishing temperature on yield stress and total elongation,
FIG. 3 is a chart showing the effect of hot rolling finishing temperature on the value and Δr value, and FIG. 4 is a chart showing the effect of Sol.Al amount on the value.

Claims (1)

[Claims] 1 In terms of weight percentage, C: 0.0030% or less, N:
0.0050% or less, Mn: 0.5% or less, Sol.Al: 0.10
After heating a steel slab with a composition of ~0.20% and the remainder essentially consisting of iron and unavoidable impurities to a temperature of 1150℃ or less, it is hot-rolled at a finishing temperature of 700 to 850℃ and a coiling temperature of 600℃ or less. Perform rolling,
A method for producing a cold-rolled steel sheet with good drawability by continuous annealing, characterized in that cold rolling is then carried out according to a conventional method and then continuous annealing is carried out.