JPH0587563B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a manufacturing method for obtaining a low carbon steel sheet with excellent press workability, especially drawability, and surface quality. Cold-rolled steel sheets and hot-rolled steel sheets are used for press working. When comparing hot-rolled steel sheets and cold-rolled steel sheets, elongation and overhang properties are almost the same, but
Hot-rolled steel sheets have poor deep drawability compared to cold-rolled steel sheets. The reason for this is that the Rankford value (), which indicates the quality of deep drawability, is that cold rolled steel sheets can develop a texture suitable for deep drawing through cold rolling and recrystallization annealing. On the other hand, hot-rolled steel sheets have a nearly random texture due to Ar ₃ transformation, and therefore hot-rolled steel sheets have significantly lower grain sizes than cold-rolled steel sheets, about 0.7 to 0.9. Therefore, if a hot-rolled steel sheet with a value of 1 or more can be manufactured, it will have the same deep drawability as a cold-rolled steel sheet, and as a result, it will be able to withstand press forming as well as a cold-rolled steel sheet, which is an advantage. There is something very big about it. The present inventors conducted various experiments to improve the deep drawability of hot rolled steel sheets, and found that by specifying the warm rolling conditions, it is possible to manufacture hot rolled steel sheets with improved press workability, especially deep drawability. Furthermore, it has been found that it is possible to produce a low carbon steel sheet with improved surface properties by further subjecting the hot rolled steel sheet to light reduction rolling after pickling. The experimental results of the inventors are shown in Figures 1, 2, and 3.
This will be explained with reference to FIGS. 4 and 5. In the experiment, C: 0.04wt%, Mn: 0.25wt%,
Al0.060wt% (All percentages below are wt%.)
Aluminium-killed steel is melted in a converter and continuously cast into slabs, then heated to 1100℃ and then heated at various temperatures.
After performing warm rolling at a rolling reduction rate, recrystallization treatment was performed under various conditions, and the value and Δr value were determined using a JIS No. 13 B test piece. Figure 1 shows the relationship between warm rolling temperature and value when the rolling reduction ratio is kept constant at 80% (recrystallization treatment is performed by maintaining the temperature at 600°C for 2 minutes after warm rolling).
As is clear from this figure, a value of 1.0 or more can be obtained at a warm rolling temperature of 700°C or less. Figure 2 shows the relationship between rolling reduction and value when the warm rolling temperature is kept constant at 650°C (the recrystallization treatment conditions are the same as in Figure 1), and as is clear from this figure, The value is 1.0 or more at a reduction rate of 80% or more. Furthermore, Figure 3 shows the relationship between the rolling reduction of the final stand and the anisotropy (△r), and from this figure, a rolling reduction of 20% or more is required to reduce the anisotropy. . Next, for the product finished at a warm rolling temperature of 680°C and a rolling reduction of 83%, the coiling temperature was varied from 365°C to 650°C, and the values were measured. The results are shown in FIG. As is clear from this figure, a value of 1.0 or more is obtained when the winding temperature is 600°C or higher. Furthermore, in order to clarify the recrystallization treatment conditions for those with a rolling temperature of 600°C or lower, the warm rolling temperature is 550°C,
Values were measured for products that were finished and rolled at 430°C with a rolling reduction of 82%, and the temperature during reheating was varied from 400°C to 750°C, and the heating time was varied from 1 to 3 minutes. The results are shown in FIG. As is clear from Figure 5, the higher the heating temperature and the longer the heating time, the higher the value.
A value of 1.0 can be secured by heating at ℃ or above for 2 minutes or more. The present invention was made based on the above knowledge,
The gist is that at least C, Mn, acid-soluble Al
At least 20 minutes have elapsed since the temperature of the aluminum killed slab or steel slab with C: 0.08wt% or less, Mn: 0.4wt% or less, and acid-soluble Al: 0.02wt% or more is lowered to below the Ar ₃ transformation point. After heating the hot slab or hot steel slab to 900 to 1150°C to precipitate aluminum nitride, the cumulative reduction rate is 80% or more in a temperature range of 700°C or lower to 400°C or higher, and Rolling is performed with a reduction ratio of 20% or more in the final pass,
A method for producing a low carbon steel sheet with excellent press workability, which is characterized by subsequent recrystallization treatment, and at least C, Mn, and acid-soluble Al, C: 0.08 wt% or less, Mn: 0.4 wt% or less, and acid. Soluble Al: 0.02wt%
The temperature of the aluminum killed slab or steel slab is
After cooling the temperature to below the Ar ₃ transformation point and allowing at least 20 minutes to pass, the hot slab or hot steel slab is
After heating to ~1150℃ to precipitate aluminum nitride, the cumulative reduction rate is 80% or more in the temperature range of 700℃ or lower to 400℃ or higher, and the final pass reduction rate is
After rolling to 20% or more, recrystallization treatment is performed,
This is a method for producing a low carbon steel sheet with excellent press workability, which is characterized in that it is then pickled and then rolled at a rolling reduction of 1 to 10%. Although a method for improving the drawability of aluminum-killed steel sheets manufactured by hot rolling has not yet been implemented industrially, in the present invention, hot rolling is performed under the rolling conditions described above, and specific treatment is performed after hot rolling. By this method, the drawability of the aluminum killed steel sheet could be improved. The present invention was achieved as a result of the systematic investigation of recrystallized grain formation by the present inventors, including the former experiments. This is because we were able to find that a steel sheet with a large rolling reduction in the ferrite region and the accumulation of strain energy necessary for recrystallization exhibits the specified characteristics that a steel sheet for drawing should have. Each requirement of the method of the present invention will be explained in detail below. First, in the present invention, precipitating aluminum nitride on aluminum killed slabs or steel slabs before warm rolling is a hindrance to growing crystals in a direction favorable to drawability during recrystallized grain formation. Add nitrogen solid solution in steel to 900 to 1150 of cast slabs or steel slabs.
This is to precipitate aluminum nitride by heating it to a temperature of ℃, thereby highly cleaning the steel. Next, the temperature range for warm rolling is 400°C to 700°C.
This is because the internal strain required for recrystallization decreases due to the recovery phenomenon at temperatures above 700℃, as shown in Figure 1, and the internal strain necessary for rolling decreases below 400℃ because it enters the blue brittle range. This is because the power increases rapidly, which is economically disadvantageous. Furthermore, the reduction ratio in this warm rolling was set to 80% or more (see Figure 2) in order to secure the strain energy necessary for sufficient recrystallization to occur in the subsequent recrystallization treatment, and to maintain a value of 1.0 or more. This is because it is necessary to control the texture to obtain the desired results, and the reason why the reduction ratio of the final stand was set to 20% or more (see Figure 3) was to keep the anisotropy within a practically harmless range. be. After the above nitrogen precipitation treatment, it is warm rolled under the above conditions to give internal strain for recrystallization, making it easy to recrystallize, so it is a hot rolled steel sheet with a high drawing value. It can be obtained by performing a simple recrystallization treatment after the warm rolling. For this recrystallization treatment, if the warm rolling temperature is 700℃ to 600℃, it is sufficient to simply roll it up and recrystallize it (see Figure 4). can get. Also, the warm rolling temperature is 600℃~300℃
In the case of ℃, it is heated to 600℃ or higher for 2 minutes or more to perform recrystallization treatment, and then rolled up. (Refer to Figure 5) The steel sheet manufactured in the above manner has excellent drawability, but also has a reduction rate of 1 to 1 after pickling.
By applying a light reduction of 10%, a deep-drawing steel sheet with a surface quality (surface roughness) comparable to that of a cold-rolled steel sheet is manufactured. The rolling ratio for light rolling was set to 1% or more and 10% or less because if it is less than 1%, it will not be possible to obtain the same roughness as a cold rolled steel sheet.
On the other hand, if it exceeds 10%, the material quality will deteriorate significantly due to processing strain. The aluminum-killed steel sheet of the present invention is cast by a normal melting process including continuous casting, and the subsequent ingot making and blooming are not particularly limited and are manufactured by normal methods. The C content of the aluminum killed steel sheet to which the method of the present invention is applied is 0.08% or less,
The reason why Mn is limited to 0.40% or less is that ductility deteriorates if it exceeds this limit, and acid-soluble Al is limited to 0.02%.
% or more in order to precipitate it as aluminum nitride and obtain the steel sheet that is the object of the present invention. Examples of the present invention will be described below in comparison with comparative examples. Example 1 Aluminum killed steel with specified contents of at least C, Mn, and Al as shown in Table 1 was melted in a converter, and a slab manufactured by a normal method was used as Example A of the present invention.
~F, after precipitation treatment of aluminum nitride, the cumulative reduction rate is 80 in the temperature range of 700℃ to 400℃.
% or more, and after warm rolling at a final pass reduction rate of 20% or more, recrystallization treatment was performed. As a result, A to F
The values of each steel sheet were 1.12 to 1.19, which exceeded 1.0, and were almost the same as those of conventional cold-rolled steel sheets for deep drawing. In comparison, C with a content within the range of the present invention,
Although it is a steel material with Mn and Al, the reduction rate of the final pass is
In the comparative example, which is a steel plate with less than 20% rolling reduction, the Δr value is 0.52 and the anisotropy is large. In the comparative example, the reduction ratio during warm rolling was outside the range of the present invention, and the accumulation of internal strain was insufficient, resulting in a value of only 0.82. In the comparative example, the temperature during warm rolling was outside the range of the present invention, and the internal strain decreased due to recovery, resulting in a value of only 0.80 to 0.85. The comparative example was a steel plate heated at a temperature of 1240°C, and since nitrogen was dissolved in solid solution during the heating stage, a value of only 0.92 was obtained even though the warm rolling conditions were within the range of the present invention.

[Table] Comparative examples are steel plates that were not subjected to recrystallization treatment after warm rolling and steel plates that were treated for a short time, and only low values were obtained for both value and elongation. Example 2 A steel plate having steel coil number A shown in Table 1 was pickled and then subjected to light reduction rolling at various reduction rates of 12% or less. Table 2 shows the surface roughness and mechanical properties of these steel plates. Coils Nos. A2 to A5 subjected to light reduction rolling at a rolling reduction within the range of the present invention have surface roughness approximately equal to that of cold-rolled steel sheets, and there is little material deterioration due to light reduction rolling. Compared to these, in the comparative example in which light reduction rolling was not performed, the surface roughness after pickling was 1.43 μm, which is very large. In the comparative example, the light reduction rolling rate was set to 12%, and although the surface roughness was good, the tensile strength and yield strength were significantly increased due to processing strain, and the elongation was deteriorated.

[Table] In the above examples, three of C, Mn, and Al are used.
Although only the components are shown, in carrying out the present invention, Ti, which is commonly used, is used to fix free nitrogen as nitride, which has a negative effect on workability.
It is preferable to add one or more of B, Nb, Zr, V, etc., and it is desirable to reduce [P] to 0.01% or less for the purpose of improving workability. As explained above, according to the method of the present invention, during the hot rolling stage, internal strain is accumulated in the crystals in the steel sheet in the {111} plane, which is advantageous for drawability, by warm rolling within a specific range of conditions. Since the growth is caused by a subsequent recrystallization treatment, hot-rolled steel sheets with the same performance as cold-rolled steel sheets for drawing, which were conventionally manufactured by performing batch annealing and continuous annealing after cold rolling, are produced. It is possible to produce a hot-rolled steel plate for press working which has a surface quality equivalent to that of a cold-rolled steel plate by subsequent pickling and light reduction. According to the method of the present invention, the manufacturing process is shorter than in the case of cold-rolled steel sheets, and steel sheets for press working can be manufactured, which not only makes it possible to reduce costs but also greatly contributes to energy conservation. Therefore, the effect brought about by the method of the present invention is very large.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between finish rolling temperature and value;
Figure 2 is a diagram showing the relationship between cumulative rolling reduction rate and value, Figure 3
The figure shows the relationship between the rolling reduction ratio of the final stand and the △r value, Figure 4 shows the relationship between the winding temperature and the value, and Figure 5 shows the reheating temperature after winding, the heating time, and the r value. It is a figure showing the relationship of values.

Claims (1)

[Claims] 1. At least C, Mn, and acid-soluble Al, C:
0.08wt% or less, Mn: 0.4wt% or less, acid soluble
After lowering the temperature of the aluminum killed slab or steel slab with Al: 0.02wt% or more to below the Ar3 transformation point and allowing at least 20 minutes to pass, the hot slab or steel slab is heated to a temperature of 900 to 1150.
After heating to ℃ to precipitate aluminum nitride,
Press workability characterized by rolling with a cumulative reduction rate of 80% or more in the temperature range from 700℃ or less to 400℃ or more, and a rolling reduction rate of 20% or more in the final pass, followed by recrystallization treatment. Excellent method for producing low carbon steel sheets. 2. A low carbon material with excellent press workability according to claim 1, characterized in that the rolling agent is rolled up at 600 to 700°C and then recrystallized by maintaining it at 600°C or higher for 2 minutes or more. Method of manufacturing steel plates. 3. Excellent press workability according to claim 1, characterized in that the material to be rolled is rolled up at 300 to 600°C, then heated to 600°C or higher and maintained for 2 minutes or more for recrystallization treatment. A method for producing low carbon steel sheets. 4 At least C, Mn, acid-soluble Al, C:
0.08wt% or less, Mn: 0.4wt% or less, acid soluble
After lowering the temperature of the aluminum killed slab or steel slab with Al: 0.02wt% or more to below the Ar3 transformation point and allowing at least 20 minutes to pass, the hot slab or steel slab is heated to a temperature of 900 to 1150.
After heating to ℃ to precipitate aluminum nitride,
After rolling with a cumulative reduction rate of 80% or more in the temperature range of 700℃ or less to 400℃ or more and a final pass reduction rate of 20% or more, recrystallization treatment is performed, followed by pickling, followed by rolling. A method for producing a low carbon steel sheet with excellent press workability, which is characterized by rolling at a rolling ratio of 1 to 10%. 5. A low carbon material with excellent press workability according to claim 4, characterized in that the rolling agent is rolled up at 600 to 700°C and then recrystallized by maintaining it at 600°C or higher for 2 minutes or more. Method of manufacturing steel plates. 6. Excellent press workability according to claim 4, characterized in that the rolling agent is rolled up at 300 to 600°C, then heated to 600°C or higher and maintained for 2 minutes or more for recrystallization treatment. A method for producing low carbon steel sheets.