JPS59133325A - Manufacture of low carbon steel sheet with superior drawability - Google Patents

Abstract

PURPOSE:To obtain the titled low carbon steel sheet without passing through a cold rolling-recrystallization annealing stage by warm rolling an Al killed ingot or steel billet having a specified composition after making the ferrite grains fine, and carrying out recrystallization treatment. CONSTITUTION:An Al killed ingot or steel billet contg. <=0.08% C, <=0.40% Mn and >=0.02% sol. Al is treated to precipitate AlN, and the ferrite grain size is regulated to No.10 or above. The billet is rolled at >=70% draft at 400-700 deg.C and subjected to recrystallization treatment. After finishing the treatment, pickling and rolling at about 1-10% draft may be carried out.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a manufacturing method for obtaining a low carbon steel sheet with excellent deep drawing properties and surface properties.

Steel sheets for deep drawing must have excellent press formability, and for this purpose, they must have a high f value, that is, the sheet orientation component (111
) and (ioo) is important. Conventionally, the manufacturing method of steel sheets for deep drawing has generally included a method of cooling as soon as possible after hot finish rolling, followed by patch annealing in a predetermined gradual heating cycle after cold rolling, and a method described in Japanese Patent Publication No. 51-28052.
As shown in No. 1, there is a method of performing warm rolling followed by cold rolling, followed by rapid heating cycles such as continuous annealing. However, since the process is long in not only patch annealing but also continuous annealing, the manufacturing period is long.
It also has the disadvantage of increasing manufacturing costs.

The present inventors conducted various studies to determine whether it is possible to control the plate surface orientation component, which is a factor that determines the drawability, only through the hot rolling process without going through such a cold rolling-recrystallization annealing process. As a result, it was found that a steel sheet with excellent deep drawability could be obtained by refining the ferrite grains, then performing warm rolling and recrystallization treatment.

The experimental results of the present inventors will be explained based on FIGS. 1, 2, 3, and 4. The experiment was conducted with C: 0.04, Mn:
Aluminum killed steel with 0.25% At and 0.060% At was melted in a converter, and after continuous casting into steel slabs, the steel was heated to 1100℃.
The material was heated and hot rolled.

The first experiment was hot rolling using the front 4 of the 7 finishing stands at a reduction rate of 55% to 85°C so that the finishing temperature after 4-gus rolling was 810°C to 940°C. After rolling, it was immediately quenched to room temperature. The cross-sectional crystal grains of this steel plate were observed using an optical microscope.

FIG. 1 shows the influence of hot finish rolling conditions on grain size points. In FIG. 1, the numbers in the figure indicate grain size points. As is clear from this figure, the grain size depends on the hot rolling temperature and the rolling reduction rate, and the grain size is the finest near the Ar3 transformation temperature shown in the figure, and the area where the rolling reduction rate becomes high. It is fine grained. Rolling reduction rate of 70% or more,
The rolling temperature is from Ar3 transformation point +20℃ to Ar3 transformation point -5
In the range up to 0°C, a crystal grain size of A10 or higher is obtained as shown by the diagonal lines.

As a second experiment, one in the rolling temperature/reduction ratio range where the grain size is 10 or higher from the first experimental level,
Select one with a rolling temperature/reduction ratio of No. 10 or lower,
Immediately after rolling, the material was rapidly cooled from 400°C to 700°C, and then the latter 3 of the 7 finishing stands were used.
Warm rolling was performed at 400°C to 700°C. Then, after rolling it up, it was heated to 600℃ for 2 minutes to recrystallize it.
The lower value was measured using an IS13 B test piece. At this time, the relationship between the post-finishing temperature and the 1 value is shown in Figure 2.As is clear from Figure 2, the finer the grain size before warm rolling, the better the lower value. It is necessary to have a grain size of JEL No. 10 or above (above the grid line), and the warm rolling temperature to obtain this is within a temperature range of 700° C. or below.

As a third experiment, the winding temperature was determined for the final finishing temperature of 880°C and rolling reduction rate of 70% for the front 4 stands and the final finishing temperature of 630°C and rolling reduction rate of 70% for the latter 3 stands from the second experimental level. 300 to 600℃, those with further heat retention, and 300 to 5℃
The V value was also measured for the film that was rolled up at 0.0°C and then heated at 650°C for 2 minutes.

FIG. 3 shows the relationship between the winding temperature or the heat treatment conditions after winding and the lower value. As is clear from this figure, when the winding temperature is 600'C or higher, the f-value significantly increases to 1.4 or higher, and by keeping it warm, the lower value can be further improved. Also, the winding temperature was unavoidably 300~6oo.
℃, the F value has not reached 1.4. In this case, the temperature should be 1.4 or higher by reheating and holding at 650°C for at least 2 minutes.

As a fourth experiment, we measured the lower values of the samples that were rolled up to 400'C in the third experiment, and changed the temperature during reheating to 400 to 700℃ and the heating time to 1 to 3 minutes. I did it. The results are shown in FIG. The relationship between the recrystallization treatment conditions, heating temperature/heating time, and 7 values is as follows.
As is clear from the figure, the higher the heating temperature and the longer the heating time, the higher the Y value improves, and a Y value of 1.4 can be secured when heated at 00° C. or higher for 2 minutes or more.

The present invention was made based on the above knowledge, and its characteristics are as follows.

(1) At least C0.08% or less, Mn 0.40%
Hereinafter, aluminum killed slabs or steel slabs containing acid-soluble At of 0.02% or more are subjected to aluminum nitride precipitation treatment at 400 to 700°C with a ferrite grain size of A10 or more.
1. A method for producing a low carbon steel sheet with excellent drawing properties, comprising rolling at a reduction rate of 70% or more in a temperature range of 70% or more, followed by recrystallization treatment.

(2) An aluminum killed slab or steel slab consisting of at least 0.08% of CO, 0.40% of Mn, and 0.02% of acid-soluble Az is heated at 400 to 700°C at a ferrite grain size of A10 or higher after aluminum nitride precipitation treatment. Production of a low carbon steel plate with excellent drawing properties, characterized in that it is rolled at a reduction rate of 70q6 or higher in a temperature range of Method.

(3) After lowering the temperature of the slab or steel slab to Ar5 or less for at least 20 minutes,
The method according to any one of the preceding items 1 or 2, wherein aluminum nitride is precipitated by heating to 0 to 1150°C.

(4) Ar5 transformation transformation temperature from 20℃ to Ar3 transformation temperature -5
The method according to any one of the preceding items 1 or 2, characterized in that a sipherite crystal grain size of A10 or more is obtained by rolling at a reduction rate of 70% or more in a temperature range up to 0°C.

(5) The method according to any one of the above items 1 or 2, characterized in that the material to be rolled is rolled at 600 to 700°C and then maintained at 600°C or higher for 2 minutes or more for recrystallization treatment.

(6) The method according to any one of the above items 1 or 2, characterized in that the material to be rolled is rolled at 400 to 600°C and then maintained at 600°C or higher for 2 minutes or more for recrystallization treatment.

No industrial proposal has yet been made to improve the drawability of aluminum-killed hot-rolled steel sheets. The present invention is the first to make this possible by applying the above-described rolling conditions and post-treatment during hot rolling.

This is because the present inventors discovered that the plate plane orientation component (111
) is generated and grows, and by refining the crystal grains before warm rolling, it was discovered that the Schiff value with a large number of (111) sheet orientation components can be improved. It's cold.

In the present invention, the ferrite crystal grain size is set to be 10 or more, which was determined from the experimental results described above.
If the number is less than 1, the generation and growth of (111) will not be sufficient and the v value will not satisfy the value of 1.4 required for a steel plate for deep drawing. In addition, the hot rolling temperature for obtaining a ferrite grain size of No. 10 or higher is from Ar3 transformation temperature + 20°C to Ar3 transformation temperature -5 (l), which was determined from the experimental results mentioned above. Regardless of whether it is high or low, it is not possible to ensure a ferrite crystal grain size of AIO number or higher.

In addition, nitrogen present in the crystal grains is a factor that inhibits the production and growth of (111), and as a countermeasure, 9
It is necessary to precipitate nitrogen in the steel sheet as aluminum nitride by heating at 00 to 115oC.

Furthermore, the reason why the reduction ratio in warm rolling is set to 70 inches or more is that internal strain is required for sufficient recrystallization to occur in the subsequent recrystallization treatment. In addition, the reason why the warm rolling temperature was set at 400 to 700°C is that if it is higher than 700°C, the internal strain required for recrystallization will be reduced.
This is because if the temperature is lower than 00°C, the power required for rolling increases rapidly, which is economically disadvantageous.

In this way, after purifying the inside of the crystal grains by precipitating nitrogen etc. by low-temperature heating, the crystal grains are refined by rolling under high pressure near the transformation point, and then the ferrite grains are re-formed. By applying internal strain for crystallization through warm rolling and leaving it in a state where it is easy to recrystallize and crystallize, a simple recrystallization treatment can be performed after that, making it possible to create high T value deep drawing steel sheets even in hot rolled steel sheets. is obtained. In the recrystallization treatment at this time, if the warm rolling temperature is 700 to 600°C, it is sufficient to recrystallize by rolling it up as it is, and by covering it with a heat insulating cover, a product with a high shift value can be obtained. In addition, when the warm rolling temperature is 600 to 300°C, even if the -i: is rolled up, it will not recrystallize, so it is recommended to perform a short-time heating/thermal recrystallization treatment by heating it to 600°C or higher for 2 minutes or more. It becomes necessary.

The steel sheet produced in the above manner has excellent drawability, but after pickling, light reduction of 1 to 10% can improve the surface properties of the cold rolled steel sheet. It becomes possible to manufacture deep-drawn steel sheets with

The reason why the light reduction rolling ratio was set to 1% or more and 10% or less is because a roughness equivalent to that of a cold-rolled steel plate can be obtained with 1 inch or more, and on the other hand, if it exceeds 10%, the material deteriorates significantly due to processing strain. .

The aluminum-killed steel sheet of the present invention is obtained by a normal melting method, and the subsequent agglomeration and blooming are not particularly limited, and normal methods can be adopted. 'Of course, continuous casting is also included in this case. Further, the reason why the steel sheet of the present invention has a C content of 0.08% or less and a Mn content of 0.40% or less is because if the content exceeds this, the ductility deteriorates. Further, AA is added for deoxidation, and in order to obtain the steel sheet targeted by the present invention, 0.02% or more of acid-soluble At is required.

The present invention will be explained in detail below.

Example 1 As shown in Table 1, a three-component aluminum killed steel was melted in a converter, and a slab produced by a conventional method was used as Invention Example A-
Regarding F, in the pre-finishing stage with AtN precipitated, the Ar5 transformation point was changed from +20°C to the Ar5 transformation point -50°C.
After rolling was carried out at a rolling reduction of 70% or more in a temperature range of , in the final stage, recrystallization treatment was performed after warm rolling at a temperature range of 700 to 400°C with a rolling reduction of 70% or more.

As a result, the lower value of each steel sheet showed a high value of 1.41 to 1.53, which was approximately the same as that of a conventional cold-rolled steel sheet for deep drawing.

Compared to this, Comparative Example (■) is a steel sheet with the same composition as the present invention, but the rolling reduction in the latter stage of finish rolling is less than 70%.
Because the rolling reduction rate was insufficient, the lower value was only 1.27L.

Comparative Example (3) had no recrystallization treatment after warm rolling and was incomplete, but only low values were obtained for both lower value and elongation.

In Comparative Example (2), the rolling temperature or rolling reduction in the first stage of finish rolling is out of the range of the present invention, but the 7 value is 1.02 to 1.141 because the crystal grains are not sufficiently refined. ．． I couldn't get it.

Comparative example ■ is a steel plate heated at a temperature of 1200°C or higher,
Since N was dissolved in solid solution during the heating stage, the lower value was only 1.12, although the finish rolling conditions were within the scope of the present invention.

Example 2 A steel plate having steel coil number A shown in Table 1 was pickled and then lightly rolled at a rolling reduction of 12% or less. Table 2 shows the surface roughness and mechanical properties of these steel plates.

Coil number A2 lightly rolled at a rolling reduction within the range of the present invention
~A5 has a surface roughness that is almost the same as that of cold-rolled steel sheets, and there is little material deterioration due to light reduction rolling.

In comparison, Comparative Example I was not subjected to light reduction rolling, and the surface roughness after pickling was 1.47 μm, which was very large.

Comparative Example (3) had a light reduction rolling rate of 12 inches, and although the surface roughness was good, it was affected by processing strain, the tensile strength and yield strength were extremely high, and the elongation was deteriorated.

As explained above, the present invention provides conditions for refining the crystal grains in the steel sheet in the hot rolling stage to the (111) plane, which is advantageous for drawing properties, before one warm rolling, and a subsequent recrystallization treatment. Since growth is achieved by combining conditions, it is now possible to produce steel plates for deep drawing in the hot rolling process that are equivalent to steel plates for deep drawing, which were conventionally produced by batch annealing and continuous annealing after cold rolling. By washing and light reduction, it is possible to produce steel plates for deep drawing that are equivalent to cold-rolled steel sheets in the hot rolling process, and by subsequent pickling and light reduction, it is possible to produce steel plates for deep drawing that have the same surface properties as cold-rolled steel plates. I got used to it. This not only makes it possible to shorten the manufacturing process and reduce costs, but also to save energy, which has very significant effects.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between rolling temperature and rolling reduction rate in the first stage of finishing on grain size, Figure 2 is a diagram showing the relationship between rolling temperature and seven values in the second stage of finishing, and Figure 3 is a diagram showing the relationship between rolling temperature in the latter stage of finishing and rolling reduction. FIG. 4 is a diagram showing the relationship between temperature, processing conditions after winding, and seven values, and FIG. 4 is a diagram showing the relationship between reheating temperature, heating time, and lower value after winding. Figure 1 Figure 2 Temperature (・C) Figure 3 Figure 4

Claims (6)

[Claims] (1) At least co, os% or less, Mn 0.40
% or less, and acid-soluble At of 0.02% or more after aluminum nitride precipitation treatment, the ferrite crystal grain size is 400 to 700 in A10 or more.
1. A method for producing a low carbon steel sheet with excellent drawing properties, which comprises rolling at a reduction rate of 70q6 or more in a temperature range of 0.degree. C., followed by recrystallization treatment. (2) At least co, os% or less, Mn 0.40%
Hereinafter, an aluminum killed slab or steel slab consisting of acid-soluble MO, 02J or more is subjected to aluminum nitride precipitation treatment, and then rolled at a temperature range of 400 to 700 ° C. at a reduction rate of 70% or more at a ferrite grain size of A10 or more, A method for producing a low carbon steel sheet with excellent drawing properties, which comprises recrystallizing, pickling, and then rolling at a rolling reduction of 1 to 10%. (3) After lowering the temperature of the slab or steel slab to Ar5 or less for at least 20 minutes,
The method according to any one of claims 1 and 2, wherein aluminum nitride is precipitated by heating to 0 to 1150°C. (4) Ar3 transformation temperature +20℃ to Ar3 transformation temperature -5
According to any one of claims 1 and 2, the sifunilite crystal grain Kanoya No. 10 or more is obtained by rolling at a reduction rate of 70% or more in a temperature range up to 0°C. the method of. (5) Any one of claims 1 and 2, characterized in that the rolled material is rolled at 600 to 700°C and then maintained at 600°C or higher for 2 minutes or more for recrystallization treatment. The method described in. (6) Any one of claims 1 and 2, characterized in that the material to be rolled is rolled at 400 to 600°C and then maintained at 600°C or higher for 2 minutes or more for recrystallization treatment. The method described in.