JPH0639620B2 - Method for manufacturing thin steel sheet with excellent formability - Google Patents

Description

The present invention relates to a method for producing a thin steel sheet having excellent formability.

(Prior art and its problems) In the current thin steel plate manufacturing process, a steel slab having a thickness of about 250 mm is cast, thinned to a thickness of about several mm by hot rolling, and then cold rolling and recrystallization annealing. It consists of applying.
If we consider future innovative manufacturing processes from the viewpoint of significantly reducing manufacturing costs due to enormous energy savings, we will greatly simplify the casting process and the subsequent two rolling processes, or we will omit some of these processes. It can be said that omission omits it. The present invention provides a method for producing a thin steel sheet having excellent press formability by an innovative thin sheet manufacturing process in which conventional hot rolling is omitted or extremely simplified.

As a thin plate manufacturing process in the future, cast thin-walled steel slabs of the thickness obtained after conventional hot rolling, omit hot rolling and directly cold-roll steel slabs, followed by recrystallization annealing. A process has already been reported, or a process of directly casting a thin steel sheet from molten steel without going through the rolling process. In the case of a process in which the hot or cold rolling step is omitted or simplified, the most serious problem is that the cast structure is not sufficiently destroyed, and the adverse effect of the cast structure is carried over to the final product, which is used for press forming. For some applications, workability, especially elongation, is extremely insufficient. Due to such a cause, good workability cannot be obtained in the process of directly casting a thin steel sheet without going through the above rolling step. Therefore, in order to obtain the formability equivalent to that of the conventional steel sheet for press forming, at least one rolling is required to destroy the cast structure. In this case, in order to impart deep drawability, it is effective to perform rolling at a temperature not higher than the recrystallization temperature to develop a rolling texture and then perform recrystallization annealing. Based on this point of view, a process of casting the above-mentioned thin-walled steel slab, directly cold rolling the steel slab, and then performing recrystallization annealing has been disclosed (for example, JP-A-59).
-43823 gazette discloses the method by controlling the average cooling rate, rolling start temperature, and rolling reduction of the steel slab at 900-700 degreeC after casting).

The present inventors, as a result of actually studying these conventional techniques,
We have found its drawbacks and the limits of its technical level. Therefore, a process of cold rolling and recrystallizing annealing a thin-walled steel slab to produce a thin steel plate, or a very simple hot-rolling, cold-rolling and recrystallization annealing of a thin-walled steel slab to produce a thin steel plate The basic research was repeated about the material control factor in the process. As a result, we have found a new finding that it is important to control the material composition, the cooling rate during solidification, the thickness of the slab, and the cold rolling rate in a composite manner. Therefore, the manufacturing technology of the thin steel sheet excellent in press formability was established.

(Means and Actions for Solving Problems) The gist of the present invention is C: 0.007% or less, Si: 0.8% or less, Mn:
1.0% or less, P: 0.10% or less, S: 0.10% or less, Sol. Al: 0.01 to 0.06%, N: 0.0
08% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/1
4) within the range of [N (%)-0.002%] <Ti (%) and Ti (%) <(48/14) N (%),
Nb is (93/12) [C (%)-0.001%]> N
b (%)> 2.00C (%), 0.003% or more and less than 0.025%, and [Ti (%) +
Nb (%)] <0.04%, B is 2 ppm or more 3
0ppm or less, continuously cast thin steel slab composed of the balance Fe, the average cooling rate from 1550 ° C to 1350 ° C is 1.0 ° C / sec or more, and the thickness of the slab is 50mm or less. A method of manufacturing a thin steel sheet having excellent formability, which comprises performing rolling at a rolling reduction of 60% or more at a recrystallization temperature or lower and then performing recrystallization annealing.

Further, according to the above method, a manufacturing method in which the average cooling rate from 1350 ° C. to 900 ° C. is 3 ° C./min or more, similarly, the steel slab is wound at a temperature of 600 ° C. or more and 850 ° C. or less after casting, or Heating the slab after casting 6
A manufacturing method in which the material is held in a temperature range of 00 ° C. or higher and 850 ° C. or lower for 5 minutes or more and then rolled, and after casting, before cold rolling, or before winding or before heat treatment, the recrystallization temperature is 20% or more. A manufacturing method of rolling at the above reduction rate is also provided.

Hereinafter, the present invention will be described in detail.

According to the present invention, in order to manufacture a thin steel sheet with high workability by performing cold rolling and recrystallization annealing by omitting or extremely simplifying hot rolling of a thin steel slab, (1) thin steel Refining the solidification structure when casting a slab, (2) refining the structure by suppressing grain growth during cooling after solidification,
(3) It is based on the knowledge that it is necessary to satisfy all of the conditions of the fracture of the cast structure by cold rolling after solidification. Each of the reasons for limitation constituting the present invention is mainly based on any of the above (1) to (3), and this will be described based on the results of implementation.

The following experiments were performed with C: 0.001 to 0.10%, Si:
0.1% or less, Mn: 0.10 to 0.15%, P: 0.
01-0.15%, S: 0.2 or less, Al: 0.01-
0.1%, N: 5-100 ppm, Nb: 0.025%
Less than, Ti: less than 0.05, B: 100ppm or less of the molten steel in the range of 1550 ~ 1350 ℃ and 135
After casting while changing the average cooling rate between 0 to 900 ° C and the thickness of the slab, hot rolling is applied to a part of the slab, and various types of winding or slab heating are performed to cool the slab. Rolling (20
˜90%), recrystallization annealing was performed at 775 ° C. for 40 seconds, and temper rolling was performed at 1%.

(1) Refining of solidification structure and suppression of grain growth after solidification As described above, in the process of simplifying the rolling process as the object of the present invention, the influence of the casting structure on the material of the final product is extremely large. growing. Among the material properties, "elongation" tends to be the worst. The fundamental reason for this is that, as described above, the cast structure is not sufficiently destroyed, so that it tends to become the starting point of cracking. This adverse effect of the cast structure becomes more remarkable as the spacing between dendrites (dendritic crystals) increases.

In order to miniaturize the cast structure, it is necessary to miniaturize the solidification nuclei by increasing the nucleation frequency and to suppress the growth of the solidification nuclei. From the viewpoint of the former, in order to increase the degree of supercooling at the time of solidification, it is necessary to limit the cooling rate at the time of solidification, and the latter also has a large effect on the cooling rate. The present inventors have found that, as the first configuration condition of the invention, the average cooling rate from 1550 ° C. to 1350 ° C. at the time of casting needs to be 1.0 ° C./sec or more. More preferably, it is 5.0 ° C./sec or more, and most preferably 20 ° C./sec or more. This is shown in FIG. 1 based on experimental data. It is clear that a good material (r value, El) is obtained only when the condition is satisfied. Among the cooling rates of 1550 to 1350 ° C., the high temperature portion corresponds to the cooling rate during solidification, and the low temperature portion is the growth of the solidification structure (growth in the δ phase region) and the γ phase during the transformation from the δ phase to the γ phase. There is a cooling rate that governs the size of.

The alloying components in steel generally widen the solidification temperature range, so that dendrites develop, which is not preferable, and C in steel is particularly prone to this. Further, since the steel sheet manufactured by the manufacturing process which is the subject of the present invention tends to be inferior in ductility among the material properties, it is necessary to lower the C content in the steel to enhance the ductility.

However, the reduction of alloying elements causes remarkable grain growth after solidification (especially when the cooling rate is small), and has the drawback of deteriorating the material.
It is not possible to simply reduce the amount of C in order to narrow the solidification temperature zone and improve the ductility.

The inventors of the present invention have described the cooling rates (1550 ° C. above) at the time of solidification, the δ phase region, the transformation from the δ phase to the γ phase and the γ phase high temperature region as described above.
These problems are caused by the refinement of the structure by limiting the cooling rate between ˜1350 ° C.) and the inhibition of grain growth by TiN, particularly in the γ phase, and the inhibition of grain growth in the α phase by NbC. I found that I could solve it. This is shown in FIG. 2 based on experimental data. That is, when Ti and Nb are not added, the composition of the extremely low carbon component becomes r due to the coarsening of the structure.
Value, El is inferior. On the other hand, when Ti and Nb are added in a combined manner, the effect of forming an extremely low carbon steel component is exhibited due to the refinement of the γ and α phases. A steel sheet with a good r value can be obtained. The amount of Ti added is preferably about the same as the amount of N in order to finely precipitate TiN. When Ti (%)> (48/14) N (%) is the amount that completely precipitates N, T
iN precipitates from a high temperature and becomes coarse, and the effect of suppressing coarsening of the γ phase becomes small. In addition, Nb alone does not provide the fineness of the γ phase due to TiN, and the material is inferior. Further, in this case, N is finely precipitated during the α phase region during cooling or during annealing after rolling, which causes deterioration of the material.

That is, the second constitutional condition of the present invention is the composition, and (1) it is an ultra-low carbon steel (C: 0.007% or less) from the viewpoint of enhancing ductility, (2) grain growth in the γ phase region. T to suppress
i is added to precipitate TiN. (3) Nb is added to precipitate grain growth in the α phase region to precipitate NbC. As shown in FIG. 2, from the viewpoint of microstructure refinement in the γ phase region, it is effective to limit the average cooling rate from 1350 ° C. to 900 ° C. in addition to the effect of TiN. In this case, the average cooling rate in the temperature range is 3 ° C / min
The above is preferable. More preferably, it is 10 ° C./min or more.

The thickness of the slab needs to be 50 mm or less. As shown in FIG. 3, when the thickness of the slab becomes thicker, the cooling rate at the center portion in the thickness direction becomes smaller, the structure becomes coarser, the material deteriorates, and the material uniformity in the thickness direction deteriorates. . Therefore, the thickness of the slab needs to be 50 mm or less. More preferably 20 mm or less, and most preferably 10 mm.
mm or less.

(2) Destruction of casting structure and provision of rolling texture by cold rolling The most problematic point in the manufacturing process as the object of the present invention is, as already mentioned, that the casting structure is not sufficiently destroyed and The adverse effect of the structure is carried over to the final product, and the workability, especially the elongation, is extremely insufficient for the application used for press molding. In order to obtain formability equivalent to that of a conventional steel sheet for press forming, at least one rolling is required to destroy the cast structure. In this case, in order to impart deep drawability, it is effective to perform rolling at a temperature not higher than the recrystallization temperature to develop a rolling texture and then perform recrystallization annealing.
In the present invention, since the structure is refined during solidification and subsequent cooling, it is possible to break the cast structure and impart a rolling texture with a low rolling rate. FIG. 4 shows the relationship between the cold rolling rate (below the recrystallization temperature) and the material. By performing rolling at 60% or more, it is possible to obtain the same formability as that of the conventional press forming steel sheet. .
Most preferably, it is 75% or more. In the present invention,
The rolling temperature is not particularly limited as long as it is lower than the recrystallization temperature.

(3) Effect of winding temperature or post-casting heat treatment In order to manufacture a thin steel sheet having good formability by the manufacturing process as the object of the present invention, as described above, the cooling rate is controlled mainly at the time of solidification. Need. C and N in steel
As described above, NbC or TiN precipitates and exerts a remarkable effect, but since the cooling rate is high, the degree of agglomeration of the precipitate is relatively small. From the viewpoint of increasing the ductility of the steel sheet and lowering the yield strength and recrystallization temperature, it is desirable to increase the degree of coagulation of precipitates. In this sense, it is effective to raise the winding temperature after casting or to heat the cast slab after casting. Specifically, the cast slab is cast at 500 ° C. or higher, preferably 60
Rewind at a temperature of 0 ° C to 850 ° C, or
After casting, the slab is heated to 600 ° C or more and 850 ° C or more before rolling.
The rolling is performed after the temperature is kept in the following temperature range for 5 minutes or more. In the latter case, the cooling rate after heating is not particularly limited (the smaller the cooling rate, the better the tendency). The winding temperature or the heating temperature is most preferably 650 ° C. or higher and 850 ° C. or lower.
This is shown in FIG. 5 based on experimental data.

When winding at a coiling temperature of 650 ° C. or higher, the coiling temperature before and after coiling in the coil length direction is suppressed, and the coiling temperature at that position is adjusted to the coiling temperature at the center of the coil length direction. A higher height is effective in terms of material uniformity in the coil length direction because the material at the front and rear ends of the coil can be improved. This kind of treatment does not impair the effects of the present invention and can be used in combination with the present invention.

(4) Effect of light hot rolling after casting Considering the future innovative manufacturing process from the viewpoint of significant reduction in manufacturing cost, it is important to greatly simplify the two rolling processes following the casting process. In the present specification, the manufacturing method in the process in which hot rolling is omitted has been described above.
However, even if the rough rolling of the conventional hot rolling process consisting of rough rolling and finish rolling is omitted and the finish rolling is simplified, the manufacturing cost is significantly reduced. That is, it is a process of performing hot rolling with a much lower rolling reduction than conventional hot rolling after casting. In this case, the material structure is improved because the casting structure is destroyed by the hot rolling. Therefore, by combining a light hot rolling with the above technical idea, a manufacturing method in which the material characteristics are improved, although the manufacturing cost is slightly increased, is extremely effective.
FIG. 6 shows the required rolling ratio (above the recrystallization temperature) based on experimental data. In the present invention, it is clear that a sufficiently good material is obtained by rolling at 20% or more, and hot rolling is sufficiently performed as compared with conventional hot rolling (reduction rate: about 95% or more). It is clear that it can be simplified. The finishing temperature at the time of hot rolling is most preferably right above the Ar ₃ point in order to reduce the γ crystal grain size before the γ → α transformation, but it is not particularly required. When the slab temperature drops below the hot-rollable temperature after casting, it is possible to perform hot rolling after heating. In this case, the heating temperature is preferably as low as possible so as not to dissolve the precipitate (mainly carbide), but it is not particularly limited.

Next, the range of component elements will be described.

C is 0.0 from the viewpoint of improving the ductility as described above.
It should be 07% or less.

Although Si may be added when producing a high-strength steel sheet, it is an element that promotes brittleness, and is also an element that inhibits chemical conversion treatment and zinc sticking property.
Should be 8% or less. When manufacturing a mild steel sheet, 0.1% or less is preferable.

Mn can also be used for increasing the strength. However, it should be 1.0% or less because it has a function of deteriorating the r value and the cost of ferroalloy is high. When manufacturing a mild steel plate, 0.3% or less is preferable.

P is an element having the largest strengthening ability and is added in the case of increasing the strength. However, if it is contained in a large amount, the amount of segregation at the grain boundaries increases, causing embrittlement, that is, secondary work embrittlement.
10%. When manufacturing a mild steel sheet, 0.03% or less is preferable.

As the amount of S increases, the required amount of alloying elements in the sulfide-forming steel increases. Therefore, the upper limit of S is 0.10%.

Al is added as a molten steel deoxidizer before adding Ti and Nb,
In order to improve the yield of Ti and Nb, it is necessary to add 0.01% or more, and excessive addition causes cost up, so the upper limit is made 0.06%.

Most of N is fixed to Ti as TiN, but when the N content is large, a large amount of Ti is required. In this case, TiN precipitates from a high temperature and becomes coarse, and the effect of refining the γ phase becomes small. Therefore, the upper limit is set to 0.008%. In order to obtain the TiN amount for exerting the miniaturization effect, the N amount of 10 ppm or more is desirable.

Ti plays a role of forming TiN and refining the γ phase and eliminating a sub-effect of precipitation of N in the steel as AlN. To achieve this effect (48/1
4) It is necessary to add it within the range of [N (%)-0.002%] <Ti (%) and Ti (%) <(48/14) N (%).

Nb is α by precipitating a part of C as NbC.
It plays a role of refining the phase and substantially eliminating the aging property of C. To achieve this effect (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
It is necessary to be within the range of less than%. 0.025%
In the above case, the recrystallization temperature becomes high. Furthermore, in order to improve the phosphate treatment (bonding treatment) performed as a coating base treatment, [Ti (%) + Nb (%)] <
It is necessary to set it to 0.04%.

B is added for the purpose of improving the secondary workability and increasing the amount of BH without deteriorating the room temperature aging when manufacturing a BH steel sheet. The effect of the addition of B is that B existing in a solid solution state in the steel.
It is due to. On the other hand, B has a major drawback that it deteriorates the ductility of the steel sheet and raises the recrystallization temperature regardless of whether it exists as a precipitate or a solid solution state. Therefore, it is necessary that the effect of B is exhibited even with a small amount of addition, and that the defect does not occur. In order to realize this, it is an essential condition that Nb and Ti are contained in a composite form as a component in the steel. That is, since Ti in the steel precipitates N in the steel as TiN as described above, it plays a role of allowing the added B to exist in a solid solution state, and the above effect can be exhibited even with an extremely small amount of B added. Become. Nb precipitates and fixes a part of C in the steel and makes a part of the C exist in a solid solution state. Only when B coexists in such a state, the BH property improving effect of B is exhibited. On the other hand, when both C and N are precipitated and fixed by Ti, the effect of B on the BH property is not exhibited,
Further, when a part of C is precipitated by Ti and the rest is present as solid solution C, the room temperature aging property becomes large, which is not preferable. The lower limit of the amount of B added is determined by the amount of solid solution B that exerts such an effect, and the upper limit is determined by the amount of the amount of solid solution B that increases and the defects appear. Specifically, the proper amount of B added is 2 ppm
It is above 30 ppm.

Next, the manufacturing conditions will be described. The casting conditions have already been described. After casting, it is not contrary to the gist of the present invention to perform descaling treatment before rolling.
Any method including mechanical treatment and chemical treatment can be applied. The rolling conditions have already been described. Depending on the rolling temperature, the scale may grow thick after rolling, and in this case as well, the descaling treatment can be performed. The annealing conditions are as follows. First, as the annealing method, it is possible to apply any method as an annealing method of a cold rolled steel sheet, for example, a box type annealing method and a continuous type zinc plating line, other continuous annealing type for performing plating. It is a continuous annealing method including a line. The annealing temperature is not particularly limited as long as it is the recrystallization temperature or higher. Performing temper rolling after annealing does not go against the gist of the present invention, and may be carried out as necessary.

Examples will be shown below.

(Example 1) A thin steel slab having the chemical composition shown in Table 1 was cast under various casting conditions shown in the table, and after that, a thin steel sheet obtained by cold rolling and annealing as described above was obtained. It was subjected to a tensile test. Its mechanical properties are shown in Table 2.

Sample steel No. 1 to 5 all show good material characteristics, and even in the manufacturing process as the object of the present invention, it was obtained by the conventional "casting-hot rolling-cold rolling-annealing" process. It was proved that almost the same material was obtained and that it had sufficient workability as a steel sheet used for press forming. On the other hand, comparative steel No. 6
Has a low cooling rate of 1550 to 1350 ° C. after casting,
No. No. 7 has a thick cast piece, No. 8 has a small cold rolling rate, and therefore is made of a good material (especially El,
r value) cannot be obtained. In addition, the sample steel No. All of 9 to 14 are different from the component range of the present invention, and similarly, the material is extremely low.

(Example 2) Sample steel Nos. Shown in Table 1 After casting a thin-walled steel slab according to the chemical composition and casting conditions of No. 1 and after performing various kinds of conditions shown in FIG. 3, the test steel No. 1 shown in Table 1 was obtained. The thin steel sheet obtained by performing cold rolling and annealing under the same conditions as in 1 was subjected to a tensile test. Its mechanical properties are shown in Table 4.

The thin steel sheets produced by the method of the present invention all show good material characteristics, and even in the production process as the object of the present invention, the conventional "casting-hot rolling-cold rolling-annealing" It was proved that a material almost equivalent to that obtained in the process was obtained, and that it had sufficient workability as a steel sheet used for press forming.

(Effects of the Invention) According to the present invention, a thin steel sheet having excellent formability can be manufactured by a hot rolling process in which the hot rolling process is omitted or simplified, and energy saving and manufacturing cost are significantly reduced. Therefore, the effect is extremely large.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the average cooling rate and the material of the present invention, FIG. 2 is a diagram showing the relationship between the components and the material of the present invention, and FIG.
FIG. 4 is a diagram showing the relationship between the thickness and material of the slab of the present invention, FIG. 4 is a diagram showing the relationship between the cold rolling rate and the material of the present invention, and FIG. 5 is the winding temperature after casting of the present invention and The figure which shows the relationship with the material at the time of reheating a cast, FIG. 6 is a figure which shows the relationship between the hot rolling rate of this invention, and a material.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI technical display location C22C 38/00 301 S 38/14

Claims (8)

[Claims] 1. C: 0.007% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.10% or less, SOl. Al: 0.01 to 0.06%, N: 0.008% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/14) [N (%)
-0.002%] <Ti (%) and Ti (%) <
Within the range satisfying (48/14) N (%), Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
%, And [Ti (%) + Nb (%)]
<0.004%, B is 2 ppm or more and 30 ppm or more, and a thin steel slab composed of the balance Fe is continuously cast,
During casting, the average cooling rate from 1550 ° C to 1350 ° C was 1.0 ° C / Sec or more, and the thickness of the slab was 50 mm.
A method for producing a thin steel sheet excellent in formability, which comprises the following steps: rolling at a rolling reduction of 60% or more at a recrystallization temperature or lower and then recrystallization annealing. 2. C: 0.007% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.10% or less, SOl. Al: 0.01 to 0.06%, N: 0.008% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/14) [N (%)
-0.002%] <Ti (%) and Ti (%) <
Within the range satisfying (48/14) N (%), Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
%, And [Ti (%) + Nb
(%)] <0.04%, and B is 2 ppm or more and 30 p
pm or less and continuously casting a thin steel slab composed of the balance Fe, and setting the average cooling rate from 1550 ° C to 1350 ° C to 1.0 ° C / sec or more during casting, from 1350 ° C to 9 ° C.
An average cooling rate up to 00 ° C. is 3 ° C./min or more, a thickness of the slab is 50 mm or less, and rolling is performed at a recrystallization temperature or less and a reduction rate of 60% or more, followed by recrystallization annealing. And a method for producing a thin steel sheet having excellent formability. 3. C: 0.007% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.10% or less, SOl. Al: 0.01 to 0.06%, N: 0.008% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/14) [N (%)
-0.002%] <Ti (%) and Ti (%) <
Within the range satisfying (48/14) N (%), Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
Is less than%, and (Ti (%) + Nb
(%)] <0.04%, and B is 2 ppm or more and 30 p
pm or less and continuously casting thin-walled steel slab composed of the balance Fe, the average cooling rate from 1550 ° C. to 1350 ° C. at the time of casting was 1.0 ° C./sec or more, and the thickness of the slab was 5
0 mm or less, and after casting, the steel slab is wound up at a temperature of 600 ° C. or higher and 850 ° C. or lower, or the cast slab is heated after casting to a temperature range of 600 ° C. or higher and 850 ° C. or lower for 5 minutes or longer. After the temperature is maintained, the rolling reduction rate is 60 below the recrystallization temperature.
%, Or more, and then recrystallization annealing is performed, followed by a method for producing a thin steel sheet having excellent formability. 4. C: 0.007% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.10% or less, SOl. Al: 0.01 to 0.06%, N: 0.008% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/14) [N (%)
-0.002%] <Ti (%) and Ti (%) <
Within the range satisfying (48/14) N (%), Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
%, And [Ti (%) + Nb
(%)] <0.04%, and B is 2 ppm or more and 30 p
pm or less and continuously casting a thin steel slab composed of the balance Fe, and setting the average cooling rate from 1550 ° C to 1350 ° C to 1.0 ° C / sec or more during casting, from 1350 ° C to 9 ° C.
The average cooling rate up to 00 ° C is 3 ° C / min or more, the thickness of the slab is 50 mm or less, and the steel slab is wound at a temperature of 600 ° C or more and 850 ° C or less after casting, or After casting, the slab is heated to 600 ° C or higher and 850 ° C
After maintaining in the following temperature range for 5 minutes or more, rolling at a recrystallization temperature of 60% or more at a recrystallization temperature or less, and then recrystallization annealing, which is a method for producing a thin steel sheet having excellent formability. . 5. C: 0.007% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.10% or less, SOl. Al: 0.01 to 0.06%, N: 0.008% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/14) [N (%)
-0.002%] <Ti (%) and Ti (%) <
Within the range satisfying (48/14) N (%), Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
%, And [Ti (%) + Nb
(%)] <0.04%, and B is 2 ppm or more and 30 p
pm or less and a thin steel slab composed of the balance Fe is continuously cast, the average cooling rate from 1550 ° C to 1350 ° C is 1.0 ° C / sec or more, and the thickness of the slab is 5
0 mm or less, and after casting, the steel slab is wound up at a temperature of 600 ° C. or more and 850 ° C. or less, or after casting, the slab is heated to a temperature range of 600 ° C. or more and 850 ° C. or less for 5 minutes or more. After holding and further rolling at a temperature of the recrystallization temperature or higher and a rolling reduction of 20% or higher after casting and before cold rolling,
A method for producing a thin steel sheet excellent in formability, which comprises performing recrystallization annealing after rolling at a reduction rate of 60% or more at a recrystallization temperature or lower. 6. C: 0.007% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.10% or less, SOl. Al: 0.01 to 0.06%, N: 0.008% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/14) [N (%)
-0.002%] <Ti (%) and Ti (%) <
Within the range satisfying (48/14) N (%), Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
%, And [Ti (%) + Nb
(%)] <0.04%, and B is 2 ppm or more and 30 p
pm or less and continuously casting a thin steel slab composed of the balance Fe, and setting the average cooling rate from 1550 ° C to 1350 ° C to 1.0 ° C / sec or more during casting, from 1350 ° C to 9 ° C.
The average cooling rate up to 00 ° C is 3 ° C / min or more, the thickness of the slab is 50 mm or less, and the steel slab is wound at a temperature of 600 ° C or more and 850 ° C or less after casting, or cast. After that, the slab is heated and kept in a temperature range of 600 ° C. or more and 850 ° C. or less for 5 minutes or more, and further, after casting, rolled at a temperature of a recrystallization temperature or more and a rolling reduction of 20% or more before cold rolling. A method for producing a thin steel sheet having excellent formability, which comprises performing recrystallization annealing after rolling at a rolling reduction of 60% or more at a temperature equal to or lower than the recrystallization temperature. 7. C: 0.007% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.10% or less, SOl. Al: 0.01 to 0.06%, N: 0.008% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/14) [N (%)
-0.002%] <Ti (%) and Ti (%) <
Within the range satisfying (48/14) N (%), Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
%, And [Ti (%) + Nb
(%)] <0.04%, and B is 2 ppm or more and 30 p
pm or less and a thin steel slab composed of the balance Fe is continuously cast, the average cooling rate from 1550 ° C to 1350 ° C is 1.0 ° C / sec or more, and the thickness of the slab is 5
After being cast to 0 mm or less and rolled at a recrystallization temperature or higher and a rolling reduction of 20% or higher, the steel slab is heated to 600 ° C.
It is wound up at a temperature of 850 ° C. or lower, or the cast slab is heated after casting and held in a temperature range of 600 ° C. or higher and 850 ° C. or lower for 5 minutes or longer, and then the rolling reduction is 60% or lower at a recrystallization temperature or lower. A method for producing a thin steel sheet having excellent formability, which comprises performing recrystallization annealing after performing the above rolling. 8. C: 0.007% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.10% or less, SOl. Al: 0.01 to 0.06%, N: 0.008% or less, and other unavoidable impurities, and further contains Nb, Ti, and B in combination, and Ti is (48/14) [N (%)
-0.002%] <Ti (%) and Ti (%) <
Within the range satisfying (48/14) N (%), Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)>
2.00 C (%) and 0.003% or more 0.025
Is less than%, and [Ti (%) + Nb (%)
<0.04%, B is 2 ppm or more and 30 ppm or less, thin-walled steel slab composed of the balance Fe is continuously cast, and the average cooling rate from 1550 ° C to 1350 ° C is 1.0 ° C / sec or more during casting. The average cooling rate from 1350 ° C. to 900 ° C. is 3 ° C./min or more, the thickness of the slab is 50 mm or less, and after casting, rolling is performed at a temperature of the recrystallization temperature or more and a rolling reduction of 20% or more. After that, the steel slab is 6
It is wound at a temperature of 00 ° C. or higher and 850 ° C. or lower, or the cast slab is heated after casting and held in a temperature range of 600 ° C. or higher and 850 ° C. or lower for 5 minutes or longer, and then the rolling reduction is performed at a temperature of recrystallization temperature or lower. A method for producing a thin steel sheet having excellent formability, which comprises performing recrystallization annealing after rolling to 60% or more.