JPH0639619B2 - 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) The current thin steel plate manufacturing process consists of casting a steel slab of about 250 mm thickness, thinning it to a thickness of about several mm by hot rolling, and then performing cold rolling and recrystallization annealing. ing.
Considering the future innovative manufacturing process from the viewpoint of significantly reducing the manufacturing cost due to enormous energy saving, either the casting process and the subsequent two rolling processes should be greatly simplified, or one of these processes should be significantly reduced. It can be said that omitting a part responds to it. The present invention discloses a method for producing a thin steel sheet having excellent press formability by such an innovative thin sheet manufacturing process that omits the conventional hot rolling.

As a thin plate manufacturing process in the future, thin-walled steel slabs with the thickness obtained after conventional hot rolling are cast, hot rolling is omitted, and the steel slabs are directly cold-rolled, followed by recrystallization annealing. It has already been reported that the process of casting or the process of casting a thin steel sheet directly from molten steel without going through the rolling process. In the case of a process in which such a rolling step is omitted or simplified, the most serious problem is that the casting structure is not sufficiently destroyed, the adverse effect of the casting structure is carried over to the final product, and it is used for press forming. The workability, especially the elongation, is extremely insufficient. From this point of view, the process of directly casting a thin steel sheet without going through the above rolling process does not provide good workability. Therefore, in order to obtain the formability equivalent to that of the conventional press forming steel sheet, at least one rolling is required in order 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 viewpoint, casting the above-mentioned thin-walled steel slab, when directly cold rolling the steel slab, a process of recrystallization annealing is disclosed (for example, JP-A-59-43823, casting. Then, a method is disclosed by controlling the average cooling rate, rolling start temperature, and rolling reduction of the steel slab at 900 to 700 ° C.).

(Purpose of the Invention) As a result of actually examining these conventional techniques, the present inventors
We have found its drawbacks and the limits of its technical level. Therefore, as a result of repeated basic research on the factors governing the material properties in the production of thin steel sheets by cold rolling and recrystallizing annealing thin steel slabs, the results show that the material components, the cooling rate during solidification, the slab thickness, and the cold We have found new knowledge that it is important to control each of the rolling ratios in a composite manner, and based on these findings, we have established a manufacturing technology for thin steel sheets with excellent press formability by this manufacturing process.

(Structure / Operation of the Invention) The present invention is: (1) C: 0.007% or less (weight% or less same) Si: 0.8% or less Mn: 1.0% or less P: 0.10% or less S: 0.10% or less sol.Al: 0.01 〜0.06% N: 0.008% or less and other unavoidable impurities, and contains Nb and Ti in combination, and Ti is (48/14) [N (%)-0.002%] <Ti (%)
And Ti (%) <(48/14) N (%) is satisfied, Nb is (93
/ 12) [C (%)-0.001%]> Nb (%)> 2.00C (%) and 0.00
Within the range of 3% to less than 0.025%, and (Ti (%) + Nb
(%)] <0.04%, and continuously casting thin-walled steel slabs with the composition consisting of the balance Fe, the average cooling rate from 1550 ° C to 1350 ° C during casting was 1.0 ° C / sec or more, and the thickness of the slabs A method for producing a thin steel sheet having excellent formability, which comprises rolling the steel sheet with a thickness of 50 mm or less and a rolling reduction of 60% or more at a recrystallization temperature or less, and then performing recrystallization annealing.

(2) The average cooling rate from 1350 ℃ to 900 ℃ after casting is 3 ℃ / m
The method for producing a thin steel sheet having excellent formability according to the above item (1) is characterized in that it is not less than in.

Hereinafter, the present invention will be described in detail.

The present invention, in order to produce a thin steel sheet having high workability by cold rolling and recrystallization annealing without performing hot rolling on a thin steel slab, when casting a thin steel slab, It is necessary to satisfy all the conditions of refining the solidified structure, refining the structure by suppressing grain growth during cooling after solidification, and breaking the cast structure by cold rolling after solidification. It is based on knowledge. Each of the reasons for limitation constituting the present invention is based on any one of the above items 1 to 3, and this will be described based on experimental results.

In the following experiments, C: 0.001 to 0.10%, Si: 0.01 to 0.10%, M
n: 0.10 to 0.15%, P: 0.010 to 0.15%, S: 0.01 to 0.20
%, Al: 0.001 to 0.100%, N: 5 to 100 ppm, Nb: 0.001 to
0.060%, Ti: Molten steel with a composition in the range of 0.001 to 0.050%,
Average cooling rate between 0 ℃ and 1350 ℃ and between 1350 ℃ and 900 ℃,
After casting with various slab thicknesses, the rolling rate is 20 to 90%
Cold rolling, recrystallization annealing at 775 ° C. for 40 seconds, and temper rolling at 1%.

(1) Minimization of solidification structure and suppression of grain growth after solidification As described above, in the process in which the rolling process as the object of the present invention is simplified, the influence of the casting structure on the material of the final product is extremely large. growing. Among the material properties, the "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. As a first constitution condition of the present invention, the present inventors set the average cooling rate from 1550 ° C to 1350 ° C at the time of casting to 1.0 ° C / sec.
The above findings have been obtained (when all other conditions satisfy the requirements of the claims of the present invention). More preferably, it is 5.0 ° C./sec or more, and most preferably 20 ° C./sec. This is shown in FIG. 1 by experimental data. Good materials (r
It is clear that the value, El) is obtained. Above 1550 ~
Of the cooling rate of 1350 ° C, the high temperature part corresponds to the cooling speed during solidification, and the low temperature part grows in the solidified structure (growth in the δ phase region) and the size of the γ phase during the transformation from the δ phase to the γ phase. Is the cooling rate that governs.

The alloying components in the molten steel are generally unfavorable because dendrites develop because they widen the solidification temperature range, 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 has a particularly strong tendency of being 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.
In order to reduce the solidification temperature zone and improve ductility, simply add C
The amount cannot be reduced.

The present inventors have already described the cooling rates during solidification, the δ phase region, the transformation from the δ phase to the γ phase, and the γ phase high temperature region (from 1550 ° C to
Refining the microstructure by limiting the cooling rate between 1350 ℃, TiN suppresses grain growth, especially in the γ phase, and NbC
It was found that these problems can be solved by suppressing grain growth in the α phase by. This is shown in FIG. 2 with experimental data. That is, when Ti and Nb are not added, the r value and El are inferior due to the coarsening of the structure in the extremely low carbon component.
On the other hand, when Ti and Nb are added in combination, the above γ and α
The refinement of the phase exerts the effect of making it an extremely low carbon steel component, and a steel sheet having a good El and r value can be obtained. The amount of Ti added is preferably approximately equivalent to the amount of N in order to finely precipitate TiN. Amount to completely precipitate N Ti (%)> (48/14) N
At (%), TiN precipitates from high temperature and becomes coarse, and the effect of suppressing coarsening of the γ phase becomes small. In addition, Nb alone cannot obtain the above-mentioned γ-phase refinement 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 constituent 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 the ductility, (2) suppressing grain growth in the γ phase region. For adding Ti, TiN
(3) Nb in order to suppress grain growth in the α phase region
The technical idea is to add N to precipitate NbC. As shown in FIG. 2, from the viewpoint of microstructure refinement in the γ phase region,
In addition to the effect of TiN, it is effective to limit the average cooling rate from 1350 ℃ to 900 ℃. In this case, the average cooling rate in the temperature range needs to be 3 ° C./min or more. More preferably, it is 10 ° C / min or more. The thickness of the slab is 50 mm
Must be: 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. It is more preferably 20 mm or less, and most preferably 10 mm or less.

(2) Destruction of casting structure by cold rolling and imparting of rolling texture As described above, the most problematic point in the manufacturing process as the subject of the present invention is that the casting structure is not sufficiently destroyed and the casting structure is The adverse effect of is carried over to the final product, and the workability, especially the elongation, is extremely insufficient for the applications 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 destroy 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 rolling at 60% or more, it is possible to obtain the same formability as that of the conventional steel sheet for press forming. . Most preferably, it is 75% or more. In the present invention, the rolling temperature is not particularly limited as long as it is the recrystallization temperature or lower.

Next, the range of component elements will be described.

C is 0.007% from the viewpoint of improving the ductility as described above.
Below.

Si may be added in the case of producing a high strength steel sheet, but it is an element that promotes brittleness, and also an element that inhibits chemical conversion treatment and galvanizability. From this viewpoint, 0.8%
Should be: When manufacturing a mild steel plate, 0.1% or less is preferable.

Mn can also be used for increasing the strength. However, since it has a function of deteriorating the r value and the cost of ferroalloy is high, it should be 1.0% or less. When manufacturing a mild steel sheet, 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 contained in a large amount, the amount of grain boundary segregation increases and embrittlement occurs, that is, secondary work embrittlement, so the upper limit is
0.10% When manufacturing a mild steel plate, 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.
0.01% or more is necessary to improve the yield of the above, and an upper limit of 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 made 0.008%. In order to obtain the TiN amount for exerting the effect, the N amount of 10 ppm or more is desirable.

Ti has the effect of forming TiN and refining the γ phase, and N in steel is A
It plays a role in eliminating the adverse effects of precipitation as lN. (48/14) (N (%)-0.002)
%] <Ti (%) and Ti (%) <(48/14) N (%) must be added.

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

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, but in this case, descaling can be performed. The annealing conditions are as follows. First, the annealing method, it is possible to apply any method as an annealing method of the cold rolled steel sheet, for example, a box-type annealing method and continuous galvanizing line, other continuous annealing type line for performing the plating. It is a continuous annealing method including the above. 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.

(Example) 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, the thin steel plate obtained by cold rolling and annealing as described above was stretched. It was submitted to the test. Its mechanical properties are shown in Table 2.

All of the test steel Nos. 1 to 5 which are examples of the present invention show good material characteristics, and even in the manufacturing process as the object of the present invention, the conventional "casting-hot rolling-cold rolling-annealing" was performed. The material obtained was almost the same as that obtained in the above process, and it was proved that the steel plate had sufficient workability as a steel sheet to be subjected to press forming. In contrast, Comparative Steel No. 6 has 15
Due to the low cooling rate of 50 to 1350 ° C, the thick slab of No.7, and the low cold rolling rate of No.8, good materials (especially El and r values) are available for the reasons described above. I can't get it. Further, all of the sample steels Nos. 9 to 13 differ from the composition range of the present invention, and similarly, the materials are extremely low.

(Effect of the Invention) According to the present invention, a hot rolling step can be omitted to produce a thin steel sheet having excellent formability, energy saving, production cost, etc. can be significantly reduced, and the effect is extremely large. is there.

[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 components and materials, and FIG. 3 is a diagram showing the relationship between the thickness of cast slab and the material. The figure shows the relationship between the cold rolling rate and the 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 (2)

[Claims] 1. C: 0.007% or less (the same as weight% 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 Consists of other unavoidable impurities, and contains Nb and Ti in combination, with Ti being (48/14) [N (%)-0.002%] <Ti (%)
And Ti (%) <(48/14) N (%) is satisfied, Nb is (93
/12)[C(%)-0.001%]>Nb(%)>2.00C(%) and 0.00
Within the range of 3% to less than 0.025%, and [Ti (%) + Nb
(%)] <0.04%, continuously casting thin-walled steel slabs with the balance being Fe, and setting the average cooling rate from 1550 ° C to 1350 ° C to 1.0 ° C / sec or more during casting. Is 50 mm or less, and rolling is performed at a rolling reduction of 60% or more at a recrystallization temperature or less, and then recrystallization annealing is performed, which is a method for producing a thin steel sheet having excellent formability. 2. The method for producing a thin steel sheet having excellent formability according to claim 1, wherein the average cooling rate from 1350 ° C. to 900 ° C. after casting is 3 ° C./min or more. .