JPH0549728B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for manufacturing a thin steel sheet with excellent deep drawability by continuous annealing. (Prior Art) Known technologies for producing cold-rolled steel sheets for deep drawing by continuous annealing using low carbon aluminum killed steel can be roughly divided into two. The first method is to set the coiling temperature after hot rolling to 700℃.
This is the so-called high-temperature winding method, in which the temperature is raised to a higher temperature.
This is aimed at the effects of coagulation of cementite and precipitation and coarsening of AlN due to high temperature winding. However, the high-temperature winding method has the disadvantage that a thick oxide film is formed on the surface of the steel sheet after winding, which significantly reduces pickling properties, and that the material inside the coil cannot be made high because the material of the inner and outer circumferential parts of the coil, where the cooling rate is high, cannot be increased. The uniformity is extremely poor and the yield is greatly reduced. JP-A No. 58-37128 discloses a method of controlling the cooling rate by inserting the coil into a heat retention furnace or attaching a heat retention cover after high-temperature winding to improve the uniformity of the material inside the coil. However, in this case as well, problems arise, such as deterioration of pickling properties, complication of the process, and increase in cost. On the other hand, the second method is based on low-temperature winding. As a method equivalent to this,
51-29696, a method of adding B to aluminum-killed steel, a method of coarsening grains after annealing by reducing the amount of Al and limiting the hot rolling reduction rate and cold rolling reduction rate (JP-A No. 51-29696). Publication No. 58-104124), B
Using additive aluminum killed steel, heating temperature during hot rolling,
Method by limiting finishing temperature
-117834) etc. are known. However, upon detailed investigation by the present inventors, it was found that with these methods, relatively large grains can be obtained, yield strength is relatively low, and elongation is quite high; The value that is an index of drawability is only low compared to the case of high-temperature winding. Therefore, although it can be adopted as a simple manufacturing technique for soft steel sheets, it is not satisfactory as a method for manufacturing deep drawing steel sheets. (Problems to be Solved by the Invention) As stated above, in the conventional method, high temperature winding is indispensable in order to obtain sufficient deep drawability (value). They have some problems such as decreased sexuality. Under these circumstances, there is a strong desire to develop a method for lowering the coiling temperature after hot rolling when producing deep drawing steel sheets by continuous annealing using aluminum killed steel. That is, the problem to be solved by the present invention is that in a method of manufacturing thin steel sheets with excellent deep drawability by continuous annealing using low carbon aluminum killed steel as a raw material, the coiling temperature after hot rolling must be raised. The disadvantage is that it does not. (Means for solving the problems) The gist of the present invention is as follows: C: 0.05% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.05
% or less, acid-soluble Al: less than 0.010%, N: 40 ppm or less, B: 2 ppm or more and less than 20 ppm, and the balance is Fe.
and steel containing unavoidable impurities is hot rolled according to a conventional method, coiled at a temperature of 400°C or more and less than 700°C, and then cold rolled at a reduction rate of 40% or more,
This is a method for producing thin steel sheets with excellent deep drawability, which is characterized by continuous annealing. In order to obtain good material quality, especially deep drawability, when producing aluminum killed steel by continuous annealing, it is important to maintain coarse cementite agglomeration during coiling after hot rolling, and to reduce N in the steel. The important point is how to precipitate coarsely as large-sized precipitates. As a result of detailed study on a method for producing thin steel sheets with excellent deep drawability by a method that does not rely on high-temperature coiling, the present inventors found that, in ordinary low-carbon steel, it is possible to add a small amount of B and to reduce the amount of Al added. The present invention was completed based on the new finding that a high value can be obtained even at low temperature winding only when the two conditions of significantly reducing it are satisfied at the same time. Specifically, first, by adding a small amount of B, N in steel can be precipitated as BN. In low-carbon aluminium-killed steel, N precipitates as AlN, but this AlN precipitates at a low ratio to the total N amount at low coiling temperatures, and precipitates extremely finely during annealing, suppressing grain growth.
This causes deterioration of deep drawability. Since BN has a greater tendency to form precipitates with N than AlN, the precipitation initiation temperature is higher, and coarse precipitation occurs even during low-temperature winding, making it possible to eliminate the adverse effects of N. B in conventional B-added aluminum killed steel also has the same role with respect to N as described above. The difference between the present invention and conventional B-added aluminum killed steel is the range of B addition amount.
There is a difference in the range of the amount of Al added, and this causes a significant difference in deep drawability. That is, B in conventional B-added aluminum killed steel
Since the amount added is in a larger range than that of the present invention, as seen in the example of Japanese Patent Publication No. 51-29696,
Although N is sufficiently extracted and fixed as BN during low-temperature winding, cementite is finely dispersed and precipitated at grain boundaries, resulting in poor deep drawability. On the other hand, in the present invention, due to the synergistic effect of the small amount of B added and the significant reduction in the amount of Al added, which will be described later, cementite precipitates coarsely in lumps even at low temperature winding. value is obtained. JP-A-58-117834, in which the amount of B added is in the same low range as the present invention;
As seen in the examples of JP-A-58-48634, the amount of Al added is significantly higher than that of the present invention, and in this case, cementite is finely precipitated and the value is at a low level. The second factor is that while adding a small amount of B, the amount of Al added is significantly reduced. As a result, cementite in the hot-rolled sheet aggregates and precipitates coarsely into lumps even during low-temperature winding, resulting in a high value after cold rolling and annealing. Although the details of the mechanism by which this phenomenon occurs only when the addition of a small amount of B and the significant reduction in the amount of Al added are satisfied at the same time are unknown, the present inventors have discovered a new finding in low carbon steel. JP-A-Sho, which coarsens the crystal grains after cold rolling and annealing by reducing the amount of Al and limiting the hot rolling reduction rate and cold rolling reduction rate.
According to the technique disclosed in Japanese Patent No. 58-104124, since B is not added, cementite does not precipitate coarsely during low-temperature winding, and although it is soft, the value is low. Based on this knowledge, the present inventors have completed a method for manufacturing a thin steel sheet with excellent deep drawability by using low carbon steel and continuous annealing at a low temperature. (Function) Hereinafter, the present invention will be explained in detail. The reason for limiting the chemical components is as follows. C is an element that determines the amount of cementite, and is 0.05
If it exceeds 0.05%, the value tends to decrease due to an increase in the amount of coarsely precipitated cementite itself.
The upper limit is %. In areas where the amount of C is less than 0.02%,
The most preferable range is less than 0.02% because the temperature at which cementite begins to precipitate is lowered. Although Si is an effective element for increasing strength,
If the content exceeds 0.8%, chemical conversion treatment properties and hot-dip galvanizing properties will decrease, so the upper limit is set at 0.8%. Mn is also an effective element for increasing strength, but
Addition of more than 1.0% is economically undesirable, and 1.0%
is the upper limit. P can be used effectively to increase strength most efficiently, but if it exceeds 0.1%, the risk of secondary processing embrittlement increases, so 0.1% is the upper limit. S is precipitated and fixed as MnS, but increasing the content will lead to an unnecessary increase in the amount of Mn added.
0.05% or less. Al is an important component of the present invention and is acid-soluble.
If the Al content is 0.010% or more, only a low value can be obtained in low-temperature coiling, similar to conventional low carbon aluminum killed steel, so the acid melt Al content is set to less than 0.010%. As shown in the examples below, the most desirable range is:
In the range of less than 0.005%. Since N tends to slightly deteriorate the material quality due to an increase in the amount of precipitated nitrides, it is set at 40 ppm or less. B is an important component of the present invention, and if it is less than 2 ppm, it has no effect on eliminating the adverse effects of N or on coarse aggregation of cementite, so the lower limit is set at 2 ppm. Moreover, if it is 20 ppm or more, the effect will be similar to that of conventional B-added aluminum killed steel, and coarse agglomeration of cementite will not occur, so the content should be less than 20 ppm. The combined effect of the two conditions of the above-mentioned significant reduction in the amount of Al added and addition of a small amount of B is the basis of the present invention, and it is essential to satisfy these two conditions at the same time. The remainder is a component consisting of Fe and unavoidable impurities. The molten steel whose chemical composition has been adjusted to the above range is made into a slab by continuous casting or ingot forming. As the hot rolling method, the slab may be cooled to a temperature below the Ar ₃ transformation point and then reheated, or it may be directly hot rolled as it is cast. There is no need to particularly specify conditions such as the temperature history and rolling reduction of the hot rolling process. There is no upper limit to the winding temperature from the viewpoint of material properties, but it is
If the temperature is higher than ℃, the pickling property will decrease, so the temperature should be lower than 700℃.
If the temperature is lower than 400°C, cementite becomes difficult to coarsely aggregate and the deep drawability decreases, so the temperature is set at 400°C or higher. The hot rolled coil obtained by this method is descaled and then cold rolled. Since a sufficient value cannot be obtained with a rolling reduction of less than 40%, the lower limit of the cold rolling reduction is set at 40%. Next, recrystallization annealing is performed, and the annealing method is continuous annealing. As continuous annealing, any process for producing various types of cold-rolled steel sheets, galvanized, tin-plated, chrome-plated, etc., is possible. As a condition for continuous annealing, there is no need to particularly specify the annealing temperature as long as the steel plate temperature is equal to or higher than the recrystallization temperature. Further, over-aging treatment for the purpose of reducing the aging properties of the steel plate may be performed as necessary. After annealing, skin pass rolling, rust prevention treatment, application of lubricant, etc. may be performed as necessary. (Example) Steel having the components shown in Table 1 is melted, made into a slab by continuous annealing, heated to a temperature range of 1000 to 1250°C, and then hot rolled at a temperature of 890°C or higher.
After being wound into a coil at a temperature of ~750℃ and pickled
Cold rolled with a reduction rate of 80%, then (600~775
A cold-rolled steel sheet was obtained by continuous annealing including soaking for 60 seconds at 350°C and subsequent overaging treatment at 350°C for 3 minutes. The pickling property was measured at the stage of hot-rolled coils, and the values were also measured. Pickling property: In 8% hydrochloric acid (80
℃) and evaluated based on the time it took for the scale to completely peel off. The manufacturing conditions and measurement conditions are shown in Table 2, and the effects of the amounts of Al and B added are extracted and shown in FIG. As can be seen from Table 2 and FIG. 1, the steel sheet produced by the method of the present invention has a high value despite being rolled at a low temperature of less than 700°C, and there is no deterioration in pickling properties that accompanies high-temperature winding.

[Table] *: Items outside the scope of the present invention

【table】

[Table] (Effects of the invention) According to the present invention, thin steel sheets with excellent deep drawability can be manufactured at a lower coiling temperature than conventional methods, and the reduction in yield associated with high-temperature coiling can be avoided. It is clear that problems such as decreased sexuality can be resolved,
It is extremely advantageous.

[Brief explanation of the drawing]

FIG. 1 is an explanatory diagram showing the relationship between acid-soluble Al and values.

Claims (1)

[Claims] 1 C: 0.05% or less, Si: 0.8% or less, Mn: 1.0% or less, P: 0.10% or less, S: 0.05% or less, acid-soluble Al: less than 0.010%, N: 40ppm or less , B: A steel containing 2 ppm or more and less than 20 ppm, with the balance consisting of Fe and unavoidable impurities, is hot rolled according to a conventional method and heated to 400°C or more.
A method for producing a thin steel sheet with excellent deep drawability, which comprises coiling at a temperature of less than 700°C, followed by cold rolling at a reduction rate of 40% or more, and continuous annealing.