JPS6237323A - Manufacture of steel sheet having superior deep drawability by continuous annealing - Google Patents

Abstract

PURPOSE:To improve the deep drawability of a steel sheet by subjecting a low carbon steel having a specified composition to hot rolling, coiling at a low temp., cold rolling and continuous annealing. CONSTITUTION:A steel having a composition consisting of <=0.05% C, <=0.8% Si, <=1.0% Mn, <=0.10% P, <=0.05% S, <0.010% acid-sol. Al, 2-<20ppm B and the balance Fe with inevitable impurities is hot rolled and coiled at 400-<700 deg.C. This hot rolled coil is descaled, cold rolled at >=40% draft and continuously anealed. By this method, a steel sheet having superior deep drawability can be manufactured at a low coiling temp. in a high yield.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) 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 a so-called high-temperature winding method in which the winding temperature after hot rolling is raised to a high temperature of 700° C. or higher. This is due to the agglomeration of cementite and precipitation of 17N due to high temperature winding.
This is aimed at the effect of coarsening. 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 significantly reduced. Japanese Patent Application Publication No. 5
Publication No. 8-37128 discloses a method of improving the material uniformity inside the coil by controlling the cooling rate by inserting the coil into a heat retention furnace or attaching a heat retention cover after high-temperature winding. In this case as well, problems arise, such as deterioration in 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 corresponding to this,
As typified by Publication No. 6, a method of adding B to aluminum killed steel, a method of coarsening grains after annealing by reducing Al and limiting hot rolling reduction and cold rolling reduction (Japanese Patent Laid-Open No. 58
-104124), a method using B-added aluminum killed steel and limiting the heating temperature and finishing temperature during hot rolling (Japanese Patent Application Laid-open No. 117834/1983), etc. are known. However, upon detailed investigation by the present inventors, it has been found that with these methods, relatively thick crystal grains can be obtained, resulting in relatively low yield strength and fairly high elongation. The 〒 value, which is an index of deep drawability, is only lower than that obtained by high-temperature winding. Therefore, although it can be adopted as a simple manufacturing technique for soft steel sheets, it is not satisfactory as a manufacturing method for deep drawing steel sheets.

(Problems to be solved by the invention) As mentioned above, in the conventional method, sufficient deep drawability (F
High-temperature winding is indispensable in order to obtain the desired value), and such methods have several problems such as a decrease in yield and a decrease in pickling properties.

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 a thin steel sheet with excellent deep drawability by continuous annealing using low carbon aluminum killed steel as a raw material, the coiling temperature after hot rolling cannot be increased. It has the disadvantage that it cannot be used.

(Means for Solving the Problems) The gist of the present invention is that a: 0.05% or less, Si 2: 0.8%
Hereinafter, Mn: 1.0% or less, p: o, io% or less,
S: 0.05% or less, acid-soluble Ag: less than 0.010%,
N: 40 ppm or less, B: 2 ppm or more 20 ppm
A steel containing less than
This method of manufacturing a thin steel sheet with excellent deep drawability is characterized by winding at a temperature of less than 0° C., then cold rolling at a rolling reduction of 40% or more, and continuous annealing.

In order to obtain good material quality, especially deep drawability (P value) when producing aluminum killed steel by continuous annealing, it is necessary to make coarse cementite agglomerates during coiling after hot rolling, and An important point is how to coarsely precipitate the N in the form of large-sized precipitates.

As a result of detailed study on a method for manufacturing thin steel sheets with excellent deep drawability using a method that does not involve high-temperature coiling, the present inventors found that the addition of a small amount of B and the amount of U added to ordinary low carbon steel 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 the 〒 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 Affl, but this AfflN has a low precipitation ratio to the total Ni at low coiling temperatures, and precipitates extremely finely during annealing, suppressing grain growth and deteriorating deep drawability. cause it to happen.

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 the conventional B-added aluminum killed steel lies in the difference in the range of B addition amount and the range of Al addition amount, which causes a significant difference in deep drawability.

That is, since the amount of B added in conventional B-added aluminum killed steel is in a larger range than that of the present invention, as seen in the example of Japanese Patent Publication No. 51-29696, N is analyzed as BN in low-temperature winding. Although it is fixed, cementite is finely dispersed and precipitated at the grain boundaries, resulting in poor deep drawability. In contrast, in the present invention, due to the synergistic effect of the small amount of B added and the significant reduction in the number of AA' added children, which will be described later,
Even with low mixed winding, cementite precipitates coarsely in lumps, resulting in a high 〒 value.

Japanese Patent Application Laid-open No. 1983-198- in which the Bm weight is in the same low range as the present invention.
117834 and JP-A-58-48634, the amount of Al added is significantly higher than that of the present invention, as can be seen in the examples, and in this case, cementite is finely precipitated and the T value is It is at a low level.

The second necessary factor is to significantly reduce the amount of Al added at the same time as adding a small amount of B. As a result, cementite in the hot-rolled sheet aggregates and precipitates coarsely into lumps even when rolled at a low temperature, resulting in a high 〒 value after cold-rolling and annealing.

Although the details of how this phenomenon occurs only when a small amount of B is added and a significant reduction in the amount of Al added are simultaneously satisfied are unknown, the present inventors have discovered a new finding in low carbon steel.

JP-A-58-1041, which coarsens crystal grains after cold rolling and annealing by reducing Alt and limiting the hot rolling reduction rate and cold rolling reduction rate
According to the technique disclosed in Publication No. 24, since there is no addition of B, cementite does not precipitate to a large extent during low-temperature winding, and although it is soft, the f value is low.

Based on this knowledge, the present inventors have perfected a method for manufacturing a thin steel sheet with excellent deep drawability by using low carbon steel and continuous annealing at low temperature winding.

(for production) The present invention will be explained in detail below.

The reason for limiting the chemical components is as follows.

C is an element that determines cementite (t), and if it exceeds 0.05%, the f-value tends to decrease due to an increase in the amount of coarsely precipitated cementite itself, so the upper limit is set at 0.05%. In a region where c4 is less than 0.02 inches, the temperature at which cementite starts to precipitate decreases, so the most preferable range is 0.
．． The range is less than 0.02%.

Si is an effective element for increasing strength, but at 0.8%
If it is contained in a larger amount, chemical conversion treatment properties and hot-dip galvanizing properties will deteriorate, so the upper limit is set at 0.8 inches.

Mn is also an effective element for increasing strength, but at 1.0%
It is economically undesirable to add more than ,1,0-.

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 if its content increases, it will lead to an unnecessary increase in the amount of Mn added, so it should be kept at 0.05% or less.

i is an important component of the present invention, and is acid-soluble A7? If the amount is 0.010% or more, will it be low in low-temperature winding like conventional low carbon aluminum killed steel? Since only a certain value can be obtained, the amount of acid-soluble Al is set to be less than 0.010. As shown in the Examples below, the most desirable range is less than 0.005%.

Since N tends to slightly deteriorate the material quality due to an increase in the amount of precipitated nitrides, the content of N is set at 40 ppm or less.

B is an important component of the present invention, and if it is less than 2 T)l)m, 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. Also 20p
pm or higher, the effect is similar to that of conventional B-added aluminum killed steel, and coarse agglomeration of cementite does not occur.
It shall 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 simultaneously satisfy the two-line property.

The remainder is a component consisting of Fe and unforeseen 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 Ar3 transformation point and then reheated, or the slab 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 70
Pickling performance decreases at temperatures above 0°C, so the temperature should be below 700°C.
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. If the rolling reduction is less than 40 inches, a sufficient value cannot be obtained, so 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 for continuous annealing, it is possible to produce piles without the process of producing various products such as cold-rolled steel sheets, galvanized, tin-plated, chrome-plated, etc. 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 casting, heated to a temperature range of 1000~12501:, hot rolled at a temperature of 890°C or higher, 7
It was wound up at a temperature of 50°C to form a coil, and after pickling, it was cold-rolled at a rolling reduction of 80%, and the next step was (600-775°C).
Continuous annealing was performed by soaking for 60 seconds and subsequently overaging at 350° C. for 3 minutes to obtain a cold rolled steel sheet. The pickling property was measured at the stage of hot-rolled coils, and the P value was also measured.

The pickling property was evaluated by immersing the sample in 8 liters of hydrochloric acid (SO°C) and measuring the time until the scale was completely peeled off.

The manufacturing conditions and measured values are shown in Table 2, and FIG. 1 shows the effects of the amount of AI added and the amount of B6Σ added. Table 2,
From Fig. 1, the steel plate manufactured by the method of the present invention is 70°C
It has a high f-value despite being wound at a low temperature of less than 100 yen, and there is no decrease in pickling properties associated with high-temperature winding.

Table 2 (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 and pickling can be avoided. It is clear that problems such as decreased sex can be solved, and it is extremely advantageous.

[Brief explanation of drawings]

FIG. 1 is an explanatory diagram showing the relationship between acid-soluble Al and Te value. ;3; Fig. 1 aooiθαraao 昭0.0060.0100.0)
:1!0.0140. (M(p-squeezing AI Cwt, %
)

Claims (1)

[Claims] 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 consists of Fe and unavoidable impurities. A method for producing a thin steel sheet with excellent deep drawability, which comprises winding the steel sheet at a high temperature, followed by cold rolling at a rolling reduction of 40% or more, and continuous annealing.