JPH02267227A - Production of cold rolled steel sheet excellent in press workability - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet excellent in deep drawability and non-ageing characteristic by subjecting a cast plate of a molten steel in which respective contents of C, Mn, S, O, and B are specified to cooling, to hot rolling, and to coiling under respectively prescribed conditions and successively applying cold rolling and continuous annealing to the above. CONSTITUTION:A steel having a composition containing, by weight, <=0.05% C, <=0.3% Mn, 0.02% S, 0.006-0.015% O, and 0.6N-1.5N B is refined. Subsequently, a continuously cast plate of the above steel is cooled at 0.5-10 deg.C/S cooling rate. Then, hot rolling is applied to the above at a temp. of >=Ar3, and the resulting hot rolled plate is coiled at >=600 deg.C, followed by cold rolling and continuous annealing.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing cold rolled steel sheets using a low-cm material as a material, which has excellent deep drawability through continuous annealing, and has little material deterioration due to aging.

(Conventional technology) Cold-rolled steel sheets are mainly used after being press-formed.

Therefore, the constituent factors of press forming, such as deep drawability and stretchability, serve as standards for material properties. Deep drawability corresponds to the recrystallization texture, and stretchability is affected by the steel composition and structure, as well as by strain aging, which changes properties due to solid solution C and H during press forming after manufacturing. Ru. For this reason, cold rolled steel sheets with excellent press formability have been manufactured by box annealing AΩ killed steel.

Since box annealing is annealed in a coil shape, the annealing time is longer.
Poor productivity. Moreover, there is a problem that there is large variation in the material depending on the coil position, and the shape of the steel plate is inferior. A continuous annealing method has been developed as a method to overcome this drawback, and a method for producing cold rolled steel sheets with excellent deep drawability using this method has been disclosed.

There are two technical ideas for these; one is a method using ultra-low C steel to which carbonitride-forming elements such as T+ are added;
The second method involves high-temperature winding of low C-A, 9-killed steel.

The former is Publication No. 44-18068, Special Publication No. 58-2
Although these are disclosed in Japanese Patent Application No. 249, etc., they have the disadvantage of high manufacturing costs due to high alloy costs for decarburization and TI addition in steel manufacturing due to extremely low carbon content. As the latter method, the special public
These techniques are disclosed in Japanese Patent Application Laid-open No. 22533, Japanese Patent Application Laid-open No. 58-6938, etc., but since both techniques require high-temperature winding, variations in the material inside the coil due to uneven thermal history after winding can occur. There is a problem that the pickling properties are large and the pickling properties are poor.

In addition, in order to reduce material deterioration due to aging, it is necessary to take a long time for overaging treatment in continuous annealing, which requires a long overaging zone, which increases equipment costs and operating costs.

(Problems to be Solved by the Invention) Problems in manufacturing steel sheets with good workability through continuous annealing as described above, namely, extremely low C, carbonitride-forming elements such as T1, etc.
, N does not increase the steel manufacturing cost by adding more than the same amount. On the other hand, in order to improve the aging property of low C steel, it is necessary to increase the overaging time of continuous annealing.

The present invention provides a method for manufacturing cold rolled steel sheets with excellent press workability that overcomes these drawbacks.

(Means for Solving the Problems) As a result of various studies on steel compositions and manufacturing conditions, the present inventor has determined that the steel composition has excellent deep drawability regardless of hot rolling conditions, and has less material deterioration due to aging. It was discovered that cold-rolled steel sheets can be manufactured by continuous annealing.

The gist of the present invention is that C in Wm%: 0.05%
Below, M n ; 0, 3% or less, S , 0.020
% or less, o, o, ooeo~0.0150%, B
, 0.6N to 1.5N is continuously cast and cooled at a cooling rate of 0.1 to 1°C/s or less.Subsequently, hot casting is performed at a temperature of Ar 3 or higher. Rolled, 6
The present invention provides a method for producing a cold-rolled steel sheet with excellent deep drawability and non-aging properties, which comprises winding at 00° C. or higher, followed by cold rolling and continuous annealing.

First, the experimental facts that revealed the steel composition, which is an important component of the present invention, will be described.

Various types of steel were melted in a laboratory, slabs were made, and the cooling rate after solidification was kept constant at lθ°C/S and cooled to room temperature. This slab was heated to 1250° C. and then hot rolled to a thickness of 3.8 mm. After pickling, this hot-rolled sheet was cold-rolled to 0.8 mm steel, and
Cooling to 680℃ after recrystallization annealing at 5℃×11n
℃/s s Then cooled at 280"C and 100'C/s, heated again to 351)℃, and heated 0.6℃/s to 250℃.
The mixture was cooled with S, and then cooled with water to room temperature. This steel plate was subjected to a 1.5% skin pass and its material properties were investigated.

At the same time, the surface condition of the cold-rolled steel sheet was visually observed to determine the degree of surface flaws. The evaluation criteria were as follows: ◎ indicates that there are almost no surface flaws, Δ indicates that there are a few surface flaws, and × indicates that there are many surface flaws.

The composition range of steel is C, 0.020% or less, Mn; 0.10
~0.19%, P, 0.08%, S; 0.005
~0.014%, N; 0.0015~0.0026%,
B; 0.0015-0.0020%, 0; 0.00
It is 10-0.023%.

Figure 1 shows the relationship between the amount of O and the material properties after cold rolling annealing.

It can be seen that both the f value and elongation become better as the O content increases up to 0.0060%. However, if it exceeds 0.015%, the elongation and F value, which are indicators of workability, are good, but
As the aging index increases, the surface quality deteriorates and the cold rolled steel sheet loses its value.

From this experimental fact, O is 0.0 OGO to 0.0 O as a condition for changing steel sheets with good workability, aging properties, and few surface defects.
It was determined to be 0.015%.

Next, the steel composition was changed to C; 0.018%, Mn; 0.17%,
S; 0.008%, P, 0.005%, 11; 0.
038%, N; 0.0018%, B, 0.00
Molten steel having a constant concentration of 1fi% and O: 0.010% was cast to obtain a slab. Average cooling rate after solidification of slab (1450℃ ~
700°C) was cooled to 0.05~B temperature, heated again to 1050°C, held at 1250°C, and hot rolled to a thickness of 3.6mm. After pickling this hot-rolled plate,
．． The material was cold rolled to a thickness of 8n+11, annealed in the same way as in the previous experiment, and after a 1.5% skin bath, the material properties were investigated.

The results are shown in Figure 2.

As can be seen from this figure, even if the components are the same, the material properties change significantly depending on the cooling rate of the slab.

It can be seen that when the cooling rate of the slab is in the range of 0.1°C/5-10°C/S, good workability and aging properties can be obtained regardless of the hot rolling heating temperature. If the cooling rate of the slab is slow, good workability can be obtained when the working temperature is low, but aging properties deteriorate. On the other hand, if the cooling rate of the slab is too fast, there are conditions in which the aging property is good, but the workability, particularly the j value, deteriorates.

Based on the above experimental facts, the cooling rate of the slab was determined. The reason why a steel plate having both good workability and aging resistance can be obtained through the appropriate relationship between Qi and the cooling rate of the slab is not clear, but the following is thought to be the reason.

O in molten steel forms an oxide with additive elements in the steel during solidification or cooling, and this oxide contributes to the size of MnS precipitation. The oxide has a specific size distribution, which directly affects crystal rotation during cold rolling and recrystallization during annealing. The average cooling rate of the slab was taken from solidification to 700°C in this experiment, but as described in later examples, it was at least 110°C.
A temperature range of 0°C or higher is sufficient.

Below, steel compositions other than O will be explained.

It has been known that the lower the C content, the better the processability. However, in order to efficiently perform C precipitation during overaging during continuous annealing, it is preferable that the C content is o, oio% or more. If the C content increases to more than 0.05%, good workability cannot be obtained.

From this point of view, the upper limit of the amount of C was determined to be 0.05%.

Like C, it is known that as the amount of Mn added increases, workability deteriorates. Therefore, even with the method of the present invention, 0.5
If the content exceeds %, processability will deteriorate.

For this reason, the upper limit of Mn1l was specified as 0.5%. Since the smaller the amount of Mn, the better the workability becomes, there is no need to particularly limit the lower limit. In the present invention, the content is mainly in the range of 0.08 to 0.20%.

S causes hot embrittlement, so in order to obtain a good steel plate, it is reduced to 0.
．． It is necessary to keep it below 0.020%.

P, N, Sl, etc. are elements harmful to workability.

Therefore, the content is preferably as low as possible.

Steel having such a composition is melted in a conventional converter, electric furnace, etc., with vacuum degassing treatment performed as necessary.

Molten steel is continuously cast to form a slab, and the cooling rate of the slab at this time is an important component, as mentioned above.

That is, it is necessary to control the average cooling rate of the slab to 0.1° C. to 10° C./S. The slab is heated for hot rolling, but if the hot rolling end temperature can be ensured at A r 3 temperature or higher,
The characteristics of the present invention are not impaired by the single-grade heating. Furthermore, the characteristics of the present invention are not impaired by the temperature of the slab before it is inserted into the heating furnace. Therefore, a hot piece such as a hot charge may be inserted into the heating furnace. Alternatively, CC-DR may be performed in which the material is hot-rolled as it is without being charged into a heating furnace.

Hot rolling is carried out in the same manner as in a conventional method, and the finishing temperature must be higher than the A ra temperature from the viewpoint of workability and operability of the steel sheet. In general, for low C steel, the higher the coiling temperature, the better the deep drawability in the case of continuous annealing. Therefore, since the present invention also aims at manufacturing a steel plate for deep drawing by continuous annealing, it is preferable to set the coiling temperature to 600° C. or higher.

The hot-rolled sheet produced in this manner is descaled and then cold-rolled. It is preferable that the cold rolling reduction ratio is 60% or more in order to ensure deep drawability, and in the present invention, it is mainly 70 to 85%.
The cold rolling rate shall be .

Annealing is carried out in a continuous manner, and the temperature may be higher than the recrystallization temperature, but the annealing temperature is high and deep drawability is good. In the present invention, the temperature is mainly in the range of 750 to 850°C. The overaging treatment after recrystallization is preferably carried out at 200 to 400° C. for 2 minutes or more in order to efficiently precipitate solid solution C. The overaging temperature does not need to be constant and may change during overaging.

The steel plate produced in this manner is subjected to temper rolling as required and then used as a product. Although the present invention was made for the purpose of manufacturing cold rolled steel sheets, ZnrZn-FB.

Even when applied as a surface-treated steel sheet plated with AΩ or the like, good deep drawability and overaging properties can be obtained without impairing the features of the present invention.

(Example) Steel having the composition shown in Table-1 was molten in a converter and manufactured under the manufacturing conditions shown in Table-1, and the results of investigation of the material properties are shown in the same table.

All manufacturing conditions other than Table 1 are the same, and the cold rolled steel plate is 0.
．． 80mm, annealing is 68mm after recrystallization at 800℃ x 111n
Cool down to 0℃ at 5℃/S, then cool at 100℃/'S for 28 hours.
A cycle of cooling to 0°C, heating again to 350°C, and cooling to 250°C in 3 minutes was carried out. Annealed plate: 1.5%
The material properties were investigated after temper rolling and accelerated aging at 100°C for 1 hour.

The material properties are shown in Table-1.

Coil No. A-1 is within the scope of the present invention both in terms of components and manufacturing conditions, and is an indicator of workability. High value, A
I is low. Coil No. B is C.

Coil No., C is Mn, : ] Illu No., D is S1 coil N (LE.

Even if F and B are outside the scope of the present invention and other manufacturing conditions are within the scope of the present invention, are they both indicators of workability? Many have a low value and a large aging index. coil) JII
Although H and O are outside the range of this component, this coil had many surface defects and could not be tested for processes after cold rolling.

Coil NαG-1 is a condition within the scope of the present invention. Coil N
CL-2 and -3 are within the scope of the present invention in terms of components, but the manufacturing conditions are outside the scope of the present invention. This coil does not have characteristics that satisfy both workability and aging properties.

Coil NαA-2 is an example of CC-DR.

In this case as well, a steel plate with good material properties was obtained, and by controlling the steel composition and the cooling rate of the continuously cast slab, a cold rolled steel plate with good workability can be obtained regardless of the hot rolling heating conditions. It can be seen that manufacturing is possible. In this CC-DR, 110
Hot rolling starts from 0℃, and the cooling rate of the slab is only controlled up to 1100℃, and it is said that the temperature range for controlling the cooling rate of the slab is 0.00℃ or higher. Recognize.

As described above, it is understood that cold-rolled steel sheets with good deep drawability and no material deterioration due to aging can be manufactured only when the steel composition and manufacturing conditions have a close and inseparable relationship.

(Effects of the Invention) The present invention does not require extremely low C or even without adding a large amount of TI to fix C and N, leaving only a trace amount of 0.
Steel sheets with excellent deep drawability and little aging deterioration are inexpensive to manufacture, and can be manufactured by continuous annealing regardless of the hot rolling heating temperature.

Therefore, the manufacturing cost is significantly lower than that of the conventional technology, and it can be manufactured regardless of the hot rolling heating temperature, meaning that there are no restrictions on production operations, making it an industrially useful invention. be.

[Brief explanation of drawings]

Figure 1 is a diagram showing the relationship between Offfi and the material properties of steel sheets.
FIG. 2 is a chart showing the relationship between the cooling rate of the slab and the material properties of the steel plate. Figure 1

Claims (1)

[Claims] C: 0.05% or less, Mn: 0.3% or less, S: 0.020% or less, O: 0.0060 to 0.0150%, B: 0.6N to Steel containing 1.5N is continuously cast, cooled at a cooling rate of 0.5 to 10℃/s or less, then hot rolled at a temperature of Ar_3 or higher, coiled at 600℃ or higher, and subsequently cooled. A method for producing cold-rolled steel sheets with excellent deep drawability and non-aging properties, characterized by inter-rolling and continuous annealing.