JPS59123721A - Production of cold rolled steel sheet having excellent processability - Google Patents

Abstract

PURPOSE:To obtain a cold-rolled steel sheet having excellent processability by casting continuously a molten steel contg. limited amt. of C and Nb in the steel compsn. and subjecting the cast steel directly to hot rolling under a limited average cooling rate and hot rolling conditions. CONSTITUTION:A molten steel contg. <=0.0030wt% C, <=0.4% Mn, 0.01-0.08% Sol Al, and <=0.023% Nb with an atom ratio in a range of 0.2<=Nb/C<=1.0 and consisting of the balance Fe and unavoidable impurities is continuously cast. The cast steel is continuously cooled, after solidification, at <=35 deg.C/min average cooling rate from 1,400 deg.C to the Ar3 transformation point. The cooled steel is subjected to rough rolling at hot at >=60% draft and after the finish rolling is completed at >= (the Ar3 transformation point-100 deg.C), the rolled steel is coiled at >=550 deg.C. The coiled steel is cold rolled and is then continuously annealed. A cold-rolled steel sheet having excellent ductility and processability is obtd. by the above-mentioned method.

Description

[Detailed description of the invention] The present invention relates to a method for manufacturing a cold rolled steel sheet with excellent workability,
In particular, it relates to a manufacturing method of ultra-low carbon Nb steel by continuous casting and direct rolling.

In general, cold-rolled steel sheets with excellent workability are usually made of low C steel of Coo, 02% or less to ensure ductility, so Ar
, the metamorphosis point is high. Therefore, when so-called direct rolling is performed in which hot rolling is repeatedly completed by reheating the slab after casting, Ar is likely to occur at the end of hot rolling, resulting in rolling in a two-phase region below the transformation point.

Generally, when two-phase region rolling is performed, a (100) plane texture develops, and even after cold rolling annealing, a strong (1, OOJ plane texture remains), which lowers the 〒1 straightness and deteriorates the laminar workability. For low-carbon cold-rolled steel sheets, where importance is placed on low-carbon cold-rolled steel sheets, slab reheating treatment is essential, and direct rolling treatment is difficult to adopt.
was.

The object of the present invention is to solve the above-mentioned problems of the prior art.

It is an object of the present invention to provide a method for manufacturing a cold rolled steel plate with excellent workability by directly rolling a continuously cast slab of ultra-low carbon Nb steel.

The gist of the present invention is as follows.

C: 0.0030% or less j'vi at a weight ratio of 5
n204% or less, 5oiAt: 0.01-0.
08%, Nb: 0.023% or less, and the atomic ratio is within the range of 0.2 < Nb/C < 1.0, and the remaining part consists of Fe and inevitable impurities; After the molten steel in the continuously cast slab is completely solidified, the continuously cast slab is continuously cooled to 1400°C in Ar at an average cooling rate of 35°C/min or less until the transformation point; After rough rolling with a reduction rate of 60% or more (A
Finish rolling is completed at the r3 transformation point -100°C) or higher, and the rolling is completed at 550
℃ or higher, a step of annealing the hot rolled steel strip after cold rolling, and a method for producing a cold rolled steel sheet with excellent workability, the method comprising: .

The reason for limiting the components in the present invention will be explained.

C: Limiting the amount of C is the most important requirement in the present invention. It has been known that the lower the C content, the better the elongation and RF workability of steel. However, when the C content is less than 0.0030%, rolling occurs in the two-phase region below the Ar point and the (100) texture develops, deteriorating the workability of the cold-rolled annealed plate. Therefore, the slab is usually reheated. By ensuring the finish rolling temperature and avoiding rolling in the two-phase region, deterioration in workability7 is prevented.

The present inventors conducted the following basic experiment. That is, by changing the C content, Mn: 0.40% or less, Si
:02% or less, Nb/c atomic ratio 0.2 < Nb/
After casting continuous cast slabs with C < 1.0, Ar
Cooling is performed at an average cooling rate of 35° C./min or less up to the point.

Rough rolling reduction rate 78%, finish rolling temperature 870°C, finish thickness 2
．． 8 wn, hot rolled at a coiling temperature of 660°C,
Then, after cold rolling to 0.8 tan, 780℃×40
Continuously annealed and temper rolled at 0.08% under conditions of sec,
The r value of the manufactured cold rolled steel sheet was measured, and the relationship between the r value and the amount of C is shown in FIG. In Figure 1, the amount of C is 0.0030%
In the following description, since the value 7 exceeds 1.5 and indicates excellent workability, the upper limit of the amount of C, 0.0030%, was set as a necessary condition for the present invention.

Mn: Mn is an element necessary to prevent the formation of FeS with a low melting point during hot rolling, but if it is added in excess of 0.4%, r@ decreases and it cannot be used as a cold rolled steel sheet for processing. Since this would be inappropriate, the upper limit of Mn was limited to 0.4%.

5otAL: 5olAt has the effect of fixing N in the value, but 0.
If it is less than 0.01%, it becomes difficult to fix lI′ltN, and solid solution N
A part of the C is consumed by Nb, and the C fixation ability of Nb decreases, so that solid solution C remains in the base. This increase in solid solution C is r
@w Decrease There is a possibility that the machinability of awning may be significantly deteriorated.

-If 5o7At exceeds O,OS%, it becomes a substitutional solid solution atom, increasing the strength of the matrix and deteriorating its ductility. For this reason, the content was limited to 0.01% to 0.08% at 5otAt.

Nb: Nb1I-f In the present invention, it is the second most important chemical component after C, and has the function of fixing the amount of C as Nb(C,N) and improving workability7, but Nb accounts for 0.023%. ``When the b/C atomic ratio exceeds 1.0%, the strength of the base increases and the ductility deteriorates.On the other hand, when the Nb/C atomic ratio is less than 0.2, the workability improvement effect of Nb addition cannot be seen. For these reasons, the Nb content was limited to 0.023% or less, and the C/C atomic ratio was limited to a range of 0.2 to 1.0.

Next, the manufacturing conditions for the steel of the present invention having the above-mentioned limiting components will be explained.

First, an experiment regarding cooling of continuously cast slabs will be described. C: 0.0025%, Si: tr, Mn: 0
．． Continuously cast slabs of 2%, 5otAt: 0.025%, Nb/c atomic ratio: 052 were cooled at various average cooling rates from 1400°C after complete solidification to Ar, and the rough rolling reduction rate was 60. % or more and less than 60%.

Finishing 1ni880t:, Finishing thickness 3. Otm, winding m
After hot rolling at K65°C and then cold rolling to a thickness of 0.8 m, continuous annealing was performed at 800°C for 40 seconds.

For reference, we have also included the so-called reheated material, which is heated in a furnace for 2 hours or more at a temperature of Ar, then cooled to below the Ar temperature point, then reheated to the Ac temperature for more than 2 hours after casting, and the components other than cooling and heating, rolling conditions, etc. The same cold-rolled steel sheet was manufactured. The r value of these cold rolled steel sheets was measured, and the relationship between the average cooling rate between points and the r value from 1400 -°C to Ar is shown in Figure 2. The cooling and rough rolling conditions in FIG. 2 are as shown in Table I''i.

From Table 1 and Figure 2, 1400℃~Ar, average cooling rate between points 35
℃/min or less and the rough rolling reduction is 60% or more, the T value exceeds 1.5, indicating that the workability is excellent. Even if the cooling rate is slow, it is difficult to obtain the desired workability when the rough rolling reduction is less than 60%. From this result, in order to obtain a steel with good workability, the average cooling rate from 1400° C. to the Ar point was set to 35° C./min or less, and the rough rolling reduction was limited to 60% or more.

Next, the finishing temperature of finishing rolling (Ar, transformation point -100℃)
The reason for this limitation is that finishing below this temperature will significantly reduce the ductility and 7-value of the cold-rolled steel sheet, resulting in poor workability (
Finish rolling was limited to finish at temperatures higher than Ar (transformation point -100°C). In addition, the coiling temperature of the hot rolled steel sheet is set at 550℃ or higher.If the coiling temperature is lower than 550℃, the ductility of the cold rolled steel sheet will deteriorate significantly as it will not be sufficiently softened even by continuous annealing after cold rolling, making it unsuitable for processing. Therefore, the winding temperature of hot rolling was limited to 550°C or higher.

Cold rolling and annealing of the hot-rolled steel strip of the present invention produced under the above conditions are not particularly limited and may be carried out by conventional methods, but a cold-rolled steel sheet with excellent workability can be obtained.

Example: Steel slabs whose chemical compositions are shown in Table 2 are continuously cast in a converter and cooled and hot-rolled under the conditions listed in Table 2 to produce hot-rolled steel with a plate thickness of 30 sheets. After pickling, the strip was cold-rolled to a thickness of 0.8 mm, continuously annealed at 800° C. and 40 SeC, and then temper-rolled at a rolling reduction of 08%. Note that the columns in which the chemical components and hot rolling conditions shown in Table 2 of Comparative Examples do not satisfy the limiting conditions of the present invention are marked with a circle.

The mechanical properties of these cold rolled steel sheets were investigated and the results are shown in Table 3. In Table 3, test steels A-F, which are examples of the present invention that satisfy the composition and manufacturing conditions of the present invention, have an elongation of 7.
Sample steel G~0, which is a comparative example that does not satisfy the limiting conditions of the present invention, has a high strength and a V value of 1, although it has a high value and excellent workability.
．． It can be seen that it is less than 5 and is not suitable for processing.

As is clear from the above examples, the present invention is applicable to components, particularly C1.
By limiting the amount of B, the average cooling rate of continuously cast slabs was reduced to 35℃/
Cold rolled steel sheets with good workability can be produced at low cost by hot direct rolling in which the hot rolling conditions are limited to less than 1 min.

[Brief explanation of drawings]

FIG. 1 is a correlation diagram showing the relationship between the C content and the 7-value, and FIG. 2 is a correlation diagram showing the relationship between the average cooling rate of the slab between 1400° C. and three Ar points and the 7-value.

Claims (1)

[Claims] (1) C:0 in weight ratio. (3030% or less, Mn: 0
4% or less, 5oLAt: 0.01-0.08%, N
Contains 0.023% or less of b2 and has an atomic ratio of 0.2 <
Continuously casting molten steel with a Nb/C ratio of 1.0 and the remainder consisting of Fe and unavoidable impurities;
°C to V, r Average cooling rate to Ar3 transformation point lff35
℃/min or less, and the cooled slab is subsequently rough rolled at a reduction rate of 60% or more (Ar, transformation point -100℃) or higher, and finish rolling is completed at 550℃ or higher. 1. A method for rolling a cold-rolled steel sheet with excellent workability, the method comprising the steps of: hot-rolling the hot-rolled steel strip; and annealing the hot-rolled steel strip after cold-rolling.