JPS63105931A - Production of cold rolled steel strip having excellent deep drawability from thin continuously cast steel strip - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel strip having excellent deep drawability without any development of rough surface by controlling the cooling velocity in the specific temp. range after solidifying in a continuous casting process under a specific steel composition, to omit the hot rolling process. CONSTITUTION:Molten steel composing of wt% of 0.0010-0.080% C, 0.05-1.0% Mn, 0.005-0.30% Si, 0.003-0.10% P, <=0.020% S, 0.005-0.070% sol.Al, 0.0010-0.0070% N, 0.0003-0.0030% B, <=0.080% Ti under satisfying Ti >=3.42N+1.5S+4C and further containing 0.003-0.050% Nb and the balance of Fe and inevitable impurities, is continuously cast. The obtd. cast strip having <=10mm thickness is cooled at >=V( deg.C/s) cooling velocity for 950-750 deg.C after solidification. Next, after winding it is cold-rolled and annealed. Herein, V( deg.C /s)=5.0X10<-4>/(Nb%XB%).

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a cold rolled steel sheet with excellent deep drawability at low cost using a thin continuous cast steel strip obtained by a continuous casting process. .

(Prior Art) Conventionally, as described in, for example, the 3rd edition of the Iron and Steel Handbook, Vol. The component steel is 200~2
The process involved continuous casting slabs or blooming slabs with a thickness of 501 m and heating and hot rolling them to a thickness of 2 to 6 nm, which required a long rolling mill and consumed a lot of energy. It was huge.

Recently, attempts have been made to make the thickness of slabs obtained by continuous casting as close as possible to the product thickness.

Specifically, cold rolled stock (hot gauge strip)
There is a method of manufacturing directly by continuous casting, and a method of manufacturing intermediate gauge material (30 to 60 m thick) by continuous casting in the hot rolling process to simplify and omit the hot rolling process.

Although the process of obtaining cold-rolled material directly by continuous casting is extremely superior in terms of process simplicity, the cold-rolled steel sheets obtained by this process are hard and have poor workability, and the crystal grains of the product are Despite its fineness, the surface becomes rough during processing, so it cannot be used for processing.

(Problems to be Solved by the Invention) The present invention solves the aforementioned problems in the prior art,
The object of this invention is to provide a manufacturing method for obtaining a cold-rolled steel sheet that has excellent deep drawability and does not cause surface roughness during processing, by a process that omits the hot rolling step.

(Means for solving problems) The gist of the present invention is as follows.

In weight%, C: 0.0010-0.0080%, Mn
: 0.05~1.0%, Si: 0.005~0
．． 30%, P: 0.003-0.10%, S:
0.020% or less, 5otkl: 0.005-0.0
70%, N: 0.0010-0.0070%, B
: 0.0003-0.0030%, Ti: 0.0
80% or less and Ti≧3.42 N + 1.53
+4 C5Nb: Molten steel containing 0.003 to 0.050%, with the balance consisting of Fe and inevitable impurity elements, is continuously cast to form a thin cast strip with a thickness of 10 mm or less, and after solidification, a 95 mm
Excellent deep drawability from a thin continuous cast steel strip characterized by cooling between 0 and 750°C at a cooling rate of V ('C/s) or more, coiling, cold rolling, and annealing. A method for producing cold-rolled steel sheets.

However, V ('C/3) -5,0X 10-'/ (%) x
B(χ)] The present invention will be described in detail below.

As a result of studying the steel composition, slab thickness, and thermal history after solidification, the inventors limited the steel composition and controlled the cooling rate in a specific temperature range after solidification of the slab in the continuous casting process. As a result, it was discovered that a cold-rolled steel sheet with excellent deep drawability and no surface roughness during processing could be produced using a process that completely omitted the hot rolling step.

First, the compositional limitations of the cold-rolled steel sheet for deep drawing of the present invention will be described.

It has long been known that the less carbon there is, the better the deep drawability of cold-rolled steel sheets will be.However, in order to reduce the carbon content to 0.0010% or less, it is extremely difficult to melt the steel. Become. On the other hand, if C is contained in excess of 0.0080%, the workability of the cold-rolled steel sheet deteriorates, and when carbonitride-forming elements are added, it is necessary to This not only increases the manufacturing cost, but also raises the problem that the recrystallization temperature becomes high and high-temperature annealing is required. In addition, the C content is 0.0010 to 0.0 because it also deteriorates the workability of cold rolled steel sheets.
It was set at 08%. The preferred range is 0.0010-0.00
It is 70%.

St, Mn, and P are effective elements for increasing the strength of cold-rolled steel sheets.

In the present invention, since not only soft cold-rolled steel sheets but also high-strength cold-rolled steel sheets are subject to the invention, Si: 0.005 to 0.
30%, Gate: O, OS ~ 1.0%,
The component range is P i O, 003 to 0.10%.

That is, when manufacturing high-strength cold-rolled steel sheets,
Sf: 0.30%, Mn: 1.0%, P: 0.10
The upper limit is %.

When the Si content exceeds 0.30%, the paintability of cold-rolled steel sheets deteriorates, and when the Mn content exceeds 1.0%, it becomes difficult to produce ultra-low C steel. , P is 0
．． When the amount exceeds 10%, not only the spot weldability of the cold-rolled steel sheet deteriorates, but also P segregates at grain boundaries, which deteriorates the secondary workability when pressing the cold-rolled steel sheet.

For these reasons, Sis M! The upper limit of 1% P was defined as above.

On the other hand, when manufacturing soft cold rolled steel sheets, Si: 0.0
05-0.02%, Mn: 0.05-0.50%, P
: It is preferable to set it as 0.001-0.020%.

When Mn exceeds 0.50%, the steel becomes hard and a cold-rolled steel sheet with high ductility cannot be produced. For the same reason, St.
o, oso% or less, P is preferably 0.020% or less.

S is an element that causes hot embrittlement.

In the present invention, since hot rolling is not performed, there is no problem of hot embrittlement, but when the slab (thin cast strip) is directly cold rolled, if there is a large amount of S, edge cracking is likely to occur. Furthermore, since the cold-rolled A-sheet becomes hard, when manufacturing a soft cold-rolled steel sheet, S needs to be 0.020% or less.

M needs to be contained in an amount of 0.005% or more as acid-soluble M in order to prevent surface scratches caused by Ti oxides. However, when the M content becomes too large,
Since alumina-based inclusions increase and surface flaws conversely increase, the upper limit of acid-soluble M is set at 0.070%. The preferred range is 0.015-0.050%.

N is fixed as TiN by Ti, but too much N increases the amount of Ti added, increasing costs and deteriorating workability, so it is preferable to have less. Therefore, the upper limit of N is 0.0070%. The lower limit is 0.0010%, which is achievable with current technology.

Ti has the deep drawability, ductility, and
To ensure non-aging property, Ti amount is 1.53 + 3.42
It is necessary to satisfy N+4C. On the other hand, the greater the amount of Ti, the better the workability, but if it exceeds o, oso%, the effect is saturated and the addition cost increases further, which is not economical, and at the same time, surface defects due to Ti oxide increase. do.

B and Nb are important components of the present invention, along with the cooling rate of the slab. We will discuss the experimental facts that revealed the reason for this limitation.

C; 0.0010-0.0070%, 5tH0.01
%, Mn; 0.1-0.3%, Ti; 0.043
~0.080%, P; 0.012%, sol
/V; 0.015-0.053%, N; 0.00
25-0.0039% B in steel; 0.0001-0
．． 0045%, Nb; 0.001-0.070%
4. Molten steel changed to 4. Immediately after solidification, the slab was cooled to 650°C by changing the cooling rate, and then kept at 650°C for 1 hour for slow cooling. This slab was cold rolled to a fin of 0.8, recrystallized at 825°C
A rough skin test was conducted after taking a skin bath. In the skin roughness test, the bulge was bulged out to a certain height, and the degree of skin roughness at that time was evaluated in three stages. A rating of 1 means that there is no rough skin, and a rating of 2 means that there is some rough skin. A rating of 3 indicates that rough skin has occurred.

fI4vi with Nb content, B content, and cooling rate of 50°C/S or more
Figure 1 shows the relationship between the skin roughness rating and the skin roughness rating. The lines in the figure indicate the boundaries of the rough skin scores of many tests, and the numbers in the figure indicate the rough skin scores in that range. As shown in FIG. 1, it can be seen that rough skin can be avoided by adding Nb and B in combination, and adding 0.0003% or more of B and 0.003% or more of Nb. On the other hand, as for the upper limit, it can be seen from FIG. 1 that the effect is saturated when added above a certain level. Therefore B
The upper limit of Nb was specified to be 0.0030%, and the upper limit of Nb was specified to be 0.050%. The grain size of the steel plate in Figure 1 is 8.0.
It was in the range of ~9.8.

Next, Figure 2 shows the relationship between NbXB, cooling rate, and skin roughness. The numbers indicate rough skin scores in that range. Second
As is clear from the figure, it can be seen that the skin roughness is influenced by NbxB and the cooling rate. In other words, when the cooling rate is fast, N
Even if bXBfi is low, skin roughness does not occur. On the other hand, when the cooling rate is slow, roughening of the surface can be prevented by increasing the amount of NbXB sufficiently. That is, the cooling rate of the slab; V('
C/S)≧5.0×10”’/ (Nl) (%)
xB (%)], skin roughness can be prevented. Based on this fact, the amounts of Nb and B, the cooling rate, and their relationships were defined.

Next, the slab was cooled to 0.8 tm by keeping the cooling rate constant at 50°C/S and changing the cooling temperature range, and then rolled to 825°C.
After recrystallization annealing of X 1 ml and 1.0% skin pass, the same rough skin test and material test as in FIG. 1 were performed. The results are shown in FIG.

Among other manufacturing conditions, the steel composition is C; 0.0035%r
Si, 0.009%, Mn; 0.21%,
P, 0.009% l Sio, 006%, A
n! ;0.51%, N, 0.0027%+71;
0.055%, Nb i O, 015%, B,
0.007%, and the slab thickness is 4.0 fl. Note that the O mark in Figure 3 indicates the cooling start temperature: 950°C or higher and the cooling end temperature: 750°C or lower, and the Δ mark indicates the cooling start temperature:
The cooling end temperature is 950°C or lower or 750°C or higher. As can be seen from FIG. 3, in order to improve the skin roughness, the cooling start temperature is 950°C or higher and the cooling end temperature is 750°C or lower. On the other hand, from the viewpoint of workability, it is preferable to set the cooling end temperature to 600° C. or higher. Based on this fact, the cooling start temperature of the slab was specified as 950°C, and the cooling end temperature was specified as 750°C.

Steel having such components is melted in a converter or electric furnace by vacuum degassing treatment as required to form a cast steel strip. The thickness of the cast steel strip is preferably 10 nm or less. The reason for this is to ensure the cooling rate of the slab and the cold rolling rate that will allow good deep drawing based on the thickness of the product. This cast steel strip was heated to 950℃
It is cooled at V ('C/s) or higher between ~750°C and wound up.

(However, V ('C/s) -5,OX 10-'/ (
%)×B(χ)〕).

Note that even if the slab is rolled by 20% or less to correct its shape, the characteristics of the present invention will not be impaired.

The winding temperature is preferably 750°C or lower and as high as possible. After cooling, the wound coil is descaled and
Cold rolled. The cold rolling temperature may be any number of times as long as it does not recover or recrystallize during rolling. With the steel composition of the present invention, the temperature at which it does not recover or recrystallize during rolling is 600°C or lower. The cold rolling rate is 50-95%. This cold rolled sheet is annealed, and the annealing may be continuous annealing or box annealing. The annealing temperature is higher than the recrystallization temperature and lower than 890°C. The steel sheet manufactured in this way is subjected to temper rolling as required and then used as a product.

Even if the steel sheet manufactured by the method of the present invention is used as a surface-treated steel sheet, the features of the present invention will not be impaired.

Example A cold rolled steel plate was manufactured using the ingredients and manufacturing conditions shown in Table 1,
The material properties and roughness of the steel plate were investigated. In the skin roughness test, the bulge was extended to a certain height, and the degree of skin roughness at that time was evaluated in three stages. Rating 1 means no skin irritation, rating 2
A score of 3 indicates that the skin is slightly rough, and a score of 3 indicates that the skin has become rough. For secondary workability, a drawing cup with a drawing ratio of 2.0 was expanded at 0°C, and samples with no vertical cracks were marked with ○, and samples with vertical cracks were marked with x.

From the results in Table 1, it can be seen that the products produced by the method of the present invention have a high r value, which correlates with deep drawability, and are also excellent in rough skin resistance. It can also be seen that the secondary processability is excellent. Coil No. 2 is an example in which 20% hot rolling was performed to correct the shape, and coils l'& t5 and 6.7 are examples of high-tensile steel. Even though the components are within the scope of the present invention, the cooling rate of the slab is outside the scope of the present invention! 'H8 not only has poor skin roughness but also poor workability. On the other hand, even though the manufacturing conditions are within the scope of the present invention, m9.10, which is outside the scope of the present invention in terms of components, not only has poor rough skin properties but also has poor processability. It can be seen that the components of the coil magnet 11 are outside the scope of the present invention, the cooling rate of the slab is also outside the scope of the present invention, and both material properties and surface roughness are inferior.

As can be seen from the above examples, the close and inseparable relationship between the steel composition and the cooling rate of the slab makes it possible to produce cold-rolled steel sheets that have excellent deep drawability and do not experience surface roughness during processing.

(Effects of the Invention) According to the present invention, as is clear from the above examples, steel with a limited chemical composition range is continuously cast into a thin slab, and by controlling the cooling process of the slab, Like,
Even if hot rolling using a powerful row of hot rolling mills is completely omitted, it is possible to produce a cold-rolled steel sheet that has excellent deep drawability and does not experience surface roughness during processing. In this way, the present invention is industrially extremely useful because it enables energy saving and significant cost reduction due to the omission of steps.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between Nb, Bii and surface roughness after cold rolling and annealing, Figure 2 is a diagram showing the relationship between NbXB, cooling rate of slab and roughness after cold rolling and annealing, and Figure 3 Figure C Cooling rate of slab; V ("C/s) = 5. OXl0
-'/(%)×B(χ)] is a diagram showing the relationship between the cooling start and end temperatures at a cooling rate that satisfies the following, the roughness of the surface after cold rolling and annealing, the r value, and the elongation. Figure 1 Nb (%) Figure 2 Nb-8 ('A) (xσ5) Figure 3

Claims (1)

[Claims] In weight %, C: 0.0010 to 0.0080%, Mn:
0.05-1.0%, Si: 0.005-0.30%,
P: 0.003 to 0.10%, S: 0.020% or less,
solAl: 0.005-0.070%, N: 0.00
10-0.0070%, B: 0.0003-0.003
0%, Ti: 0.080% or less, and Ti≧3.42N
Molten steel containing +1.5S+4C, Nb: 0.003 to 0.050%, with the balance consisting of Fe and unavoidable impurity elements, is continuously cast into a thin cast strip with a thickness of 10 mm or less, and after solidification is heated at 950 to 750°C. A cold-rolled steel sheet with excellent deep drawability made from a thin continuous cast steel strip, which is characterized by being cooled at a cooling rate of V (° C./s) or more and then coiled, followed by cold rolling and annealing. Production method. However, V (℃/s) = 5.0 x 10^-^4/[Nb (%) x
B (%)]