JPH08158007A - Production of cold rolled steel sheet excellent in surface characteristic and workability - Google Patents

Abstract

PURPOSE: To produce an extra low carbon cold rolled steel sheet excellent in tensile characteristics and deep drawability and minimal in surface defects by specifying the relation among casting velocity, mold amplitude, and vibrational frequency of casting. CONSTITUTION: The steel has a composition consisting of, by weight, 0.0005-0.0035% C, <=1% Si, 0.05-0.6% Mn, <=0.10% P, 0.005-0.10% Al, <=0.0050% N, $0.0050% O, and the balance iron with inevitable impurities. This steel is cast continuously, hot-rolled, cold-rolled, and annealed, by which a cold rolled steel sheet is produced. At the time of this continuous casting, when casting velocity, mold amplitude, and vibrational frequency of casting are represented by V(m/min), (a)(m), and N(/min), respectively, the mold amplitude satisfies a=0.006(m) and also the time (t)(min) represented by t = (1/2πN)COS<-1> (--V/2 aπN) satisfies 0.1sec<=(60/N)-120t<=0.22sec. By this method, this cold rolled steel sheet can be suitably used for the purposes where the attractiveness of surface is strictly required.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a cold-rolled steel sheet having excellent surface properties and workability, and more particularly, to parts to be pressed such as automobile bodies, especially for outer panels of the body. For example, the present invention relates to a method for manufacturing a cold-rolled steel sheet that can be suitably used for manufacturing a member that requires an excellent appearance.

[0002]

2. Description of the Related Art Surface defects found in cold-rolled steel sheets include
Usually, there are point defects called blow holes and blisters, and linear defects called sliver. Many of these defects are caused by gas trapped in molten steel during the production of steel ingots from molten steel after the completion of refining in a converter or the like, and non-metallic inclusions present near the surface of the steel sheet, such as alumina. The inclusions and the like cause the above-mentioned defects during hot rolling and cold rolling of the steel sheet.

As a method of preventing these defects, as described in Japanese Patent Publication No. 49-11972,
Cr is contained in the molten steel, or as described in Japanese Patent Publication No. 51-4935, Si or Al which is a deoxidizing agent.
The amount of the inclusions in the molten steel is floated, and the inclusions are separated to reduce the amount, and JP-A-59-1297.
As described in Japanese Patent Publication No. 14, the composition of MgO—CaO / CaF ₂ flux is added to the molten steel after adjusting the components so that the inclusions in the steel are ductile and easily broken or dispersed. It is known how to do.

However, according to these methods, the elements necessary for controlling the material of the steel sheet are limited, or unnecessary addition of raw materials causes deterioration of the material and an increase in manufacturing cost. In recent years, due to the progress of steelmaking degassing technology, it has become possible to easily produce ultra-low carbon steel having a C content of 0.01% or less. Moreover, for example, Japanese Patent Publication No. 44-1
As described in Japanese Patent No. 8066, so-called IF steel (Interstitial Free steel, for example, Japanese Examined Patent Publication No.44-), in which carbon and nitrogen in the steel are precipitated and fixed in the form of carbide or nitride.
No. 18066) is suitable for production in a hot dip galvanizing line because it does not require over-aging treatment when it is produced in a continuous annealing process like a conventional Al-killed steel. Combined with the fact that the industry has improved the corrosion resistance of car bodies and its excellent press formability, the production volume has increased dramatically, and it is widely used in various industrial fields. However, IF steel requires addition of Ti, Nb, etc., and has a high component cost.

In particular, the amount of carbon, which has a great influence on the properties of the steel sheet, has been steadily decreasing, and recently, it has been about 0.
The amount of 003% is drastically reduced. As a method of removing carbon in steel, oxygen is blown during melting in a converter to remove oxygen in the steel as carbon monoxide gas, and further a suction type vacuum degassing device, for example, a RH method, etc. It is common to reduce the carbon content to the specified value.

However, in order to reduce the amount of carbon in the steel, the amount of blown oxygen required to remove it as carbon monoxide gas increases, and the deoxidation added to remove excess oxygen in the steel after refining is required. Since a large amount of the agent is also required, non-metallic inclusions produced by the reaction of the elements in the steel with oxygen will increase. As described above, the amount of non-metallic inclusions increases as the carbon content in steel decreases, and the surface defects caused by this increase are used for members whose surface condition is required to be beautiful, mainly for automotive outer panel applications. In recent years, it has become a big problem for steel sheets.

[0007]

As described above, in the prior art, it is necessary to unnecessarily limit the components in the steel and consider the deterioration of the material, regardless of the decrease in the oxygen content in the steel. Moreover, it is difficult to obtain a cold-rolled steel sheet having both good surface properties and high workability. The inventors of the present invention have earnestly studied in order to solve the above-mentioned problems in the production of a cold-rolled steel sheet having both conventional surface properties and workability, and as a result, in order to improve workability, an ultra-low carbon Al killed By using steel as a raw material and further appropriately controlling the conditions in continuous casting, it was found that a cold-rolled steel sheet having both excellent surface properties and workability can be obtained, and the present invention was achieved. is there.

Therefore, an object of the present invention is to provide a method for producing a cold-rolled steel sheet which is excellent in both surface properties and workability without causing unnecessary restrictions on the components and increase in cost.

[0009]

A method for producing a cold-rolled steel sheet having excellent surface properties and workability according to the present invention is C 0.0005 to 0.0035% by weight%, Si 1.0% or less,
Mn 0.05 to 0.6%, P 0.10% or less, Al 0.
005 to 0.10%, N 0.005% or less, and O
In a method for continuously casting a steel having 0.0050% or less and a balance of iron and unavoidable impurities, and producing a cold-rolled steel sheet through hot rolling, cold rolling and annealing, the casting speed at the time of continuous casting Is V (m / min), template amplitude is a
(M) and the casting frequency are N (/ min), the mold amplitude is a ≦ 0.006 (m), and t = (1 / 2πN) cos ⁻¹ (−V / 2aπN). It is characterized by satisfying the condition that the represented time t (minute) is 0.1 (second) ≤ (60 / N) -120t ≤ 0.22 (second).

First, the reasons for limiting the chemical composition of steel in the present invention will be explained. In the method of the present invention, the upper limit of the amount of C is set to 0.0035% in order to obtain workability, especially deep drawability (r value), with Al killed steel. Especially 0.002
By setting the content to 5% or less, workability is further improved.
You can get the desired results. However, the amount of C is 0.0
If it is less than 005%, it takes a long time for degassing in steel making, and the productivity decreases. Preferably, the lower limit is 0.00
10%.

Si is an element that enhances the strength without impairing the workability, and is added in an appropriate amount according to the required strength. However, when added in excess, the electroplating property and the pickling property of the hot-rolled steel sheet deteriorate, so the upper limit of the addition amount is 1.
0% However, in the case of mild steel plate, the addition amount is 0.02%
The following are preferred. Mn is added at least 0.05% in order to prevent hot brittleness due to S during hot rolling. However, excessive addition causes deterioration of workability and increase of refining cost, so the addition amount is set to 0.6% or less. Preferably, it is in the range of 0.10 to 0.30%.

P is an element that enhances the strength without impairing the deep drawability, and is added in an appropriate amount according to the required strength.
However, if added too much, the steel sheet becomes brittle, so the upper limit is made 0.10%. In the case of mild steel plate, the addition amount is
The range of 0.005 to 0.015% is preferable. Al is added in an amount of 0.005% or more for adjusting the deoxidation of steel.
When adding too much, alumina-based inclusions increase,
Since the ductility is deteriorated, the upper limit of the added amount is 0.10%. In particular, when box annealing is adopted as the annealing method and the cold-rolled steel sheet according to the present invention is manufactured, Al that precipitates during heating
In order to improve the deep drawability due to N precipitates, the addition amount is preferably in the range of 0.020 to 0.060%.

Since N is a component that affects the deep drawability and strain aging, the upper limit is 0.0050%. As described above, when adopting box annealing as the annealing method, in order to improve the deep drawability due to the AlN precipitates that precipitate during heating, as in the case of the above Al, 0.0020 to 0.0050 is used.
% Range is preferred. On the other hand, when continuous annealing is adopted as the annealing method, since the heating rate of the steel sheet is high, it is not possible to expect improvement in deep drawability by AlN as in box annealing. Therefore, when the amount of N is large, the deep drawability deteriorates. Therefore, the lower the amount of N is, the more preferable it is.
The range of 010 to 0.0025% is optimal.

O is a very important element when decarburizing molten steel during refining in a converter or RH. However, if it remains unnecessarily in the molten steel, it will combine with other elements in the steel and increase non-metallic inclusions, causing deterioration of the surface properties,
In addition, the mechanical properties of the final product steel sheet are also deteriorated. For these reasons, the O content is 0.0050% or less, especially 0.0
030% or less is preferable.

Next, a method of manufacturing the cold rolled steel sheet according to the present invention will be described. The steel having the above-described chemical composition is melted by a usual method, but according to the present invention, it is necessary to satisfy the conditions according to the present invention when casting the molten steel.
That is, according to the present invention, when continuously casting molten steel,
When the casting speed is V (m / min), the mold amplitude is a (m) and the casting frequency is N (/ min), the mold amplitude is a ≦ 0.006 (m), and t = (1 It is necessary to set the time t (minutes) represented by / 2πN) cos ⁻¹ (−V / 2aπN) to 0.1 (seconds) ≦ (60 / N) −120t ≦ 0.22 (seconds).

The reason will be described below. The main cause of the surface defects of steel plates such as outer plates of automobiles whose surface is required to be beautiful is the inclusions existing in the surface layer of the slab. According to the research conducted by the present inventors, most of the inclusions in steel float and separate during continuous casting, but they cannot be completely separated, and some of the inclusions in the slab rise during continuous casting. After the rolling process such as hot rolling, cold rolling, etc., the surface of the sliver is caught by the so-called claws formed at the early stage of solidification in the surface layer, which is caused by the inclusions captured by the claws. It turned out to cause defects.

Therefore, based on such knowledge, the inclusions in the steel are floated and separated without being trapped by reducing the claws formed on the slab during continuous casting. By reducing the amount remaining in the slab and reducing the inclusions present on the slab surface, it is possible to manufacture a steel sheet with few surface defects.

The inventors of the present invention have conducted extensive studies to prevent the formation of the claws, and as a result, have determined that the template amplitude a is 0.00.
By setting the length to 6 (m) or less and improving the lubrication between the mold and the solidified slab, the formation of claws that hinder the floating and separation of inclusions is significantly suppressed, and hot rolling, cold rolling, etc. It was found that the surface defects such as sliver after the rolling process of No. 1 were reduced.

That is, according to the present invention, a ≦ 0.006
(M) and 0.1 (seconds) ≦ (60 / N) −120
When t ≦ 0.22 (second) is satisfied, the claw portion is reduced.
Electric mold type, electric cam type, and hydraulic type are practically used as the vibration source of the mold, but the hydraulic type allows the amplitude and frequency to be arbitrarily selected by remote control during casting. There is an advantage that the mechanical structure near the mold can be simplified. In addition, in order to prevent disturbance to the vibration waveform, the vibration fulcrum has a structure that is independent of the surrounding machinery and the cast floor frame,
Care has been taken to increase the natural frequency of the entire vibration system and maintain a correct vibration waveform for a long period of time.

According to the present invention, the slab obtained by the above continuous casting is hot-rolled, pickled, cold-rolled and annealed according to a conventional method to obtain the intended cold-rolled steel sheet. However, preferably, while continuously casting molten steel under the above-mentioned conditions, hot rolling, cold rolling and annealing are performed under the conditions described below, whereby not only the surface texture but also the workability It is possible to obtain an excellent cold rolled steel sheet.

In the present invention, the heating temperature of the slab prior to hot rolling is not particularly limited, but it is usually
It may be 1150 ° C or higher. In particular, when the cold-rolled steel sheet according to the present invention is manufactured by continuous annealing, low temperature heating at 1000 to 1100 ° C. may be performed in order to further enhance the deep drawability. Moreover, you may perform the direct hot rolling which rolls a slab without passing through a heating furnace, and the hot charge rolling which heat-rolls without cooling to room temperature.

However, in the present invention, the finishing temperature of hot rolling is set to Ar ₃ point or higher. When the finishing temperature is lower than the Ar ₃ point, crystal grains in the hot rolled steel sheet become coarse, and a processed structure is formed, resulting in deterioration of deep drawability in the final product. The upper limit of the finishing temperature is preferably 950 ° C. When the finishing temperature exceeds 950 ° C, in the hot rolled steel sheet,
Not only does the crystal grains become coarser and the deep drawability deteriorates, but it also takes a long time to heat before hot rolling, resulting in a high energy cost. Particularly, according to the present invention, the finishing temperature is set to A
It is preferable to set the temperature in the range of r ₃ to (Ar ₃ +50) ° C., whereby the crystal grains of the hot rolled steel sheet can be made finer and the deep drawability can be further enhanced.

In ultra low carbon steel, crystal grains are likely to grow, and when cooling after hot rolling finish is slow, coarse grains are formed, and deep drawability deteriorates and material anisotropy deteriorates. In order to make the crystal grains finer, it is better that the cooling rate is faster. Therefore, in the present invention, after finish rolling, the film is cooled to the coiling temperature at an average cooling rate of 30 ° C./sec or more. On the other hand, when the cooling rate is too fast, the shape of the steel sheet deteriorates, which hinders productivity. Particularly preferably, in the present invention, this average cooling rate is in the range of 50 to 120 ° C./sec.

Next, if the winding temperature is too high, the pickling property after hot rolling is impaired, so the upper limit is 720 ° C. After cold rolling, when annealing is carried out by continuous annealing, the coiling temperature is preferably in the range of 650 ° C to 720 ° C, and when carried out by box annealing, it is preferably 650 ° C or lower, and particularly precipitation of AlN occurs. A lower temperature is preferable, but it is usually 400.
The range of to 550 ° C is preferable.

Then, the coil of the hot-rolled steel sheet thus obtained is pickled by a conventional method to remove the scale, and then subjected to cold rolling. Although the cold rolling ratio is not particularly limited, since ultra-low carbon steel is used in the present invention, the deep drawing property is improved as the cold rolling ratio is higher. Therefore, the cold rolling rate is preferably a high cold rolling rate of 70 to 85%.

After such cold rolling, the steel sheet is annealed at the recrystallization temperature or higher and the Ac ₃ point or lower. The annealing method may be box annealing or continuous annealing. In the case of box annealing, either tight coil or open coil annealing may be used, and the atmosphere gas is not particularly limited. In the case of continuous annealing, the cooling method after soaking, the conditions, and the overaging temperature are not particularly limited. The annealing temperature is higher than the recrystallization temperature, but in the case of box annealing, it is 60
0 to 700 ° C., and in the case of continuous annealing, 700 to 850 ° C. is desirable.

After the annealing, temper rolling of 0.2 to 1.5% can be applied to correct the shape and eliminate the elongation at yield.
The cold-rolled steel sheet after annealing may be subjected to surface treatment such as electric or hot dip galvanization, tin plating, and chromium plating, if necessary.

[0028]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples.

Example 1 Steels having the chemical compositions shown in Table 1 were melted in a converter and were cast under the casting conditions specified in the present invention, that is, the casting speed V was 1.4.
m / min, mold amplitude a is 0.003 m, mold frequency N is 19
A slab was formed at 2 / min and T = 60 / N-120t = 0.117 seconds. This slab was hot-rolled under the conditions shown in Table 2 and wound up to obtain a hot-rolled steel sheet having a thickness of 3.6 mm. This hot-rolled steel sheet was pickled, cold-rolled to a thickness of 0.8 mm, continuously annealed under the conditions shown in Table 2 or recrystallized by box annealing, and then tempered at 0.8%. It was rolled.

Table 2 shows the results of an examination of the tensile properties (yield point, tensile strength, elongation and n value) and deep drawability (r value) of the steel sheet thus obtained. The occurrence rate of surface defects determined by the evaluation criteria described later was 2% or less in most of the test examples, and a cold rolled steel sheet having excellent surface properties was obtained. However, steel 1
Since only No. 1 had a large amount of oxygen, it had many surface defects, especially sliver flaws, and had poor surface properties.

In Table 2, the tensile test was based on JIS No. 5 tensile test. The r value is (r _L + r _C + 2r _N ) /
Defined in 4. Here, L, N and C indicate the rolling direction, the 45 ° direction and the right angle direction, respectively. In Table 2, the invention steels 1 to 5 have tensile strength and ductility (elongation and n
Value) and has a deep drawability. On the other hand, steel 7 having a chemical composition outside the range specified in the present invention
Nos. 11 to 11 are inferior to the elongation, at least one of the n value and the r value, and do not have the target characteristics. In particular, steel 7 is a solid solution C
The amount was large and the aging resistance was poor.

Next, the influence of manufacturing conditions was examined for Steel 1 having a chemical composition within the range specified in the present invention. Steel 1
All of A to 1C have the same chemical composition as Steel 1 shown in Table 1, but the production conditions were changed as shown in Table 2. However, the casting conditions are the same as above and are within the range specified in the present invention. The test results are also shown in Table 2.

As is clear from Table 2, steels 1A and 1B
Since the finishing temperature and the cooling rate are outside the scope of the present invention,
The deep drawability is inferior. On the other hand, since the coiling temperature of Steel 1C is too high, not only the pickling property is inferior, but also the crystal grains in the hot rolled sheet are coarsened, the anisotropy is large, and the deep drawability is also inferior.

[0034]

[Table 1]

[0035]

[Table 2]

Example 2 Steel having a chemical composition within the range specified in the present invention shown in Table 1 was continuously cast under the conditions shown in Table 3. The steel numbers in Table 3 correspond to the steel numbers in Table 1. The resulting slab is hot rolled, cold rolled,
The surface properties of the cold-rolled steel sheet obtained by annealing were investigated.

For the evaluation of surface texture, a coil hot-rolled from the same slab was cold-rolled, annealed, and then a test piece having a width of 500 mm and a length of 500 mm was cut, and the surface thereof was observed. The evaluation criteria are as follows. ⊚ indicates that no surface defects are observed, ◯ indicates that the occurrence rate of surface defects is 2% or less, Δ indicates that the occurrence rate of surface defects is 2 to 5%, and x indicates that the occurrence rate of surface defects is 5% or more. The results are shown in Table 3.

The test numbers 1, 4, 6, 8 and 10 produced from the slabs cast under the condition that the time T (second) determined by the continuous casting conditions in the continuous casting satisfy the present invention have few surface defects. Excellent surface quality. On the other hand, other test numbers 2, 3, 5, 7, 9 and 11 in which the time T (second) is out of the range defined by the present invention have many defects such as sliver, and the surface properties are inferior.

[0039]

[Table 3]

[0040]

As described above, according to the present invention, it is possible to produce an extremely low carbon cold-rolled steel sheet which is excellent in tensile properties and deep drawability and has very few surface defects. The rolled steel sheet can be suitably used for applications in which the beauty of the surface is strictly required, such as the outer panel of an automobile.

Claims (2)

[Claims] 1. C. 0.0005 to 0.0035%, Si 1.0% or less, Mn 0.05 to 0.6%, P 0.10% or less, Al 0.005 to 0. Steel having 10%, N 0.0005% or less, and O 0.0050% or less, the balance of which is iron and unavoidable impurities, is continuously cast, and subjected to hot rolling, cold rolling and annealing to obtain a cold rolled steel sheet. In the manufacturing method, in continuous casting, when the casting speed is V (m / min), the mold amplitude is a (m) and the casting frequency is N (/ min), the mold amplitude is a ≦ 0.006 ( m) and the time t (minute) represented by t = (1 / 2πN) cos ⁻¹ (−V / 2aπN) is 0.1 (seconds) ≦ (60 / N) −120t ≦ 0.22 (Sec) is satisfied, and a method for producing a cold-rolled steel sheet having excellent surface properties and workability, which is characterized by satisfying the following condition. 2. A continuous casting slab is hot-rolled at a finishing temperature of at least ₃ Ar points and then immediately cooled at an average cooling rate of 30.
Cool at ℃ / sec or more and wind at a temperature of 720 ℃ or less,
Next, pickling, cold rolling, and annealing at a recrystallization temperature or higher and an Ac ₃ point or lower, are carried out, and the method for producing a cold rolled steel sheet according to claim 1.