JPH0716718A - Manufacture of cold rolled steel sheet excellent in surface property - Google Patents

Abstract

PURPOSE:To improve workability and surface property without causing the excess restriction of component and increase of cost at the time of manufacturing a cold rolled dead soft steel sheet by continuously casting under specified casting conditions. CONSTITUTION:A cold rolled steel sheet contains, by weight, <=0.01% C, <=0.5% Si, <=1,0% Mn, <=0.02% P, <=0.02% S, 0.01-0.1% Al, 0.01-0.08% Ti and satisfies (C/12+N/14)<=Ti/48 and consists of the balance Fe with inevitable impurities is manufactured. As the conditions at the time of continuously casting, when casting velocity is expressed by V (m/min), amplitude of casting mold by (a) (m) and frequency of casting mold N by (min<-1>), (a) <=0.006 (m) and time (t) (min) that is expressed by the equation I should satisfy 0.1<=(60/N)-120t<=0.22 (s). Then, by reducing nail part which is formed in the surface layer of slab at the time of continuous casting, inclusions in steel floats and separates without being captured, so the steel sheet having few surface defects is manufactured.

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, and more particularly, a pressed part such as a body of an automobile, particularly a body panel of a car body, which is required to be beautiful. The present invention relates to a method for manufacturing a cold rolled steel sheet used for a part.

[0002]

2. Description of the Related Art As surface defects found in cold-rolled steel sheets,
Point defects usually called blowholes and blister,
There are linear defects called sliver. Many of these defects are caused by those existing near the surface of gas and non-metallic inclusions captured during the production of steel ingot from molten steel after the completion of refining in a converter etc. It also causes the above-mentioned defects during cold rolling.

As a method for preventing these defects, a method of containing Cr in molten steel (Japanese Patent Publication No. Sho 49-11972) and a method of defining the amounts of Si and Al which are deoxidizing agents (Japanese Patent Publication No. 51- 4935), a technique for floating, separating, and reducing inclusions in steel, and MgO for molten steel after composition adjustment.
There is known a method (Japanese Patent Laid-Open No. 59-129714) in which the inclusions in the steel themselves are made to be ductile and easily broken or dispersed by adding a flux of —CaO / CaF ₂ . In these technologies, elements necessary for controlling the material of the steel sheet are restricted, deterioration of the material due to addition of unnecessary raw materials, and an upper limit of cost are incurred.

In recent years, C≤
It has become possible to easily manufacture 0.01% ultra-low carbon steel. Further, so-called IF steel (see, for example, Japanese Patent Publication No. 44-18066) in which carbon and nitrogen in steel are precipitated and fixed in the form of carbides and nitrides is subjected to a continuous annealing process like conventional Al-killed steel. It is suitable for production in hot dip galvanizing line because it does not require over-aging treatment when it is manufactured.In addition, the automobile industry has been improving the corrosion resistance of car bodies and its excellent press formability. The production amount has increased dramatically and is widely used in various industrial fields.

Further, in order to complicate the design of an automobile, reduce the weight of a vehicle body and reduce the number of assembling steps, there is an increasing number of integrally formed press working methods for integrally press forming a plurality of parts, which has been conventionally used. User needs for improving material properties are increasing year by year.

In particular, the amount of carbon, which has a great influence on the properties of the steel sheet, has been steadily decreasing and is now about 0.00
3% and its content have decreased sharply. As a method of removing carbon in steel, a method of blowing oxygen during melting in a converter to remove carbon in steel as CO gas, and further reducing the carbon content to a predetermined value in an RH vacuum degassing device or the like is common. Is.

However, in order to reduce the carbon content in the steel, the amount of blown oxygen required to remove CO gas increases, and the deoxidizing agent added to remove excess oxygen in the steel after refining is also added. A large amount is required, which results in an increase in non-metallic inclusions produced by the reaction of the elements in the steel with oxygen.

Since the amount of non-metallic inclusions increases as the amount of carbon in the steel decreases in this way, surface defects caused by this increase occur in areas where the surface condition is required to be beautiful, mainly for automobile outer panel applications. In recent years, it has become a big problem for the steel sheets used.

[0009]

As described above, in the conventional technique, it is necessary to unnecessarily limit the components in the steel and consider the deterioration of the material, and it is preferable to reduce the oxygen content in the steel. It is difficult to obtain a cold-rolled steel sheet having both excellent surface properties and high workability.

Under the circumstances, the present invention solves the above-mentioned drawbacks of the prior art, and does not cause unnecessary restriction of components and increase of cost, and is excellent in workability and surface properties. It is intended to provide a method for manufacturing a rolled steel sheet.

[0011]

Means for Solving the Problems The inventors of the present invention have diligently studied a method capable of solving the above-mentioned problems in an ultra-low carbon cold-rolled steel sheet, and as a result, have found that there is a close contact between continuous casting conditions and surface texture. The present invention has been completed by newly discovering that there is such a relationship.

That is, according to the present invention, C ≦ 0.01%, Si
≤ 0.5%, Mn ≤ 1.0%, P ≤ 0.02%, S ≤ 0.0
2%, Al: 0.01 to 0.1%, Ti: 0.01 to 0.08
%, And (C / 12 + N / 14) ≦ Ti / 48 is satisfied, and if necessary, Nb: 0.003 to 0.05% and B: 0.0.
In producing a cold-rolled steel sheet containing 0005 to 0.002% of 1 type or 2 types and the balance of Fe and unavoidable impurities, the conditions for continuous casting are V (m
/ Min), the mold amplitude is a (m), and the mold frequency is N (min ^-1 ), a ≤ 0.006 (m), and

[Equation 2] The time t (min) represented by is 0.1 ≦ (60 / N) −12
The gist is a method for producing a cold-rolled steel sheet having excellent surface properties, which is characterized in that continuous casting is performed under the condition that 0t ≦ 0.22 (s) is satisfied.

[0013]

The present invention will be described in more detail below. First, the reasons for limiting the chemical composition of steel in the present invention will be described.

C: C is an element that greatly affects the properties of the steel sheet. The lower the C content, the better the material.
If the amount is large, the material deteriorates and excellent mechanical properties cannot be obtained, so the upper limit is made 0.01%.

Si, Mn: The smaller the content of Si, Mn, the softer the steel, the more the elongation, and the better the deep drawability. Addition of a large amount of both elements deteriorates the characteristics, so the upper limit of the Si content is 0.5%,
The upper limit of the Mn content is 1.0%.

P and S: P and S are all impurity elements, so it is desirable to reduce them as much as possible.
It is acceptable if it is about 2% or less.

Al: Al is an element useful as a deoxidizer at the time of refining molten steel, and needs to be added in an amount of 0.01% or more.
However, addition of a large amount leads to an increase in refining cost, so the upper limit is made 0.1%.

Ti: Ti is an element useful for improving the material by fixing and precipitating solid solution components such as C, N, S in steel. However, if the content is less than 0.01%, the effect of addition is poor. On the other hand, if the content is more than 0.08%, the effect cannot be expected to increase. Therefore, the Ti content is limited to the range of 0.01 to 0.08%.

Further, in the present invention, Ti is (C / 12 + N / 14) so that C and N can be completely precipitated and fixed by Ti.
Add in a range that satisfies ≦ Ti / 48.

In addition to the above essential components, the following components can be added in appropriate amounts, if necessary.

Nb: Nb has an effect of fixing C like Ti, and has an effect of complementing the effect of the present invention.
However, the addition of Nb causes a cost increase, and the effect cannot be expected to increase even if added in a large amount. Therefore, the content thereof is limited to the range of 0.003 to 0.05%.

B: B has a function of improving the secondary workability, so that the effect of the present invention is further improved, but the addition thereof in a large amount lowers the deep drawability, so the content thereof is 0.1.
It is limited to the range of 0005 to 0.002%.

The steel having the above chemical composition is melted by a usual method, but in the present invention, it is necessary to limit the casting method of the molten steel to the continuous casting method and continuously cast it under a specific condition.

That is, as the conditions for continuous casting, the casting speed is V (m / min), the mold amplitude is a (m), and the mold frequency is N (mi).
n ⁻¹ ), a ≦ 0.006 (m), and

[Equation 3] The time t (min) represented by is 0.1 ≦ (60 / N) −12
It is necessary to satisfy the condition of 0t ≦ 0.22 (s). The reason for the limitation will be described below.

The main cause of the surface defects of steel plates such as automobile outer plates whose surface is required to be beautiful is the inclusions present in the surface layer of the slab. Most of the inclusions in the steel float during continuous casting and separate, but they cannot be completely separated.Some of the inclusions are the parts that are formed in the surface layer of the slab called claws at the early stage of solidification. It turned out to be captured. The inclusions trapped in the claws cause the surface defects such as sliver after the rolling process such as hot rolling and cold rolling.

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

The inventors of the present invention have studied the means for preventing the claw formation and found that the mold amplitude a is 0.006.
(m) or less, and by improving the lubrication between the mold and the solidified slab, the inclusions are floated, the formation of claws that hinder the separation is significantly suppressed, and the rolling process such as hot rolling and cold rolling is performed. It was found that the surface defects such as the later sliver are reduced.

That is, a ≦ 0.006 (m), and

[Equation 4] The time t (min) represented by is 0.1 ≦ (60 / N) −12
It was found that the claw portion is reduced when 0t ≦ 0.22 (s) is satisfied. If continuous casting is performed under the conditions that do not satisfy these relationships, the above effect cannot be obtained.

The slab produced by continuous casting is subsequently hot-rolled, pickled, cold-rolled and annealed, but these conditions are not particularly limited, but the following production conditions are used. Is preferred.

First, in the hot rolling, the slab is cooled to room temperature and then inserted into the hot rolling heating furnace. At that time, the HCR method may be used in which the slab is not cooled to room temperature but is inserted into the heating furnace. The effect of the present invention is not impaired at all, not only by rolling the slab without reheating it, but also by rolling the slab after heat retention for a short time and / or partial heating. At that time, the heating temperature of the slab may be normally 1000 to 1250 ° C., but it is desirable to be as low as possible if the finishing temperature can be secured.

The hot rolling finishing temperature is preferably in the range of 870 to 950 ° C. If it is lower than 870 ° C., a texture that is disadvantageous to deep drawability after annealing develops, which is not preferable, and if it exceeds 950 ° C., grain growth in the austenite region is remarkable, and the ferrite grain size after transformation becomes coarse, so that after annealing. Not suitable for deep drawability. 50-100 immediately after finish rolling
It is desirable to make the hot-rolled sheet finer by starting strong cooling at ℃ / s.

The coiling temperature is preferably 700 ° C. or lower. At a temperature higher than 700 ° C., the variation of the material in the longitudinal direction of the coil becomes large, and it is not desirable from the viewpoint of descaling.

The rolled hot-rolled steel sheet is pickled by a usual method and then cold-rolled. Cold rolling rate is 60-9
5% is preferable. If the rolling ratio is less than 60%, the texture preferable for deep drawability does not sufficiently develop after annealing, and 95%.
If it exceeds, the in-plane anisotropy increases, which is not preferable.
A more preferable range is 75 to 90%.

The cold rolled coil is annealed. Recrystallization annealing may be either box annealing or continuous annealing, and surface treatment such as electroplating or hot dip galvanizing may be performed.

Next, examples of the present invention will be described.

[0036]

[Example 1]

Sample steels having the chemical compositions shown in Table 1 were melted in a converter and made into slabs by the continuous casting method under the conditions shown in Table 2, and the slabs were subjected to hot rolling, cold rolling and recrystallization annealing. Then, the mechanical properties were investigated. The results are also shown in Table 2.

In Table 2, No. 1, No. 3 to 6, No. 14 to
21 is an example of the present invention. Since all the steel sheets were manufactured under the chemical composition and process conditions within the scope of the present invention, they show very good characteristics (high r value). In addition, no surface defect was observed in any of them. On the other hand, No. 2 is manufacturing conditions (finishing temperature)
Is outside the preferred conditions of the present invention, and Nos. 7 to 13 have chemical compositions outside the scope of the present invention, and therefore the properties of the steel sheet are inferior.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

Example 2

Among the test steels shown in Table 1, some steels were continuously cast into slabs under the conditions shown in Table 3,
Hot rolling, cold rolling, and recrystallization annealing were performed to investigate the surface properties. The results are shown in Table 3 and FIG.

FIG. 1 shows the time T (second) determined by the mold amplitude when the steel shown in Table 1 was continuously cast under the conditions shown in Table 3 and the continuous casting conditions (casting speed, mold amplitude, mold frequency).
And the surface texture of the annealed steel sheet shown in Table 3 is evaluated, the steel sheet produced from the slab continuously cast under the condition that the time T satisfies the range of the present invention. It has very few surface defects (especially sliver scratches) and has a beautiful surface. On the other hand, the steel sheet continuously cast under the condition that the time T is out of the range of the present invention has many defects such as sliver, and has a poor surface property.

[0044]

[Table 3]

[0045]

As described above, according to the present invention, the defects on the surface of the steel sheet can be remarkably reduced. And an excellent effect can be obtained.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between a mold amplitude during continuous casting in an example and a time T (second) determined by continuous casting conditions (casting speed, mold amplitude, mold frequency).

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C21D 8/02 A 7412-4K 9/46 G C22C 38/00 301 S 38/14 (72) Invention Hideo Kageyama 1 Kanazawa-cho, Kakogawa-shi, Hyogo Prefecture Kobe Steel Works Kakogawa Works

Claims (3)

[Claims] 1. In weight% (hereinafter the same), C ≦ 0.01.
%, Si ≦ 0.5%, Mn ≦ 1.0%, P ≦ 0.02%, S
≤ 0.02%, Al: 0.01 to 0.1%, Ti: 0.01 to
In producing cold-rolled steel sheet containing 0.08% and satisfying (C / 12 + N / 14) ≦ Ti / 48, and the balance Fe and unavoidable impurities, the conditions for continuous casting are respectively When the velocity is V (m / min), the mold amplitude is a (m), and the mold frequency is N (min ^-1 ), a ≤ 0.006 (m) and The time t (min) represented by is 0.1 ≦ (60 / N) −12
A method for producing a cold-rolled steel sheet having excellent surface properties, which comprises continuously casting under conditions satisfying 0t ≦ 0.22 (s). 2. A hot temperature of 1000 to 1250 ° C. and a finishing temperature of 8 when hot rolling the continuously cast slab.
The hot rolling is performed under the conditions of 70 to 950 ° C. and the winding temperature of 700 ° C. or less, after pickling, cold rolling is performed at a rolling rate of 60 to 95%, and annealing is performed at a recrystallization temperature or higher to 900 ° C. The method according to 1. 3. The steel further comprises Nb: 0.003 to 0.0.
5%, B: 0.0005-0.002% 1 or 2 types are contained, The method of Claim 1 or 2.