JP2910543B2 - Steel sheet with excellent surface properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an ultra-low carbon steel sheet having excellent surface properties used for automobiles and electric parts.

[0002]

2. Description of the Related Art With today's advances in steelmaking degassing technology, ultra-low carbon steels in which the amount of carbon in steel has been reduced to 30 ppm or less have become relatively inexpensive and large-scale. So-called IF having excellent formability by further adding carbonitride forming elements such as Nb and Ti.
Steel is becoming widely used.

[0003] By the way, in oxidation of steel, C in steel is
Combines with O from the atmosphere to become CO or CO ₂ gas,
When they move from the inside of the steel to the atmosphere, they are taken in the way, making the scale porous and facilitating peeling. However, ultra-low carbon steel has an extremely low C, and the above effects cannot be expected. Therefore, the peelability of the scale is poor as compared with ordinary low carbon steel, and the scale that has not been completely removed by descaling is removed during hot rolling. Frequently penetrates steel plates and causes surface defects.

As a countermeasure against this, it is conceivable to enhance the descaling ability. However, with the increase in equipment costs and the decrease in surface temperature due to the descaling, the occurrence of surface cracks due to the decrease in hot ductility and the finishing temperature can be secured. Another problem arises, such as disappearance.

[0005] The present invention has been made in view of such circumstances, and has as its object to provide a steel sheet having reduced surface defects and excellent surface properties, based on ultra-low carbon steel.

[0006]

In order to solve the above-mentioned problems, the present invention firstly provides a method for producing a C: 0.
003% or less, Mn: 0.05-2.2%, Si: 0.
8% or less, P: 0.1% or less, S: 0.015% or less,
sol. Al: 0.01 to 0.08%, N: 0.003
%, O: 5 to 30 ppm , Sn: 5 ppm or more, 5
0 ppm , B: 2 to 9 ppm , B (pp
m) A steel sheet which satisfies the relationship of ≧ 0.1 × Sn (ppm) -1 and is excellent in surface properties characterized by being composed of the balance of Fe and unavoidable impurities.

Second, in terms of% by weight, C: 0.003% or less, Mn: 0.05 to 2.2%, Si: 0.8% or less,
P: 0.1% or less, S: 0.015% or less, sol. A
l: 0.01 to 0.08%, N: 0.003% or less,
O: 5 to 30 ppm , Sn: 5 ppm or more, 50 ppm
Less, B: contains 2~9ppm, B (ppm) ≧ 0 .
1 × Sn (ppm) -1 is satisfied, and Ti:
Provided is a steel sheet having excellent surface properties, characterized by containing 0.005 to 0.12%, the balance being Fe and unavoidable impurities.

Third, Cu: 0.01-0.5% and Ni: 0.01-0.5% with respect to the first or second steel sheet.
%, Cr: 0.01 to 0.5% is provided, and a steel sheet having excellent surface properties is provided.

In the present invention, an important constituent requirement is to add a composite of Sn which promotes scale peeling to improve surface properties and B which suppresses surface layer boundary cracking during hot rolling.

[0010] The ultra-low carbon steel to which B is added is disclosed in
19442, JP-A-2-163323, JP-A-2-163
No. 267231 discloses an ultra-low carbon steel to which Ti-B is added.
Although these are disclosed in 149943, they are not intended to improve the surface properties by adding Sn. Further, with respect to the Sn-containing ultra-low carbon steel, Japanese Unexamined Patent Publication No.
No. 371528 discloses that when Sn contains 0.005 to 0.020%, a winding temperature or a slab heating temperature is defined to minimize the adverse effect of Sn on deep drawability. However, it is not intended to actively add Sn to improve the surface properties. In fact, the amount of Sn in the examples is 0.007%, which is higher than that of the present invention, and the object and the range of the amount of Sn are completely different from those of the present invention. Further, Japanese Unexamined Patent Publication No.
No. 56 discloses a steel to which Sn and B are added. The purpose of the steel is to improve the magnetic properties.
And the addition amount of B is 0.05% or more and 30 ppm or more, respectively, which are extremely higher than those of the present invention, and are completely different from the present invention.

As described above, although there are prior arts in which Sn, B, etc. are added to ultra-low carbon steel, the fundamental idea is different from the present invention. That is, the concept of obtaining an ultra-low carbon steel having excellent surface properties by adding Sn-B in combination is the first finding of the present inventors.

Hereinafter, the present invention will be described in detail. First, Sn among the above-mentioned additional elements is concentrated at the interface between the base material and the scale in an oxidizing atmosphere, and reduces the adhesion of the scale.
It has the effect of promoting scale peeling by descaling and improving the surface properties. However, on the other hand, Sn precipitates at austenite grain boundaries, reduces the hot ductility of the steel sheet, and increases scale defects caused by hot-rolled grain boundary cracks in the surface layer.

In the present invention, B is added to solve this problem. B segregates at the austenite grain boundary similarly to Sn, but strengthens the grain boundary contrary to Sn and cancels the adverse effect of Sn. In particular, when the amount of Sn added is large, the amount of B added also needs to be increased. However, if B is added excessively, it causes slab cracking in the continuous casting stage and deteriorates the surface properties. Therefore, the addition amount has an appropriate range.

FIG. 1 shows the amount of Sn added and the amount of B added and the defect rate of the scale defect of the hot-rolled-pickled coil (defects having a length of 2 cm or more are assumed to be 0.5 m per piece, and the total sum is divided by the coil length. FIG. In the figure, a white circle indicates a defective rate of 0.06 to 0.08%, and a white square indicates a defective rate of 0.
01-0.05%, black triangle is defective rate 0.16-0.52
%. When the amount of Sn added is less than 5 ppm, scale peeling failure occurs, and B is less than 2 ppm or 0.1 × S
When the value is less than n-1 and the content of Sn is more than 50 ppm, hot rolling cracks are generated, and in each case, the scale defect occurrence rate is high. Therefore, from the viewpoint of improving surface properties, Sn: 5pp
m, less than 50 ppm , B: defined as 2 to 9 ppm ,
In addition, it is necessary to satisfy the relationship of B (ppm) ≧ 0.1 × Sn (ppm) −1.

The next important element is O. O exists as an oxide in steel and suppresses grain growth of austenite. Since hot-rolled grain boundary cracks become more remarkable as austenite is coarser, O is added in an amount of 5 ppm or more from that viewpoint. However, if it is added excessively, B segregates on the oxide surface, the effective B amount contributing to grain boundary strengthening decreases, and hot-rolling grain boundary cracks become evident. Is specified at 30 ppm or less where no odor occurs.

The lower the content of C, the more advantageous the moldability. Therefore, in the present invention, the upper limit of C is set to 0.0030.
%. Although Mn is preferably low in material, it is effective for improving hot ductility. Therefore, from that viewpoint, the amount of added Mn is specified in the range of 0.05 to 2.2%.

It is preferable that Si is lower in material, and the upper limit is set to 0.8%. P is a grain boundary embrittlement element, and is preferably low from the viewpoint of hot ductility, and its upper limit is set to 0.1%.

If S is too large, it weakens the grain boundaries and causes cracking during hot rolling. Therefore, it is desirable that S is small, and the upper limit is made 0.015%. Al needs to be added to deoxidize steel. However, if a large amount is added, Al ₂ O
₃ to increase the grain boundary segregation amount of B effective for improving hot ductility. 0.0 as Al content
It is specified in the range of 1 to 0.08%.

N is preferably low from the viewpoint of moldability, and the upper limit is made 0.003%. The above are the basic components of the present invention. In the present invention, Ti may be added to these basic components as necessary, or one or two of Cu, Ni, and Cr may be used instead of Ti or together with Ti. Seeds or more may be added.

Ti is a useful element that fixes C and N and improves formability. In particular, TiN formed by combining with N suppresses austenite grain coarsening by a pinning action and improves hot ductility. To obtain such an effect, it is necessary to add 0.005% or more.
Even if it is added in excess of 0.12%, the effect is saturated and economic efficiency is impaired. Therefore, when adding Ti, the amount is specified in the range of 0.005 to 0.12%.

Cu, Ni, and Cr have the effect of changing the properties of the scale to those that are easy to peel off and enhancing the surface properties. If any of them is less than 0.01%, the effect is not recognized, and if the addition exceeds 0.5%, the effect is saturated. Therefore, when these are added,
The added amount is specified to be 0.01 to 0.5%, respectively.

The present invention can be applied to any of a hot-rolled steel sheet, a cold-rolled steel sheet, and a surface-treated steel sheet, and the effects of the present invention can be obtained in any case. The steel sheet of the present invention having the above chemical composition is usually melted and cast in a converter or an electric furnace in accordance with a conventional method, and then hot-rolled or pickled and then cold-rolled into a steel sheet having a desired thickness.

Although the production method is not particularly limited, hot rolling is performed at a heating temperature of 1150 ° C. or more and a finishing temperature of ₃ points or more. Is ensured, the effect of the present invention can be maximized. The effect of the present invention is not impaired even when hot rolling is performed directly after casting or when rough rolling is not performed in hot rolling.

Annealing after pickling or after cold rolling may be box annealing, continuous annealing, or may be performed using a continuous galvanizing line. It does not matter whether or not the alloying treatment is performed in the latter case. Further, the effects of the present invention are not impaired when surface treatment such as electroplating, organic composite film formation, or chemical conversion treatment is performed alone or in combination after temper rolling after annealing.

[0025]

EXAMPLE The steel of the present invention and the comparative steel having the component compositions shown in Table 1 were continuously cast and then directly hot-rolled (steel 1,
7, 13, 16, 19, 24, 33) or a slab is cooled to room temperature and then reheated to 1200 to 1300 ° C and hot-rolled (steel 2 to 6, 8 to 12, 15, 17, 17, 18,2
0 to 23 , 25 to 32 , 34 , 35) , 1.8 to 4.0 mm by cooling-winding process on the run-out table
A hot-rolled steel sheet having the following thickness was obtained. The average finishing temperature was 900 ° C and the winding temperature was 640 ° C.

In the case of forming a cold-rolled steel sheet, the hot-rolled steel sheet is pickled, then cold-rolled to a sheet thickness of 0.6 to 1.6 mm, and continuously annealed at 760 to 850 ° C.-0.5% Temper rolling.

The hot-dip galvanized steel sheet is cold-rolled, then continuously annealed at 820 ° C., cooled to 460 ° C., and then plated with 55 g / m ² of hot-dip zinc per side, and subsequently alloyed at 500 ° C. Was done. After the temper rolling of 1.0%, a part of the steel was further 3 g / m ² per side.
80% Fe-Zn alloy is subjected to upper electrogalvanizing,
A hot-dip galvanized steel sheet was used.

For the electroplated steel sheet, after temper rolling,
Electroplating of 88% Zn-Ni alloy at 30 g / m ² per one side was performed. With respect to the organic-coated steel sheet, composite coating was further performed on the electroplating with a chromate layer of 50 g / m ² and a resin layer of 1 μm.

Table 1 shows the types of these steel sheets.
It was also described in. The scale defect rate of these steel sheets was determined. As the scale defect defect rate, one defect of 2 cm or more at the stage of the hot-rolled steel sheet was set to 0.5 m, and the value obtained by dividing the total by the length of the hot-rolled coil was used. Table 1 shows the results.

[0030]

[Table 1]

As is clear from Table 1, the steels 19 to 35, which are comparative steels, have at least one of the Sn, B, and O contents out of the proper range.
The steels 1 to 18 of the present invention in which the content is 16% or more and the content is in the appropriate range have a scale defect rate of 0.08% or less, and good surface properties are obtained. In particular, steels 7 to 18 to which one or more of Ti and / or Cu, Ni, and Cr are added have a further lower defect rate of scale defects of 0.05% or less, and more excellent surface properties can be obtained. Was confirmed.

[0032]

As described above, according to the present invention,
It is possible to reduce the scale defect which is a problem in the ultra-low carbon steel sheet without installing new equipment and without changing the manufacturing process, and thus it is possible to obtain a steel sheet having excellent surface properties. The present invention can be applied to any of a hot-rolled steel sheet, a cold-rolled steel sheet, and a surface-treated steel sheet, and excellent surface properties can be obtained in any of them.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the amounts of Sn and B and the scale defect rate.

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshihiro Hosoya 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Tomoyoshi Ohkita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Inside Kokan Co., Ltd. (72) Inventor Yasuyuki Takada 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Japan Kokan Co., Ltd. (56) References JP-A-4-371528 (JP, A) JP-A-4-131357 ( JP, A) JP-A-2-93047 (JP, A) JP-A-60-162715 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00 301 C22C 38 / 06 C22C 38/50

Claims (3)

(57) [Claims] 1. The method according to claim 1, wherein C: 0.003% or less, M
n: 0.05 to 2.2%, Si: 0.8% or less, P:
0.1% or less, S: 0.015% or less, sol. Al:
0.01 to 0.08%, N: 0.003% or less, O: 5
-30 ppm , Sn: 5 ppm or more, less than 50 ppm ,
B: contains 2 to 9 ppm, and B (ppm) ≧ 0.1 × S
A steel sheet having excellent surface properties, which satisfies the relationship of n (ppm) -1 and is composed of a balance of Fe and inevitable impurities. 2. In% by weight, C: 0.003% or less, M
n: 0.05 to 2.2%, Si: 0.8% or less, P:
0.1% or less, S: 0.015% or less, sol. Al:
0.01 to 0.08%, N: 0.003% or less, O: 5
-30 ppm , Sn: 5 ppm or more, less than 50 ppm ,
B: contains 2 to 9 ppm, and B (ppm) ≧ 0.1 × S
n (ppm) -1 and further Ti: 0.0
A steel sheet having excellent surface properties, characterized in that the steel sheet contains 0.05 to 0.12% and the balance consists of Fe and unavoidable impurities. 3. The steel sheet according to claim 1, further comprising:
u: 0.01 to 0.5%, Ni: 0.01 to 0.5%,
Cr: A steel sheet having excellent surface properties, characterized in that it contains one or more of 0.01 to 0.5%.