JP2761179B2 - Method for producing thin steel sheet with extremely good 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 a method for producing a thin steel sheet having extremely good surface properties, and relates to a method for producing an ultra-low carbon steel sheet which is widely used at present. Therefore, the present invention relates not only to the steelmaking technology for manufacturing the steel sheet but also to a wide range of industrial fields such as automobiles, home appliances, and building materials using the steelmaking technology.

[0002]

2. Description of the Related Art After refining steel, carbon is reduced to several tens of ppm by a vacuum degassing device such as RH, and the remaining carbon is reduced to Ti.
Steel (an interstitial free steel: Interstitial-free steel) which is fixed by adding a carbonitride forming element such as iron or Nb
Free Steel) now occupies a firm foothold as the main type of steel sheet. However, this IF steel has significant disadvantages in production. It concerns surface defects or surface defects. IF steel tends to have surface defects for reasons of steelmaking, and furthermore, IF steel is soft, and handling flaws are inevitably generated, for example, the hot rolling temperature and the annealing temperature must be increased. For this reason, the yield is significantly reduced and the delivery date is delayed, which is a problem in terms of production management in which manufacturing is performed according to a schedule aimed at a target quality.

[0003] Such a serious situation is considered to be due to the fact that IF steel was also considerably taken up at the discussion meeting of "Surface flaw prevention technology in hot rolling and plate rolling" at the 126th Autumn Lecture Meeting of the Iron and Steel Institute of Japan. Understandable (eg, CAMP
-ISIJ, vol. 6, p. 1328-1331 and p. 1332-1335). Despite the analysis and examination of this debate, the measures are on-site and symptomatic, and have not yet reached a radical solution.

On the other hand, many inventions have been filed for steel sheets having multiple layers as in the present invention. For example, there are the inventions described in JP-A-4-191330 and JP-A-4-191331. However, these inventions are alloy steels in which at least one component contains a large amount of C, Mn, Si, P, and the like, and are characterized by having high strength, such as dent resistance, for the purpose.

[0005]

DISCLOSURE OF THE INVENTION The present invention fundamentally eliminates the defects related to surface flaws or surface defects of IF steel described in the section of the prior art, and has a solid foundation in terms of both performance and economy. An object of the present invention is to provide a method for producing a thin steel sheet having a very good surface property capable of sublimating into a material.

[0006]

That is, the gist of the present invention is as follows: (1) C: ≦ 0.0050%, Mn: 0.1 to 0.4%, P: ≦ 0.08%, Al: 0.01 to 0.10%, Ti: 0.01 to 0.08% and N
b: containing 0.01 to 0.08% of one or two kinds,
When the steel consisting of the remaining Fe and unavoidable impurities is tapped and then continuously cast, an electromagnetic brake is provided at the lower part of the mold, and carbon containing powder is added to the surface of the slab (steel slab) using powder containing carbon on the upper surface of the mold. A method for producing a thin steel sheet having extremely good surface properties, characterized in that it is contained in an amount of from 0.01 to 0.08%, and (2) the carbon content of the powder is from 0.5 to 5%. The method for producing a thin steel sheet having extremely good surface properties according to the above (1) is provided. (3) The steel to be tapped further has B: 0.0001 to
(1) characterized by containing 0.0010%.
The present invention relates to a method for producing a thin steel sheet having an extremely good surface property described in the section .

[0007]

According to the present invention, it is intended to effectively utilize a powder containing carbon used for keeping the temperature of a hot water surface in a continuous casting operation. An electromagnetic brake is provided at the bottom of the continuous casting mold, the inner layer is separated by the effect of the electromagnetic brake, and the carbon mixed from the powder penetrates only into the surface layer of the slab by the action, and hot rolling, cold rolling, It is intended to drastically solve surface defects or other surface defects generated during annealing. Although there are various types of surface defects, in addition to ordinary scale defects and liquid film embrittlement defects, very small cracks on the surface layer during hot rolling, minute scale defects, or minute scale residue This is effective for a wide range of surface defects such as those that become apparent as flaws or surface defects in a later step.

Further, it is expected that surface defects can be eliminated based on the improvement of the solidification state. That is, in the conventional IF steel,
The solid-liquid coexistence region is extremely small in composition, and small temperature fluctuations appear as fluctuations in the solidified shell thickness, which affects segregation at the solid-liquid interface, and tends to cause defects such as powder entrainment and slab vertical cracks. Was. However, adding 0.01 to 0.08% of carbon to the surface layer and solidifying it with a mold as in the present invention is the solidification of the Fe-C alloy.
A solid-liquid coexistence area is sufficiently ensured, such non-uniformity is unlikely to occur, and surface defects based thereon are greatly reduced.

The powder used in the present invention contains 0.5% carbon.
It may be any one having 5% free. As an example of the constituent components, SiO ₂ ; 30%, Al ₂ O ₃ ;
CaO; 30%, Na ₂ O; 10%, F ⁻ ; 7%, C;
3%. The standard output of the electromagnetic brake is 0.5 to 5 T in terms of magnetic flux density. The thickness of the surface layer having a high carbon concentration is controlled to 0.5 to 10 mm based on the drawing speed, the powder component, and the output of the electromagnetic brake. If the thickness is less than 0.5 mm, there is not much effect on eliminating surface defects. On the other hand, if the thickness exceeds 10 mm, the thickness of the final product is not negligible in terms of material, resulting in material defects such as hardening. When a hot scarf or a cold scarf is applied after the slab is formed, a high carbon layer thickness should be controlled in consideration of the scarf margin.

In the present invention, a surface layer having an increased carbon concentration is obtained in this way to reduce the source of surface defects. Then, at the time of hot rolling, unlike a component that is soft and easy to seize on a roll or the like like ultra-low carbon steel, it is a component containing ordinary carbon, so that hot rolling flaws are hardly generated. Also, unlike ultra-low carbon steel, it has moderate high-to-medium-to-normal temperature strength, so it is expected that the occurrence of handling flaws in each of the lower steps will be significantly reduced.

The bulk component is a component as a general IF steel. That is, C must be 0.0050% or less. If it exceeds 0.0050%, it is difficult to obtain sufficient workability, particularly, r value and elongation value. Mn is also set to 0.40% or less from the same viewpoint. The lower limit of Mn is S which is an impurity.
Is fixed as MnS. For more stability, S is set to 0.010% or less, and Mn / S is set to 1 or less.
It is preferably 0 or more.

Since P has a large solid solution strengthening ability, it is added in an amount of 0.08% or less when forming a high-strength thin steel sheet. 0.
What is necessary is just to design with a strengthening capacity of 10 N / mm ² per 01%. In mild steel, P is unnecessary and should be 0.02% or less. Al is used as a deoxidizing agent. If it is less than 0.01%, sufficient deoxidation is not performed, and the inclusions of steel increase, resulting in appearance of surface defects or deterioration of the material. 0.1
% Of Al rather deteriorates the cleanliness of the steel, and also appears as surface defects and internal defects.

[0013] Ti and / or Nb is used as an element fixing solid solution carbon as carbide. Each content is set to 0.01 to 0.08%. If it is less than 0.01%, a sufficient effect of addition cannot be obtained, so that a sufficient material as IF steel cannot be obtained. If it exceeds 0.08%, the improvement of the material is saturated. Further, both may be added in combination. In order to secure sufficient materials, it is preferable that C is set to 0.0030% or less and Ti is set to 0.025% or more. Further, it is preferable to add 0.01 to 0.02% of Nb to this system. Further, it is preferable to use Ti addition in a cold rolled steel sheet and Nb addition in a hot-dip galvanized steel sheet or a system in which 0.01 to 0.02% of Ti is added thereto in order to take advantage of the respective characteristics.

In the case of a thin steel sheet, when subjected to secondary processing such as flaring after deep drawing, secondary processing brittleness in which the formed wall is brittlely broken or a specific defect called a vertical crack occurs. May be. This is considered to be due to the deterioration of the grain boundary strength of the IF steel, and as a countermeasure, 0.0001 to 0.0010% of B is added as necessary. If it is less than the lower limit, there is no secondary workability improvement effect. Further, B impairs processability, so that when added, the content is limited to a very small amount of 0.0010% or less.

In order to obtain such a component, first, steel is converted into ultra-low carbon by a vacuum degassing device such as RH after converting into steel in a converter, then deoxidized, and Ti and / or Nb is added.
Thereafter, continuous casting is performed to obtain a steel slab. Next, hot rolling is performed. In hot rolling, it is wound into a coil through heating, rough rolling, finish rolling, cooling in a run-out table, and the like. In hot rolling, direct hot rolling (DR), which does not pass through a heating furnace, or hot strip insertion (HCR), in which a hot piece is inserted into a heating furnace, may be used. When passing through a heating furnace, the heating temperature is usually 105
It is about 0 to 1200 ° C. The surface carbon addition zone is largely scaled off during this heating process. Therefore, there is no adverse effect on the material while maintaining the original thickness ratio until the final step. However, if the scale-off is performed too much, the effect of preventing surface defects is reduced. Further, it is preferable to perform heating at a low temperature of 1100 ° C. or lower, because a further improvement in the material can be obtained.

The hot rolling may be any of a continuous type, a semi-continuous type, or an intermediate type. The finishing end temperature is usually equal to or higher than the Ar ₃ transformation point, but Ar ₃ may be cut off within a range where ridging-like surface roughness is not present. The winding temperature is a major factor in the material, and the higher the temperature, the better the material. In that sense, the winding temperature is desirably 680 ° C. or higher. On the other hand, in high-temperature winding, poor pickling properties are likely to occur, and the ends of the hot-rolled coil are rapidly cooled, which deteriorates the material of the portion and causes unevenness of the material. For that purpose, it is preferable that the winding temperature be approximately 550 to 650 ° C. After winding, the coil is cooled, pickled if necessary, subjected to appropriate refining treatment, turned into a hot-rolled steel sheet or a hot-rolled coil, and shipped.

The pickled hot-rolled coil is cold-rolled and then subjected to recrystallization annealing to form a cold-rolled coil. Cold rolling rate is 60 to 85 as usual
%. Recrystallization annealing is performed by box annealing or continuous annealing. 650-750 ° C for box annealing, 1-20hr
s is the annealing condition. In the case of continuous annealing, 700 to 90
The condition is 0 ° C., 10 seconds to 10 minutes. The types of box annealing and continuous annealing are not questionable. The cold-rolled coil becomes a product as it is as a cold-rolled steel sheet, or becomes an electro-galvanized steel sheet by passing through an electro-galvanizing line. Electrogalvanizing may be performed by zinc-based alloy zinc plating such as Zn-Ni in addition to ordinary pure zinc plating.

The coil may be passed through a continuous hot-dip galvanizing line while being cold rolled to form a hot-dip galvanized steel sheet. The heating conditions in that case are the same as the conditions in continuous annealing. In hot-dip galvanizing, after being immersed in a zinc pot, it may be so-called alloyed hot-dip galvanizing that reheats to 500 ° C. and changes the plating layer to an alloy phase with Fe. In this case, the adhesion of zinc and the state of alloying (in the case of galvannealing) are affected by the relationship with the components in the steel. And alloying conditions).

[0019]

EXAMPLES Four types of steels shown in Table 1 were melted in a converter-RH vacuum degassing process, and then continuously cast. In the continuous casting machine, an electromagnetic brake device 4 is provided below the mold 3 as shown in FIG. The powder is SiO ₂ : 2
9%, Al ₂ O ₃ : 7%, CaO: 3%, Na ₂ O: 1
^{3%, F -: 7%} , C: 2.5% was used as a main component. The electromagnetic force of the electromagnetic brake was 1.5T. The slab withdrawal speed was 1.9 min, and the slab thickness was 280 mm. In the figure, 1 is a ladle, 2 is a tundish,
3 is a mold, 4 is an electromagnetic brake, and 5 is an immersion nozzle.

In this way, the C content of the surface layer is reduced from 0.026 to
0.030%. As a result of taking a part of the slab and observing the cross section, the surface layer (upper part) was 6.6 mm in the total thickness of 280 mm.
The thickness was 3 to 7.7 mm and the surface layer (lower portion) was 6.6 to 7.4 mm (however, the range was variation in the slab width direction). As an example, FIG. 2 shows the distribution of steel components in the slab 1/4 width portion of the steel A of the present invention (chemical analysis was performed by cutting 1 mm at a time).

The slab thus manufactured is heated at a temperature of 1800 to 1170 ° C. without performing a scarf on the surface layer.
Finishing temperature 890-926 ° C, winding temperature 650-70
Hot rolling was performed at 0 ° C. The hot-rolled sheet thickness was 4.0 mm. After pickling, two coils were selected, subjected to temper rolling of 1%, sampled, and then subjected to a material / quality investigation. Also, in order to check the surface quality of the coil, a low-speed sheet rolling was performed on an inspection line. The inspection was performed on both sides.

The remaining pickling hot-rolled coil was cold-rolled to 0.8 mm (cold rolling rate 80%). Eight coils were appropriately selected from the cold-rolled coils, and annealing was performed by continuous annealing. Another 9
The coil passed through a continuous galvanizing line. The continuous annealing conditions are as follows: heating rate 10 ° C / sec, soaking 850 ° C, 50se
c, Temper rolling 0.8%. The continuous hot-dip galvanizing line consists of a non-oxidizing heating furnace, a reduction furnace, a Zn pot, an alloying furnace, and a temper rolling mill.
Maximum reaching temperature 850 ° C, penetration temperature into the pot 460 ° C,
Alloying conditions: 500 ° C., 10 sec, temper rolling 0.8%, zinc basis weight 45 g / m ² .

All the coils were passed through the inspection line at a low speed, and the front and back surfaces were inspected in detail. Table 2 shows the results of surface flaw inspection and material test. Here, the first class is a standard for an automobile outer panel, which is the most severe application.

The steel used for comparison is an IF steel which is manufactured as a standard, and the component is C: 0.0018 to
0.0031%, Mn: 0.12-0.18%, Al:
0.029-0.036%, N: 0.0018-0.0
029%, B: hot rolled at 0.0002-0.0006%,
The conditions of cold rolling, annealing and size were selected as close as possible to those of the steels of the examples. It is an average value of data of at least 50 coils or more.

[0025]

[Table 1]

[0026]

[Table 2]

As shown in Table 2, the mechanical test values showed little difference between the steel of the present invention and the comparative steel, or were slightly reduced. You can see that the material is inherited. The cause of the slight decrease in the mechanical test value of the present invention is considered to be due to the addition of carbon to the surface layer.
A layer of about μm or less was recognized as a carbon-added region.

Next, although it is a surface flaw, it is clear that the steel of the present invention has remarkably improved in any of the hot-rolled steel sheet, the cold-rolled steel sheet and the galvannealed steel sheet.
In comparison with the IF steel, the hot-rolled steel sheet had a rejection rate of several percent, and the cold-rolled steel sheet and the hot-dip galvanized steel sheet had a rejection rate of more than 10%, whereas the rejection rate of the steel of the present invention was at most 2%. These defects include those caused by extremely minor defects such as those caused by plating, and the fundamental solution was almost impossible with the conventional IF steel. The surface defects of the cold-rolled steel sheet and the hot-dip galvanized steel sheet of the present invention are as follows:
Most are post-process flaws, which can be further reduced significantly by improved operating conditions.
Until now, there was a problem of surface defects peculiar to IF steel, and no matter how much the operating conditions were improved, it did not lead to the elimination of the specific defects, and it was difficult to take countermeasures. Therefore, it is expected that these operation measures will be easier to take in the future, and that the rejection rate will further decrease significantly.

[0029]

According to the present invention, surface defects, which are one of the major defects of IF steel, have been eliminated, and economical efficiency during production has been established. As a result, it can be said that the company has firmly established itself as a major general-purpose material for automobiles.

[Brief description of the drawings]

FIG. 1 is a schematic view of a continuous casting machine having an electromagnetic brake used in an embodiment.

FIG. 2 is a diagram illustrating a component distribution in a thickness direction of a quarter width portion of a slab of an example.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Ladle 2 Tundish 3 Mold 4 Electromagnetic brake 5 Immersion nozzle

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/14 C22C 38/14

Claims (3)

(57) [Claims] C .: 0.0050%, Mn: 0.1 to 0.4%, P: 0.08%, Al: 0.01 to 0.10%, one or two types of Ti: 0.01 to 0.08% and Nb: 0.01 to 0.08%, the balance being Fe and unavoidable impurities. When the steel is made of steel and then continuously cast, an electromagnetic brake is provided at the lower part of the mold, and the surface of the slab is made to contain 0.01 to 0.08% carbon by using a powder containing carbon on the upper surface of the mold. A method for producing a thin steel sheet having extremely good surface properties. 2. The powder has a carbon content of 0.5 to 0.5.
The surface texture according to claim 1, which is 5%.
Is a very good method for producing thin steel sheets. 3. The steel according to claim 1 , further comprising : B: 0.00
Method for producing a very good thin steel sheet surface texture according to claim 1, characterized in that it contains from 01 to 0.0010%.