JP4781172B2 - Manufacturing method of coated steel sheet with excellent surface appearance - Google Patents

Description

  The present invention relates to a coated steel sheet for use in automobile exterior parts such as automobile wind moldings and roof moldings, household appliances such as washing machines and refrigerators, kitchen appliances, and electrical appliances such as audio equipment, and a resin coating film formation. Even if heat treatment is applied at some times, the yield elongation is suppressed to 0.2% or less, and a coated steel sheet that can prevent the appearance from deteriorating due to the occurrence of stretcher strain caused by extremely slight bending is manufactured at low cost. Regarding the method.

In recent years, hot dip galvanized steel sheets, particularly alloyed hot dip galvanized steel sheets, are widely used in automotive exterior parts, home appliances, kitchen equipment, electrical products, and the like because of their excellent corrosion resistance and workability after painting. Not only hot dip galvanizing and alloying hot dip galvanizing, but also hot-dip galvanized-aluminum plated, hot-dip galvanized-aluminum-magnesium plated, or hot-dip galvanized steel sheets are used in the same manner.
Such a hot-dip plated steel sheet is usually a cold-rolled steel sheet having a predetermined component composition, pickled as necessary, then annealed in a reducing atmosphere, and then immersed in the required plating bath. It is manufactured by.

  And the above hot-plated steel plate is used as a base plate, and it is used for an outer panel of home appliances or the like after being subjected to a paint baking process in order to enhance corrosion resistance. For this reason, the beauty of the appearance of the surface of the coated steel sheet that has been subjected to a paint baking process is naturally emphasized. The appearance of the surface may be caused by the painting process, such as biting of foreign matter and surface flaws, but the mechanical properties of the base steel before painting often affect the appearance of the surface. It has come to occur. That is, among the mechanical properties of the base steel before painting, stretcher strain and the like, which are likely to occur when the yield elongation is too large, often reduce the surface appearance of the coated steel sheet after bending.

By the way, stretcher strains and the like are problems of cold-rolled steel sheets that have been known for a long time, but recently, due to changes in the shape of exterior panels for automobiles and exterior parts for automobiles, they are also regarded as a problem with painted steel sheets. It became so. In recent years, exterior parts for automobiles such as wind moldings and roof moldings are subjected to extremely light processing such as stretch processing with a processing rate of several percent and bending processing with a bending radius of 100 cm in accordance with the design change of the vehicle body. More cases.
Therefore, it is necessary to ensure and maintain a beautiful surface appearance even if the coated steel sheet subjected to extremely light processing is processed after coating.

As a steel plate which does not generate yield elongation, there is a so-called IF steel in which Ti or Nb is added to ultra low carbon steel and solid solution C and N are fixed in the form of carbonitride. For example, Patent Documents 1 and 2 also propose a coated steel plate that uses IF steel as a base plate and has improved workability or excellent surface quality after press working.
The present inventors also reduced the amount of solid solution C and N in steel, which is a material factor affecting the yield elongation, and coarsened the crystal grains in Patent Document 3, and after coating and baking. It has been found that the yield elongation can be suppressed, and B containing N / B and satisfying B / N ≧ 11/14 × 1.1 is contained at 0.0002 to 0.000020 mass%, and the ferrite crystal grain size is 20 It was proposed that the yield elongation can be reduced to 3% or less by adjusting the structure in the range of ˜80 μm.
Japanese Patent Laid-Open No. 10-140317 JP 2005-187835 A Japanese Patent Application No. 2005-228412

However, so-called IF steel is a very expensive material compared to low carbon aluminum killed steel. Further, if the yield elongation is simply set to 3% or less, deterioration of the appearance due to the generation of stretcher strain cannot be suppressed under extremely slight bending conditions. Furthermore, in the coated steel plate, there are cases where the customer further coats the resin and the resin is cured. Since the yield elongation is restored again at the stage where the coating film is baked or the coating resin is cured, stretcher strain and the like are more likely to occur.
Therefore, the present invention provides a resin further applied to a coated steel sheet that has almost the same productivity as that of a general material without using an expensive material such as IF steel, and has been subjected to hot dipping and subsequent normal paint baking treatment. Even if heat treatment such as coating is applied, the yield elongation is kept to 0.2% or less, and even if a slight bending process is applied, a coated steel sheet that does not cause stretcher strain or the like and does not deteriorate the surface appearance is provided at low cost. The purpose is to do.

In order to achieve the object, the coated steel sheet having excellent surface appearance according to the present invention has C: 0.01 to 0.10% by mass, Si: 0.3% by mass or less, Mn: 1.0% by mass or less, P: 0.05% by mass or less, S: 0.02% by mass or less, Al: 0.04% by mass or less, N: 0.004% by mass or less, and B: 0.0002 to 0.000020% by mass, the balance being A slab having a composition composed of Fe and inevitable impurities is subjected to hot rolling at a hot rolling coiling temperature of 550 to 750 ° C., and then an annealing temperature is set to 650 to a cold rolled steel sheet that has undergone pickling and cold rolling. After performing the hot dipping treatment at 850 ° C., the hot dipped steel sheet is subjected to light reduction with a total elongation of 1.5 to 4% by skin pass rolling and a tension leveler, and then subjected to a paint baking treatment. Apply tension leveler processing of 0.3% or less again. And butterflies.
As the hot dip plating, any one of hot dip galvanizing, alloying hot dip galvanizing, hot dip zinc-aluminum plating, hot dip zinc-aluminum-magnesium plating, or hot dip aluminum plating is employed.

The coated steel sheet with excellent surface appearance provided by the method of the present invention is subjected to a coating baking treatment after applying a low pressure by skin pass rolling and a tension leveler with a total elongation of 1.5 to 4% after hot dipping, Furthermore, the tension leveler processing of 0.3% or less is given. For this reason, even if heat treatment such as resin coating is applied to the coated steel sheet, the yield elongation can be reduced and maintained at 0.2% or less.
Therefore, even if the coated steel sheet manufactured by such a method is subjected to an extremely light bending process after the resin coating and its curing heat treatment have been applied, there is no occurrence of stretcher strain, etc., and an excellent surface appearance A molded product maintaining the above can be obtained. That is, a processed product made of a coated steel sheet having an excellent surface appearance can be obtained at a cost substantially equal to that of a general material, so that the use of the coated steel sheet can be greatly increased.

The inventors of the present invention have a high yield elongation after a paint baking process and a subsequent heat treatment process such as resin coating, etc., because the stretcher strain is likely to occur when bending a coated steel sheet. In view of the above, various studies were made on material factors that can suppress yield elongation even after heat treatment such as resin coating.
As a result, after hot-dip plating, the skin pass rolling with a predetermined elongation rate and a light reduction process by a tension leveler are performed, and after coating baking, a tension leveler process is performed again, even after a heat treatment step such as a subsequent resin coating, It was found that the yield elongation can be maintained and suppressed to 0.2% or less.
Details will be described below.

In order to obtain a coated steel sheet having an excellent surface appearance after bending according to the present invention, first, the component composition of the steel is determined as follows.
C: 0.01-0.10 mass%
C is usually a component effective for increasing the strength, but it is not necessary to increase the strength in the coated steel sheet provided by the method of the present invention. Accordingly, the C content is regulated only from the viewpoint of manufacturability and yield elongation of the coated steel sheet. Making it less than 0.01% leads to an increase in production cost in consideration of the current steelmaking decarburization capability. Moreover, if it exceeds 0.10%, even if hot rolling coiling temperature is 550-750 degreeC and high temperature coiling, the amount of solid solution C which affects yield elongation cannot be suppressed to an appropriate amount.

Si: 0.3% by mass or less Si is usually added as a deoxidizer and is an effective component for increasing the strength, but it is not necessary to increase the strength in the coated steel sheet provided by the method of the present invention. Deoxidation is also preferably performed by adding Al. If the Si content exceeds 0.3% by mass, the hot dipping properties deteriorate and surface defects such as non-plating occur.

Mn: 1.0% by mass or less Mn is also an effective component for increasing the strength, but it is not necessary to increase the strength in the coated steel sheet provided by the method of the present invention. Similar to Si, when its content exceeds 0.5%, hot dipping properties deteriorate and surface defects such as non-plating occur.
P: 0.05 mass% or less P not only causes an increase in strength and lowers the ductility, but also increases the P segregation in the steel and deteriorates the bending workability with a decrease in local ductility. Such harmful effects become prominent when the content exceeds 0.05%.

S: 0.02 mass% or less S forms inclusions of MnS and degrades the bending workability as the local ductility decreases. Such harmful effects become prominent when the content exceeds 0.02%.

Al: 0.04 mass% or less Al is an alloy component added as a deoxidizer and is an element effective for fixing solute N as AlN. The effect of lowering the yield elongation by reducing the solute N is exhibited. However, if added over 0.04%, not only the manufacturing cost is increased, but coarse AlN may be formed to cause deterioration of bending workability.

N: 0.004 mass% or less N, like C, is usually an effective component for increasing the strength, but the coated steel sheet provided by the method of the present invention does not need to be increased in strength. From the viewpoint of reducing the amount of solute N, it is preferable to reduce as much as possible. If it exceeds 0.004%, there are many situations that cannot be fixed even by the above-described addition of Al or addition of B described later, and it becomes difficult to control the amount of solute N involved in the reduction of yield elongation.

B: 0.0002 to 0.0019 mass%
It is an element that has a strong effect of fixing N as precipitates BN. Al is also added as an element for fixing N, but Al is rather expected to act as a deoxidizer. Comparing the solubility products, B is easier to fix N than Al. In order to exert the N fixing action, at least 0.0002% of B is required. However, even if it is added to an amount exceeding 0.0010%, the fixing action of B is saturated and only increases the manufacturing cost.

Next, the built-in conditions will be described.
There are no particular restrictions on the method of melting, casting and solidifying the steel material, the cold rolling method after hot rolling, and the hot dipping method. The usual method may be used. If necessary, pickling may be performed before cold rolling or before hot dipping. It is preferable to perform annealing before hot dipping in a reducing atmosphere as usual. As for the hot dip plating, ordinary hot dip galvanizing, hot dip aluminum plating, alloying hot dip galvanizing, hot dip zinc-aluminum plating or hot dip zinc-aluminum-magnesium plating is performed.
In the present invention, in order to limit the yield elongation to 0.2% or less after coarsening the crystal grain size, coating baking, and subsequent heat treatment such as resin coating, It is necessary to prescribe the annealing temperature before hot dipping, the light rolling processing conditions after hot dipping, and the tension leveler application rate after paint baking.

Hot rolling temperature: 550-750 ° C
Control of the carbide of the hot-rolled sheet is important for reducing the amount of solute C related to yield elongation. Specifically, it is necessary to coarsely precipitate massive coarse carbide or pseudo pearlite in the vicinity of the crystal grain boundary. For this reason, a lower limit temperature of 550 ° C. is required. However, winding at a temperature exceeding 750 ° C. is very difficult as a real problem. For example, a strong and thick hot-rolled scale that cannot be removed by normal pickling, or a material that becomes too soft and is crushed by the weight of the hot-rolled coil frequently occurs.
Further, the coarsening of the crystal grain size of the hot-rolled sheet is also involved in the coarsening of the crystal grain size of the final coated product, and is effective in reducing the yield elongation. For this reason, it is preferable to perform hot rolling at a relatively high temperature within the above range.

Annealing temperature: 650-850 ° C
Basically, it is recrystallization annealing after cold rolling. In order to finish the recrystallization and obtain the desired good mechanical properties, it is necessary to perform annealing at a temperature of at least 650 ° C. or higher. Further, the coarsening of the crystal grain size is effective in reducing the yield elongation. For this reason, it is preferable to raise the annealing temperature. However, annealing at a temperature higher than necessary not only saturates the effect, but also leads to an increase in manufacturing cost, so the annealing temperature is set at 850 ° C. as the upper limit.
The steel sheet annealed at a temperature within the above range is subjected to the hot dip galvanizing, hot dip galvanizing, galvannealed hot dip galvanizing, hot dip galvanizing-aluminum plating or hot dip galvanizing aluminum-magnesium plating. Is given.

Light reduction with skin pass rolling and tension leveler: Elongation 1.5-5%
Basically, in order to eliminate the yield elongation after hot dipping, skin pass rolling and light reduction by a tension leveler are applied. In order to finally suppress the target yield elongation to 0.2% or less, it is necessary to make the total of the elongation rate of the skin pass rolling and the elongation level of the tension leveler at this stage at least 1.5%. Further, since giving a large amount of pre-strain is effective for eliminating the yield elongation, the higher the elongation at this stage, the better. However, if the elongation rate is increased too much, it leads to a decrease in total elongation and causes deterioration of workability, so the upper limit is made 4%.

Tension leveler application ratio after paint baking: 0.3% or less Applying a normal resin coating to a steel sheet that has been hot-dip plated and then lightly pressed, and then baking at about 100 to 300 ° C. as usual to obtain a coated steel sheet It is done. Even after baking at such a temperature, the extremely mild yield elongation is recovered. For the purpose of eliminating the yield elongation and correcting the shape of the coated steel sheet, a tension leveler process of about 0.2% is performed. When tension leveler processing is not applied at this stage, if heat treatment such as resin coating is applied in a later step, the yield elongation is further recovered and the beautiful appearance is impaired. If the application rate of this tension leveler processing becomes too large, the surface deterioration of the coated steel sheet will be caused. Therefore, the upper limit of the tension leveler processing application rate after baking is 0.3%.

Since the yield elongation can be suppressed to 0.2% or less by applying tension leveler processing again after painting and baking, a very small amount of this steel sheet is subjected to heat treatment such as resin coating at the customer's site. Even if an extremely slight processing to which the elongation is applied is applied, stretcher strain or the like is not generated, and a processed product maintaining a beautiful surface appearance can be obtained.
For this reason, it supplies suitably for manufacture of exterior parts for motor vehicles, such as a wind molding and a roof molding in which a very small amount of elongation processing, such as a stretch of several% and a bending radius of 100 cm, is performed.

A steel slab having the composition shown in Table 1 is formed into a hot-rolled steel plate having a thickness of 2.3 mm by hot rolling at the coiling temperature shown in Table 2, and after pickling and cold rolling, the thickness is 0.35 mm. A cold-rolled steel sheet was obtained. Subsequently, after passing through a hot dip galvanizing line having an annealing temperature shown in Table 2, skin pass rolling with a total elongation shown in Table 2 and tension leveler processing were performed to produce a steel sheet for coating.
The obtained steel plate for coating was used as a base plate, baked at the temperature shown in Table 2, and then subjected to a tension leveler processing of 0.2%. These coated steel sheets were further subjected to a heat treatment for heating to the temperatures shown in Table 2 assuming a heat treatment such as resin coating performed at the customer site.

The coated steel sheet subjected to the second heat treatment was first subjected to a tensile test at room temperature using a JIS Z2201 No. 5 test piece.
Moreover, about this coated steel plate, it was confirmed by the following means whether the beauty of an external appearance was not impaired by generation | occurrence | production of a stretcher strain etc. at the time of a bending process.
《Lacquer coating tensile test》
Using the JIS Z2201 No. 5 test piece of the coated steel sheet obtained above, a commercially available black lacquer was further applied, an elongation of 1% was given at a tensile speed of 10 mm / min, and the appearance was visually observed. NG was defined as the surface relief associated with the occurrence of hip folding patterns and stretcher strains.
<< Bending test with bending radius of 100cm >>
Similarly to the above, a commercially available black lacquer was further coated on a coated steel plate having a width of 3 cm and a length of 50 cm, a bending test was performed with a bending radius of 100 cm, and the appearance was visually observed. NG was defined as the surface relief associated with the occurrence of stretcher strain.
The results are also shown in Table 3.

As is clear from these test results, the total of No. 11 steel with no B added, No. 9 steel with a low hot rolling temperature outside the specified range of the present invention, and skin pass rolling and tension leveler outside the specified range. The No. 10 steel with a small amount of elongation may be out of the conditions specified in the present invention, and the yield elongation after the resin coating treatment is clearly large. Then, it can be seen that the poor aesthetic of the surface after processing as compared with the No. 2 to 8 of the test strip of the present invention range. The No. 1 steel is shown as a reference example because the B content is too high.

Claims (2)

  C: 0.01 to 0.10% by mass, Si: 0.3% by mass or less, Mn: 1.0% by mass or less, P: 0.05% by mass or less, S: 0.02% by mass or less, Al: 0.04% by mass or less, N: 0.004% by mass or less, and B: 0.0002 to 0.0020% by mass, with the balance being Fe and inevitable impurities, and a hot rolling coiling temperature. Is subjected to hot rolling to 550 to 750 ° C., and then subjected to hot dip plating treatment to an annealing temperature of 650 to 850 ° C. on the cold-rolled steel plate that has undergone pickling and cold rolling, and then the skin pass is applied to the hot dip plated steel plate. The surface is characterized in that the total elongation by rolling and tension leveler is reduced by 1.5 to 4%, subjected to paint baking, and then subjected to tension leveler processing of 0.3% or less again. A method of manufacturing coated steel sheets with excellent appearance.   2. The coated steel sheet having excellent surface appearance according to claim 1, wherein the hot dip plating is any one of hot dip galvanizing, alloying hot dip galvanizing, hot dip zinc-aluminum plating, hot dip zinc-aluminum-magnesium plating, or hot dip aluminum plating. Production method.