JP3058911B2 - Method for producing hot-dip galvanized steel sheet with good workability and excellent perforation corrosion resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a hot-dip galvanized steel sheet having bake hardenability and excellent perforation corrosion resistance.

[Conventional technology]

In the field of thin steel sheets used for automotive steel sheets, etc., in addition to excellent workability, in order to improve dent resistance, the property that the yield stress of the steel sheet increases after baking paint, that is, bake hardenability is required. Often. Demand for surface-treated steel sheets is increasing from the viewpoint of corrosion resistance. A typical example of the surface-treated steel sheet is a hot-dip galvanized steel sheet. In this case, a hot-dip galvanized steel sheet with an increased coating weight is applied to reduce perforated corrosion.

Conventionally, this type of hot-dip galvanized steel sheet is made of low-carbon aluminum-killed steel sheet, ultra-low-carbon steel-based steel sheet with added Ti, or Si, Mn,
The use of high-strength steel sheets with increased strength by adding P, Cr, etc. is typical, and there are many proposals for these.

For example, Japanese Patent Publication No. 1-54413 discloses a hot-dip galvanized steel sheet in which P is added to a low carbon aluminum killed steel. JP-A-58-31035 discloses a galvanized steel sheet having bake hardenability by adding Si and P to a low carbon aluminum killed steel. Japanese Patent Publication No. 1-37468 discloses that Si,
A galvanized steel sheet to which Mn and P are added in combination is disclosed.

[Problems to be solved by the invention]

Generally, in hot-dip galvanizing, molten zinc reacts with the base iron to form an alloy layer, and the alloy layer bonds the zinc layer and the base iron. In addition, those which are positively subjected to alloying treatment to make the entire plating layer into an alloy layer are often used because the coating adhesion is improved. This alloy layer is hard and brittle.

Therefore, when the thickness of the coating layer increases, the workability of the hot-dip galvanized steel sheet is governed by the workability of the plating layer rather than the workability of the base metal base metal. When the plating layer with poor workability is peeled or damaged during processing, the base metal base material is exposed and rusts, which is a problem of so-called perforated corrosion. In addition, the plating layer peels off in powder form during press molding, and this also causes the generation of surface defects called stars.

As described above, the perforated corrosion of hot-dip galvanized steel sheet is derived from the hard and brittle alloy layer, and when it is subjected to deep drawing as in the case of steel sheets for automobiles, solving this problem requires good workability and This is an important issue at the same time as having bake hardenability.

When a low-carbon aluminum-killed steel is used as a base metal, very high bake hardenability can be obtained, but there is a problem that elongation is greatly deteriorated by aging at room temperature. Baking hardenability cannot be obtained when the base steel is made of ultra-low carbon Ti-added steel. On the other hand, as described in Japanese Patent Application Laid-Open No.
When baking hardening is imparted by adding i and P, the problem of perforated corrosion resistance cannot be solved by this alone, so the plating layer must be thickened.

In view of such circumstances, the present invention provides a hot-dip galvanized steel sheet that can simultaneously satisfy workability, bake hardening property, and plating adhesion while improving the most important corrosion resistance of hot-dip galvanized steel sheet. It was made for the purpose of obtaining.

[Means to solve the problem]

According to the present invention, C; 0.001 to 0.010%, Si; 0.10% or less, Mn; 0.05% to 1.8%, P; 0.03 to 0.20%, S; 0.02% or less, Cu; 0.05 to 0.86% by weight%. , Sol.Al; 0.005-0.100%, N; 0.007% or less, Ni; 0.008% or less, Cr; 0.08% or less, with the balance being iron and unavoidable impurities.
Alternatively, C; 0.001 to 0.010%, Si; 0.10% or less, Mn; 0.05% to 1.8%, P; 0.03 to 0.20%, S; 0.02% or less, Cu; 0.05 to 0.86%, sol.Al; 0.10%, N; not more than 0.007%, and up to 1% Ni, up to 1% Mo or up to 7% Cr, containing one or more of the following, the balance being a steel slab consisting of iron and unavoidable impurities After hot rolling and cold rolling, it is passed through a continuous galvanizing line with or without iron plating on a continuous electroplating line, and the annealing temperature in this line is 670-950 ° C.
It is intended to provide a method for producing a hot-dip galvanized steel sheet having good workability and excellent perforation corrosion resistance, which comprises hot-dip galvanizing.

At that time, the hot-dip galvanizing process has a galvanized layer of 400
An alloying process at a temperature of 650 ° C. may be included.

[Action]

The present invention has achieved the above object by properly considering the component composition of the steel of the base steel sheet and adopting appropriate manufacturing conditions. First, the reasons for regulating the content ranges of the respective components of the base steel sheet will be described together with their actions.

C is preferred as it is small, since it significantly deteriorates ductility. However, if C is less than 0.001%, sufficient bake hardenability cannot be obtained. On the other hand, when C exceeds 0.010%, ductility is significantly deteriorated by aging at room temperature. Therefore, C is set to 0.001 to 0.010%.

Although Si is a preferable element for improving the strength of steel without impairing the workability, according to the study of the present inventors, the content of Si in steel when passed through a continuous hot-dip galvanizing line of the Sendzimer type is considered. If it exceeds about 0.10%, non-plating occurs. Therefore, the content of Si is set to 0.10% or less.

Mn is effective in preventing hot embrittlement due to S, and therefore, at least 0.05% or more is necessary. In addition, it is a desirable element for improving the strength of steel. However, 1.8
%, Ductility and deep drawability decrease. For this reason, the lower limit of the Mn content is 0.50% and the upper limit is 1.80%. P and Cu are characteristic elements in the present invention. The combined addition of both elements provides an effect of remarkably improving the perforated corrosion resistance of the hot-dip galvanized steel sheet. For this, P
Must be 0.03% or more, and Cu must be 0.05% or more. But P
Even if Cu exceeds 0.20% and Cu exceeds 1.5%, the effect of improving pitting corrosion resistance saturates and ductility deteriorates. In particular, when the content of Cu is up to 0.86%, the effect of sufficiently improving the corrosion resistance to perforation is exhibited. Therefore, P is 0.03% to 0.
20% and Cu are contained in the range of 0.05% to 0.86%.

S is an element which is essentially harmful to the base steel and is preferably as small as possible. However, in the case of the steel of the present invention, up to 0.02% is permissible.

Al not only plays a role as a deoxidizer but also functions to fix N in steel as AlN and prevent aging at room temperature due to solid solution N. For this purpose, 0.005% or more of Al is necessary, but if it exceeds 0.10%, inclusions such as Al ₂ O ₃ increase,
The lower limit is 0.005 because it deteriorates workability and surface quality.
%, And the upper limit was 0.10%.

N is an element that is essentially harmful to the steel of the present invention.
% Or less.

In addition, up to 1% Ni, up to 1% Mo or up to 7% Cr
It is preferable to include one or more of
Thereby, the strength and the perforated corrosion resistance of the steel sheet can be improved.

Ni effectively acts to prevent hot brittleness by Cu and to improve pitting corrosion resistance, but its effect is saturated even if it exceeds 1%. Therefore, the production cost is unnecessarily high. Therefore, the upper limit is set to 1%.

Mo effectively acts to increase the strength of the steel sheet and to improve the corrosion resistance to perforation. However, even if added over 1%, the effect is saturated and the production cost increases, so the upper limit is made 1%.

Cr effectively acts to improve the pitting corrosion resistance, but if it exceeds 7%, the production cost becomes extremely high, so the upper limit is made 7%.

In the present invention, a slab of steel having the above composition is hot-rolled and cold-rolled to produce a cold-rolled steel strip,
This is passed through a continuous hot-dip galvanizing line to apply hot-dip galvanizing. At that time, it is also advantageous to pass an appropriate amount of iron plating by passing the sheet through a continuous electroplating line before passing the sheet through the continuous galvanizing line. This makes it possible to eliminate the occurrence of non-plating, thereby improving the corrosion resistance. The amount of iron plating may be as thin as ² g / m ² .

For the continuous hot-dip galvanizing, a well-known in-line annealing type equipment (Senzimer-type continuous hot-dip galvanizing equipment) can be used, but it is important that the annealing temperature be 670 to 950 ° C. If the temperature is lower than 670 ° C, recrystallization of the steel strip does not proceed sufficiently, which makes it difficult to develop good workability. However, even at an annealing temperature exceeding 950 ° C., the effect of improving the workability is saturated, and surface defects are easily generated, making it difficult to obtain a good product.

Further, in the present invention, the alloying treatment of the coating layer of the hot-dip galvanized steel sheet can be positively performed. As described above, when the alloying treatment of the plating layer is performed, the plating layer becomes hard and the problem of perforation corrosion generally occurs during processing. In the case of the present invention, this problem is solved and the coating film of the galvanized steel sheet is removed. Since the adhesion and the lap resistance weldability are improved, it can be said that the treatment is a preferable treatment in the application intended by the present invention. Usually, the alloying is carried out in a continuous hot-dip galvanizing line.
Heating to a temperature in the range of ℃ 650 ° C. makes alloying necessary and sufficient. That is, if the temperature is lower than this temperature range, alloying is insufficient, and if the temperature is higher, alloying becomes excessive and the adhesion of the plating layer is impaired.

 Hereinafter, the effects of the present invention will be specifically described with reference to examples.

〔Example〕

A steel having the chemical composition values shown in Table 1 was hot-rolled under the conditions shown in Table 2, and then cold-rolled into a cold-rolled steel strip having a thickness of 0.8 mm. The obtained cold-rolled steel strip was annealed in a continuous hot-dip galvanizing line under the conditions shown in Table ² and subjected to hot-dip galvanizing with a coating weight of 30 g / m ² , with or without alloying treatment. A plated steel strip was manufactured, and then subjected to skin pass rolling at an elongation of 0.8%. The mechanical properties and corrosion resistance of the obtained hot-dip galvanized steel sheet were investigated, and the results are shown in Table 2. For the mechanical properties, a JIS Z 2201 No. 5 test piece was used. For the corrosion resistance test, 70 × 150 mm test pieces were cut out and subjected to a composite corrosion test. The composite corrosion test was performed according to the salt spray test of JIS Z 2371. That is, a total of 24 hours of [salt spray test in which salt water concentration was changed to 1.0% for 5 hours] → [60 ° C. hot air drying for 4 hours] → [50 ° C. wet test for 14 hours] → [blast drying for 1 hour] The maximum erosion depth due to corrosion after 30 cycles of one cycle was measured. The bake hardenability (BH) was measured by applying a 2% tensile pre-strain using a JIS No. 5 tensile test piece, then performing artificial aging at 170 ° C for 20 minutes, and applying the pre-strain applied stress and the post-artificial aging yield stress. Was determined as BH.

As can be seen from the results in Table 2, the hot-dip galvanized steel sheet manufactured using the No. 1 comparative steel having a high C and a low P and Cu content less than the amounts specified in the present invention has a low tensile strength (TS). High yield point (YS) and low elongation (EL). In addition, baking hardenability (BH) is too high, so that room temperature aging occurs. And the corrosion resistance is poor.

The hot-dip galvanized steel sheet manufactured using the No. 2 comparative steel containing less P and containing P has relatively good tensile strength (T
S), elongation (EL) and bake hardening (BH),
Again, the corrosion resistance is poor.

On the other hand, the hot-dip galvanized steel sheet manufactured using the No. 3 to 11 steels having the component composition within the range specified in the present invention has a low yield point (YS) and a high elongation despite its high tensile strength (TS). (EL), good bake hardenability (BH), and excellent corrosion resistance.

Continuation of the front page (51) Int.Cl. ⁷ Identification code FI C22C 38/58 C22C 38/58 (72) Inventor Teruo Tanaka 11-1 Showa-cho, Kure-shi, Hiroshima Pref. 56) References JP-A-2-190443 (JP, A) JP-A-57-1443464 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 9/46-9/48 C23C 2/00-2/40 C22C 38/00-38/60

Claims (3)

(57) [Claims] C. 0.001 to 0.010%, Si; 0.10% or less, Mn; 0.05% to 1.8%, P; 0.03 to 0.20%, S; 0.02% or less, Cu; 0.05 to 0.86%, sol.Al; 0.005 to 0.100%, N; 0.007% or less, Ni; 0.08% or less, Cr; 0.08% or less. After rolling, the steel sheet is passed through a continuous galvanizing line with or without iron plating on a continuous electroplating line.
A method for producing a hot-dip galvanized steel sheet having excellent bake hardenability and pitting corrosion resistance, which comprises hot-dip galvanizing at 670 to 950 ° C. 2. In% by weight, C; 0.001 to 0.010%, Si; 0.10% or less, Mn; 0.05% to 1.8%, P; 0.03 to 0.20%, S; 0.02% or less, Cu; 0.05 to 0.86%, sol.Al; 0.005 to 0.100%, N: 0.007% or less, containing one or more of Ni up to 1%, Mo up to 1% or Cr up to 7%, the balance being iron and unavoidable impurities After hot rolling and cold rolling a steel slab consisting of
It consists of passing through a continuous galvanizing line with or without iron plating on a continuous electroplating line and galvanizing at an annealing temperature in this line of 670 to 950 ° C. A method for producing hot-dip galvanized steel sheets with excellent workability with excellent pitting corrosion resistance. 3. The hot-dip galvanizing treatment is performed by adding a galvanized layer to 400 galvanized layers.
The method according to claim 1 or 2, further comprising a process of alloying at a temperature of 650 ° C.