JP3105533B2 - Method for producing hot-dip galvanized steel sheet with excellent bake hardenability and pitting corrosion resistance - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing a hot-dip galvanized steel sheet having bake hardening properties 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. JP-A-2-111841 discloses a galvanized steel sheet having bake hardenability by adding P and a large amount of Mn to ultra-low carbon Ti-added steel.

[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 the form of powder in the press mold, which 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 by C cannot be obtained in the case where the base steel is made of extremely low carbon Ti-added steel.
On the other hand, in the case of adding baking hardenability by adding Si and P to low carbon aluminum killed steel as disclosed in Japanese Patent Application Laid-Open No. Sho 58-31035, the problem of perforated corrosion resistance cannot be solved by itself, so that the plating layer Need to be thicker. Similarly, JP-A-2-
Even if P and a large amount of Mn are added to ultra-low carbon Ti-added steel to impart bake hardenability as in 111841, the problem of perforated corrosion resistance cannot be solved with 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 for solving the problem]

The present inventors have conducted various studies on a method for producing a hot-dip galvanized steel sheet having excellent workability, bake hardenability and corrosion resistance. As a result, the C and Ti contents of the ultra-low carbon Ti-added steel were properly regulated. Can add bake hardenability, and the combined addition of Cu and P can significantly improve the corrosion resistance of the steel base metal, which can solve the problem of perforated corrosion of hot-dip galvanized steel sheets. The addition of Mn and the addition of Ni, Cr, Mo, etc., can increase strength and improve corrosion resistance, resulting in a highly workable hot-dip galvanized steel sheet with excellent bake hardening and pitting corrosion resistance. I knew that it could be done.

That is, in the present invention, C; 0.001 to 0.010%, Si; 1.5% or less, Mn; 0.05% to 1.8%, P; 0.03 to 0.20%, S; 0.02% or less, Cu; 0.05 to 1.5%, sol.Al; 0.005 to 0.100%, N; 0.005% or less, Ti; 0.002 to 0.04%, and [Ti] according to the following formula, [Ti] = Ti−48 / 14 · N−48 / 32 · S Satisfies the relationship of 0 or less, and in some cases, up to 1% Ni, up to 1% Mo or 7%
One or two or more types of Cr up to and including B; 0.0004 to 0.003%, with the balance being iron and unavoidable impurities. After that, iron plating is performed in a continuous electroplating line, passed through a continuous hot-dip galvanizing line, and hot-dip galvanized by setting the annealing temperature in this line to 700 to 950 ° C. An object of the present invention is to provide a method for producing a hot-dip galvanized steel sheet having excellent corrosion resistance.

In the production method, in particular, the Si content of steel
A hot-dip galvanized steel sheet characterized in that the slab of the steel is hot-rolled, cold-rolled, and then passed through a continuous hot-dip galvanizing line without iron plating. It is intended to provide a manufacturing method.

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, if it exceeds 0.010%, the amount of solid solution C increases, and very high bake hardenability can be obtained, but aging at room temperature occurs, leading to rapid deterioration of ductility. For this reason, C must be within the range of 0.001 to 0.010%. However, in the present invention, the content of C is limited to the range of 0.001 to 0.005%, which is a particularly preferable range of the C content under the condition that [Ti] is 0 or less, as described later.

Although Si is a preferable element for improving the strength of steel without impairing the workability, according to the study of the present inventors, in the Sendzimer type continuous galvanizing line, when the Si content exceeds about 0.10%, Was found to occur. Therefore, it is desirable to make the content 0.10% or less, but this problem can be solved by applying iron plating with an adhesion amount of about 2 g / m ^{2 by} electroplating prior to passing the Zenzimer-type continuous galvanizing line. However, if Si exceeds 1.5%, it becomes hard and ductility deteriorates, so the upper limit was made 1.5%.

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, P and Cu are characteristic elements in the present invention, with the lower limit of the Mn content being 0.05% and the upper limit being 1.80%. 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. Therefore, P is 0.03% ~ 0.20%, Cu is 0.05% ~
It is contained in the range of 1.5%.

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 acts as a deoxidizing agent, and for this purpose
5% or more is necessary, but if it exceeds 0.10%, inclusions such as Al ₂ O ₃ increase and workability and surface quality deteriorate. Therefore, the lower limit was set to 0.005% and the upper limit was set to 0.10%.

N is an element which is essentially harmful to the base steel, and is preferably as small as possible.

Ti is an element necessary for ensuring excellent workability, but its effect is not recognized at less than 0.002%. And
If the Ti content exceeds 0.04% at the C content specified in the present invention, that is, 0.001 to 0.005%, baking hardenability cannot be obtained.
Therefore, Ti is contained in the range of 0.002 to 0.04%.

However, since Ti has the effect of fixing C, S, and N as precipitates such as TiC, TiS, and TiN, baking hardenability cannot be obtained if more Ti is added than can fix the entire amount of C. Therefore, it is necessary to satisfy the relation that [Ti] is 0 or less according to [Ti] = Ti−48 / 14 · N−48 / 32 · S. Under the condition that [Ti] is 0 or less, C Especially 0.001
It is preferably about 0.005%.

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%.

B effectively acts to prevent embrittlement during secondary processing, and improves secondary workability. For this purpose, 0.0004% or more is necessary, but even if it exceeds 0.003%, the effect is saturated. Therefore, B is preferably added in the range of 0.0004 to 0.003%.

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. Especially among the steel strips in the above chemical composition value range
This iron plating is advantageous for those having a high Si content (Si ≧ 0.1%) and those having a high Cr content (Cr ≧ 1.0%). 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 700 to 950 ° C. At temperatures below 700 ° C, recrystallization of the steel strip does not proceed sufficiently, and it is 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 1 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 to form 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) is measured by applying a 2% tensile prestrain using a JIS No. 5 tensile test specimen, then performing artificial aging at 170 ° C for 20 minutes, and applying the pre-strained stress and the yield stress after the artificial aging. 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 higher Ti content and lower P and Cu (without addition) than specified in the present invention has a tensile strength. Although the properties are good, baking hardenability (BH) cannot be obtained and the corrosion resistance is poor.

In addition, hot-dip galvanized steel sheets manufactured using the No. 2 comparative steel with a higher C content than specified in the present invention have low elongation (EL) and too high bake hardenability (BH), resulting in aging at room temperature. . And the corrosion resistance is poor.

On the other hand, the hot-dip galvanized steel sheet manufactured using the steels of Nos. 3 to 8 having the composition in the range specified in the present invention has a low yield point (YS) in spite of the high tensile strength (TS). ,
It has good elongation (EL) and bake hardenability (BH) and excellent corrosion resistance.

[Example 2] Steel having the chemical composition values shown in Table 3 was hot-rolled under the conditions shown in Table 4 and then cold-rolled into a cold-rolled steel strip having a thickness of 0.8 mm. The obtained cold-rolled steel strip was subjected to Fe-0.01% B plating with an adhesion amount of 2 g / m ² in a continuous electroplating line, and then annealed in a continuous galvanizing line under the conditions shown in Table 4 and then deposited. Hot-dip galvanized steel strip was manufactured with or without alloying treatment with a galvanizing amount of 30 g / m ² . Thereafter, skin pass rolling was performed at an elongation of 0.8%. The mechanical properties and corrosion resistance of the obtained hot-dip galvanized steel sheet were evaluated in the same manner as in Example 1. The results are shown in Table 4.

As can be seen from the results in Table 4, the hot-dip galvanized steel sheet manufactured using the comparative steel No. 10 containing a larger amount of Ti as specified in the present invention has a relatively good tensile strength ( T
Although S) and elongation (EL) are shown, baking hardenability (BH) cannot be obtained due to the large amount of Ti added. Further, since the contents of Cu and P are low (no addition), and Ni, Cr, Mo and the like are not added, the corrosion resistance is poor.

On the other hand, the hot-dip galvanized steel sheet manufactured using the steels of Nos. 9 and 11 to 13 having the chemical component values in the range specified in the present invention has a high yield strength (YS) despite its high tensile strength (TS). ), Good elongation (EL), good bake hardenability (BH), and excellent corrosion resistance.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI C23C 2/02 C23C 2/02 2/06 2/06 (72) Inventor Teruo Tanaka 11-1 Showa-cho, Kure-shi, Hiroshima Nisshin Steel Co., Ltd. (56) References JP-A-4-141554 (JP, A) JP-A-3-107426 (JP, A) JP-A-3-111519 (JP, A) Patent 3058911 (JP, B2) Tokiko Hei 1-54413 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/02-9/46 C22C 38/00-38/60 C23C 2 / 02,2 / 06

Claims (5)

(57) [Claims] C. 0.001 to 0.005%, Si; 1.5% or less, Mn; 0.05% to 1.8%, P; 0.03 to 0.20%, S; 0.02% or less, Cu; 0.05 to 1.5%, sol.Al; 0.005 to 0.100%, N; 0.005% or less, Ti; 0.002 to 0.04%, and [Ti] according to the following formula, [Ti] = Ti−48 / 14 · N−48 / 32 · S The steel slab, which satisfies the relationship of 0 or less, with the balance being iron and unavoidable impurities, is hot-rolled and cold-rolled, then is subjected to iron plating by a continuous electroplating line, and is subjected to a continuous hot-dip galvanizing line. A hot-dip galvanized steel sheet with excellent bake hardenability and pitting corrosion resistance, comprising hot-dip galvanizing with an in-line annealing temperature of 700 to 950 ° C. 2. In% by weight, C; 0.001 to 0.005%, Si; 1.5% or less, Mn; 0.05% to 1.8%, P; 0.03 to 0.20%, S; 0.02% or less, Cu; 0.05 to 1.5%, sol.Al; 0.005 to 0.100%, N; 0.005% or less, Ti; 0.002 to 0.04%, and [Ti] according to the following formula, [Ti] = Ti−48 / 14 · N−48 / 32 · S A slab of steel that satisfies the relationship of 0 or less and contains one or more of Ni up to 1%, Mo up to 1%, or Cr up to 7%, with the balance being iron and unavoidable impurities. Hot-rolled and cold-rolled, and then subjected to iron plating in a continuous electroplating line, passed through a continuous hot-dip galvanizing line, and hot-dip galvanized at an annealing temperature in this line of 700 to 950 ° C. A method for producing a hot-dip galvanized steel sheet having excellent bake hardenability and pitting corrosion resistance, comprising: C. 0.001 to 0.005%, Si; 1.5% or less, Mn; 0.05% to 1.8%, P; 0.03 to 0.20%, S; 0.02% or less, Cu; 0.05 to 1.5%, by weight%. sol.Al; 0.005 to 0.100%, N: 0.005% or less, B; 0.0004 to 0.003%, Ti; 0.002 to 0.04%, and the following formula: [Ti] = Ti−48 / 14 · N−48 / 32 Satisfies the relationship that [Ti] according to S is 0 or less, and in some cases further contains one or more of Ni up to 1%, Mo up to 1% or Cr up to 7%, with the balance being A steel slab consisting of iron and unavoidable impurities is hot-rolled and cold-rolled, then iron-plated in a continuous electroplating line, passed through a continuous hot-dip galvanizing line, and the annealing temperature in this line is adjusted. A method for producing hot-dip galvanized steel sheet with excellent bake hardenability and pitting corrosion resistance, comprising hot-dip galvanizing at 700 to 950 ° C. 4. The method according to claim 1, wherein the slab of steel having a Si content of 0.10% or less is hot-rolled, cold-rolled, and then continuously subjected to iron plating. A method for producing a hot-dip galvanized steel sheet, comprising passing the hot-dip galvanized steel sheet through a hot-dip galvanizing line. 5. The hot-dip galvanizing treatment is performed by adding a galvanized layer to 400 galvanized layers.
5. The production method according to claim 1, further comprising a process of alloying at a temperature of up to 650.degree.