JP3052835B2 - Galvannealed steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a galvannealed steel sheet having excellent adhesion of a plating layer to a base steel sheet and excellent press workability, and particularly suitable as a steel sheet for automobiles.

[0002]

2. Description of the Related Art An alloyed hot-dip galvanized steel sheet is one in which the plating layer on the surface of the steel sheet is an Fe-Zn alloy, and the average Fe concentration of the plating layer (Fe in the plating layer usually has a high concentration on the steel sheet side). (Because there is a gradient, the average concentration of the entire plating layer thickness) is generally 7 to 12% by weight. The reason for alloying is that when a coating is further applied on the plating layer, the adhesion of the coating is superior to that of ordinary hot-dip galvanizing, and the corrosion resistance after coating is significantly improved. This alloyed hot-dip galvanized steel sheet is usually subjected to continuous hot-dip galvanizing, followed by a heat treatment furnace for alloying.
It is manufactured by heating at a material temperature of about 80 to 600 ° C., and reacting the galvanized layer with the base material to form an Fe—Zn alloy.

[0003] As a base steel sheet of an alloyed hot-dip galvanized steel sheet, besides a conventionally used low-carbon Al-killed steel, an ultra-low carbon IF (Inte-
rstitial Free) steels, and steels with good workability, in which the strength of IF steels is increased by addition elements and the like, have come to be used in many cases. In IF steel, C in steel is made as low as possible at the steelmaking stage, and the remaining C and N which forms an interstitial solid solution in the same manner as C in steel are fixed by adding Ti or Nb. N
Is hardly in a solid solution state. As a result, even under the conditions of rapid heating and rapid cooling as in the conventional production of hot-dip galvanized steel sheet, a base steel sheet having sufficiently excellent press workability and non-aging property can be obtained. The hot-dip galvanizing method is a manufacturing method that can make the thickness of the plating layer, which governs corrosion resistance, more economical than the electroplating method. It has come to be used frequently.

[0004] When an alloyed hot-dip galvanized steel sheet is used as a steel sheet for automobiles, the adhesion of the plating layer is required at the time of press working or when used in a vehicle body. The problems with poor adhesion are powdering and low temperature chipping.

[0005] Powdering is a phenomenon in which a plating layer peels off in a powdery form in a region where compression deformation occurs during pressing of a steel sheet. Not only does the corrosion resistance of that part deteriorate, but also the peeled-off powder becomes a metal mold. On the surface of the molded product. The plating layer is changed to a Zn-Fe alloy by the alloying treatment after the deposition of zinc, but it is said that powdering becomes remarkable if a particularly hard and brittle intermetallic compound phase (Γ phase) develops too much. To suppress the occurrence of powdering, control the structure and composition of the alloy layer by limiting the basis weight of plating, controlling the Al concentration in the plating bath, limiting the degree of alloying, and selecting the alloying temperature and time, and improving it. Has come off.

[0006] Low-temperature chipping means that when pebbles collide with a car at a high temperature of about 0 ° C or less at high speed, the surface of the outer plate is coated together with the coating film on the galvannealed steel sheet. This is a phenomenon in which a layer separates from the interface with the base material. This is also considered to be due to the brittleness of the galvannealed galvanized layer.

[0007] The extremely low C IF steel contains only a small amount of C, which is the most effective for strengthening, in a normal steel material. Therefore, it is soft and preferable for press forming, but has insufficient strength after being incorporated into a vehicle body. . Therefore, S as a strengthening element
i, Mn or P is added. Si tends to increase surface flaws or cause non-plating, and Mn has a small strengthening effect and, when added in a large amount, deteriorates deep drawability. On the other hand, P is preferably used because it has a great effect of increasing the strength when added in a small amount, has a small effect on press workability, hardly causes non-plating, and has a low raw material cost. However, for low temperature chipping resistance,
It is known that P present in a base steel sheet has an adverse effect.

[0008] The adhesion of the plating layer to the base steel sheet is determined by the fact that the interface between the plating layer and the steel sheet is rough and the contact area is large, and the Zn of the plating on the ferrite grain boundary of the steel sheet during the alloying process.
It is thought that the number of invaders increases as invasion progresses. The penetration into the ferrite grain boundaries embrittles the grain boundaries of the base steel sheet. However, when subjected to an impact, it is presumed that cracks traveling at the interface between the plating layer and the steel sheet are dispersed and propagated also to the ferrite grain boundaries, and as a result, cracks at the interface are suppressed and peeling is prevented. .

[0009] One of the reasons that P in steel reduces the adhesion of the plated layer after alloying is that Fe in the alloying process.
This is considered to be due to the fact that the roughness of the interface tends to be reduced in order to reduce the crystal orientation dependency of the base material during the growth of the Zn intermetallic compound. Another reason was considered to be segregation at the ferrite grain boundaries of the steel sheet to prevent penetration of Zn into the grain boundaries. Furthermore, when the P is reduced simply in the ultra-low carbon steel,
Rapid alloying from the ferrite grain boundary portion, that is, an outburst reaction is likely to occur, and a brittle alloy phase such as a Γ phase rapidly grows, and the powdering resistance decreases.

[0010] The presence of Si in the steel is due to the presence of Zn during the plating process.
It is said that the steel has an effect of promoting the penetration of the steel into crystal grain boundaries, enhancing the so-called anchoring effect of invading Zn into the base material, and improving the adhesion of the plating layer. Using this,
A method for alleviating the adverse effect of the low-temperature chipping resistance deterioration of P has been considered, and an example thereof is the invention of Japanese Patent Application Laid-Open No. 6-41707. The present invention relates to the use of Si in steel
The amount is 0.02-0.10%, and P is (0.1 × Si%) + 0.005
%. However, even if the maximum content of Si is 0.10%, P is 0.015%.
It is necessary to keep the content below, and since the content is lower than the content that is usually mixed as an impurity, it is almost impossible to use P addition for improving the strength of steel.

As an example of an alloyed hot-dip galvanized steel sheet taking high strength into consideration, Japanese Patent Application Laid-Open No. 6-81099 discloses that Si is 0.02-1.00%, Mn is 0.5-2.5%, and Mn is
(%) ≧ 0.25 × Si (%), and an invention using an ultra-low carbon base steel having a P of 0.007% or less has been proposed. In this case, Si is included for strengthening the steel, and the occurrence of non-plating due to an increase in Si can be reduced by Mn content. However, P, which degrades low-temperature chipping resistance, is set to 0.0.
In order to suppress the content to as low as 07% or less, Si must be increased to improve the strength. If the Si content is increased, non-plating and uneven plating are increased, and surface flaws and uneven alloying are increased. And even if the problem caused by the increase in Si can be dealt with by adding Mn, the press workability tends to deteriorate due to the increase in the Mn content.

As described above, the deterioration of the low-temperature chipping resistance of the alloyed hot-dip galvanized steel sheet due to the inclusion of P in the base steel sheet cannot be adequately dealt with, and this hinders the production of a high-strength steel sheet that is rational or economical. Has become.

[0013]

SUMMARY OF THE INVENTION The present invention relates to a galvannealed steel sheet mainly used for automobiles, and more particularly to a steel sheet which is excellent in low-temperature chipping resistance and powdering resistance of a plated layer thereof. Is what you do.

[0014]

Means for improving the powdering resistance of the plating layer is effective in improving the chipping resistance, but is not enough to prevent deterioration due to P present in the base steel sheet. Did not reach. Although it is effective to reduce the P content in steel, it is not preferable from the viewpoint of strength and economy. Therefore, various studies have been made to obtain a steel sheet having excellent low-temperature chipping resistance in a range where P is a normal impurity level or slightly higher than the normal impurity level.

As a result, it is apparent that in the presence of a certain amount of P, the chipping resistance is improved by positively adding an appropriate amount of Si, rather than merely reducing the P content. became. It was also found that this also had the effect of improving the powdering resistance.

[0016] Si in the base steel sheet is liable to cause non-plating in the hot-dip plating of Zn on a thin steel sheet, so that its addition is usually avoided. This is attributed to the fact that during the process of raising the temperature of the plating line and performing the reduction annealing, the surface is concentrated as an oxide before plating. However, it was found that in the presence of about 0.02% of P, the presence of a small amount of Si had an effect of promoting the penetration of Zn by plating into the ferrite grain boundaries of the steel sheet. In this case, Mn in the steel also has the effect of promoting the above-mentioned outburst reaction, and it has been found that it is not preferable to contain a large amount of Mn.

The present invention was completed by further studying the limit of such an interaction between P and Si that most effectively improves the adhesion. That is, the gist of the present invention is that the chemical composition of the steel base material is 0.006% or less of C and 0.05% or less of Si in weight%
0.25%, Mn: 0.06 to 0.48%, P: 0.015% or more and satisfies the following expression, and the sum of one or two of Ti and Nb: 0.005 to 0.2%, Cr: 0.3% or less, B: 0.0
03% or less, and sol. Al: 0.005 to 0.05%, the balance is composed of unavoidable impurities and Fe, and the plating layer has an average F
e-concentration: 7 to 15%, and Al: 0.05 to 0.5%, the balance being substantially Zn ;
This is an alloyed hot-dip galvanized steel sheet with excellent resistance to powdering and powdering .

P (%) ≦ [Si (%) / 20] +0.0175...

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The reasons for limiting the chemical composition of the base steel sheet and the chemical composition of the plating layer in implementing the present invention are described below. The chemical compositions of the following steels and plating layers are all by weight.

(1) Base material steel sheet C: C significantly deteriorates the workability of the steel sheet, so the smaller the better, the better. Particularly in the hot dip galvanizing step where the steel sheet is rapidly heated and cooled because of continuous treatment, solid solution C tends to remain, which causes strain aging,
In order to deteriorate mechanical properties such as drawability and ductility, Ti or Nb is usually added and fixed. However, when C is increased, a large amount of these elements must be added, and the resulting carbides and the like also deteriorate mechanical properties. For this reason, the C content is set to 0.006% or less.

Due to the interaction with Si: P, the interface between the plating layer and the steel sheet is moderately roughened in the course of the alloying treatment to increase the surface area, and furthermore, the development of the brittle Γ phase is suppressed, and the adhesion,
In particular, it improves the low-temperature chipping resistance. Content 0.05
If it is less than 0.25%, the effect is not sufficient. If it exceeds 0.25%, the oxide on the surface layer of the base steel sheet increases, and non-plating and uneven alloying are likely to occur. Can no longer be used. Therefore, the content of Si is 0.05
To 0.25%. Particularly preferred is 0.10-0.15%.

Mn: To prevent hot brittleness caused by S mixed into steel as an inevitable impurity, Mn must be contained in an amount of 0.06% or more. In addition, increasing the content has the effect of increasing the strength and improving the adhesion of the plating layer, and in particular, suppressing non-plating due to an increase in Si. Therefore, when the content of Si is 0.15% or more, the content of 0.3% or more is desirable. . However, an excessive content not only deteriorates the press workability, but also promotes the outburst reaction in the alloying process, and causes
One phase increases, reduces powdering resistance, and further deteriorates low-temperature chipping resistance. Therefore, the content of Mn is set to 0.06 to 0.48%.

P: Usually, P degrades adhesion, especially low-temperature chipping resistance. However, coexistence with Si improves low-temperature chipping resistance. In order to obtain the effect, the content must be 0.015% or more. However, if the amount is too large, the low-temperature chipping resistance deteriorates and cannot be improved even by the presence of Si. Therefore, the upper limit of P varies depending on the amount of Si, and P (%) ≦ [Si (%) / 20] +0.0175...

Further, this P is 0.015% or more [Si (%)
/20+0.0175]% or less, alloying can be sufficiently performed even at a relatively low temperature of 530 ° C. or less, and Γ phase which is likely to appear at a high temperature can be suppressed. Desirably, P is 0.022% or more and Si is 0.1 to 0.15% and the range satisfies the formula. In this range, the low-temperature chipping resistance is particularly excellent.

Ti and Nb: Included for the purpose of fixing solid solution C of ultra-low carbon steel in order to obtain a steel sheet having good workability even in the treatment of hot-dip galvanizing production equipment.
In that case, if the total content of one or two of Ti and Nb is 0.005% or less, the effect is insufficient, and if the content exceeds 0.1%, no more effect is obtained, Poor workability. In addition, the growth of the Γ phase is promoted in the process of alloying the plating layer, and the powdering resistance is reduced. Therefore, the total content range of these two elements is
0.005 to 0.1%.

Sol. Al (acid-soluble Al): It is added for deoxidization to obtain a slab without defects and for fixing N, which is an unavoidable impurity mixed in steel. For this purpose, 0.0
A content of less than 05% has no effect, while a content of more than 0.05% saturates the effect. In addition, an excessive content causes non-plating. Therefore, the content range is 0.005 to 0.
Limited to 05%.

B: Although B may not be added, embrittlement due to press working of a very low carbon base material can be suppressed.
Include if necessary. In that case, the content of 0.0005% or more is desirable in order to obtain the effect of the addition. However, an excessive content not only saturates the effect but also impairs the workability of the base material, so the upper limit is made up to 0.005%.

Cr: Cr does not need to be added because Cr lowers the surface activity and easily causes non-plating and uneven alloying. However, it has the effect of improving the powdering resistance, and is added as necessary. If added, the content is 0.03
% Or more is desirable, but at most 0.3% as a range where the harm is not noticeable.

Inevitable impurities: In addition to the above component elements, the content of elements that are inevitably mixed in the production of steel often deteriorates various properties such as workability, brittleness, and corrosion resistance of steel. Use as little as possible. In particular, S and N not only deteriorate the workability of steel as non-metallic inclusions,
To fix Ti, Nb or the like added for the purpose of fixing, and increase the necessary addition amount thereof. In the case of the present invention, S
Is preferably 0.02% or less, and N is preferably 0.005% or less.

(2) Plating layer Here, the plating layer refers to a coating mainly composed of Zn, including an intermetallic compound layer formed on the surface of the base steel sheet. The coating layer of the galvannealed steel sheet has a coating weight of 20 ~
70 g / m ² is common, and the present invention is also directed to a plating layer of a similar degree. The average Fe concentration of this plating layer is 7% or more and 15% or less, and the Al concentration is 0.05% or more and 0.5% or less.

If the average Fe concentration is less than 7%, unalloyed Zn remains, which deteriorates spot weldability, deteriorates corrosion resistance after coating, and causes a material breakage during press working due to a large friction coefficient. Or Also, the average Fe
When the concentration exceeds 15%, the powdering resistance deteriorates. For normal hot-dip galvanizing without alloying,
Al suppresses the formation of an alloyed layer of Fe-Zn.
A small amount is added to the galvanizing bath. Therefore, when performing alloying treatment, there is an effect of delaying alloying,
This has the effect of improving the powdering resistance. To obtain this effect, the plating layer needs to contain 0.05% or more of Al. However, when the amount is too large, the alloying reaction becomes non-uniform, and the surface roughness becomes abnormally large, resulting in poor surface clarity after coating.Therefore, the upper limit of the amount of Al contained in the plating layer is set to 0.5. %.

The degree of unevenness at the interface between the base steel sheet and the plating layer becomes more severe as the alloying progresses, and the adhesion is improved. In particular, when low-temperature chipping resistance is required, the Fe concentration is reduced to 9%. % Or more, and if necessary, 10% or more.

(3) Plating method and alloying treatment method In the present invention, the base material of the plated steel sheet is mainly a work-hardened cold-rolled coil after cold rolling, but the annealed steel sheet, Alternatively, it may be a hot-rolled steel sheet from which scale has been removed, and is manufactured using generally used hot-dip galvanizing and alloying-treated steel sheet manufacturing equipment.

The plating conditions may be the same as those usually performed. For example, the base steel sheet is preheated to about 600 ° C., and subsequently reduced at a high temperature of 600 to 900 ° C. in an atmosphere containing hydrogen. Next, after cooling to a temperature near the hot-dip zinc plating bath, it is immersed in the bath to deposit a zinc film and perform plating. The alloying treatment may be performed under the same conditions as those used in the past.
After rapid heating to a temperature of ~ 600 ° C to cause alloying, rapid cooling is performed.

[0035]

EXAMPLE A 0.8 mm thick ultra-low carbon steel sheet having a chemical composition shown in Table 1 was used as a base material, and 10 wt% NaOH at 75 ° C.
Degreasing and washing with an aqueous solution
After pre-heating to 60 ° C, reduction annealing by heating in an atmosphere of 20 vol% H2 balance N2 and a dew point of -37 ° C for 60 s, then treatment at 550 ° C for 60 s in an atmosphere of 20 vol% H2 balance N2 and a dew point of -47 ° C rear,
After cooling to 480 ° C, it was immersed in a molten zinc bath containing 0.11 to 0.13% of Al at 460 ° C for 2 seconds, and the adhesion amount was 55 g / m ² per side.
Was obtained.

After plating, each steel plate was heated to a temperature range of 500 to 575 ° C. at a heating rate of 40 ° C./s by an induction heating method, kept for 18 seconds, and cooled at a rate of 15 ° C./s.

[0037]

[Table 1]

The presence or absence of streak-like surface patterns was evaluated by observing the surface appearance of the obtained alloyed hot-dip galvanized steel sheet specimen. Next, the plating layer was dissolved by immersion in 6 wt% hydrochloric acid containing 0.5 vol% of an inhibitor (Ibit 710N, manufactured by Asahi Chemical Co., Ltd.), and the amounts of Fe and Al therein were analyzed.

For powdering resistance, a disc having a blank diameter of 60 mmφ was punched from a test piece, and the blank was pressed with a wrinkle pressure of 3.92 kN.
Perform cylindrical drawing with a punch of 33 mmφ, and examine the peeling resistance with the adhesive tape on the outer surface of the side wall of the obtained cup.
It was evaluated by the peel weight.

Regarding the low-temperature chipping resistance, first, the width
70mm, 150mm long alloyed hot-dip galvanized steel sheet specimens are treated with phosphate by Chemifil CF168, and then coated with cationic electrodeposition paint (Uniprime by PPG).
Specimens of three coatings of a base coat of 0 μm, a middle coat of 15 μm in thickness (epoxy ester paint by PPG), and a top coat of 45 μm in thickness (acrylic enamel paint by PPG) were prepared. This painted test specimen was cooled to -20 ° C,
Using a gravelometer that complies with the SAE J400 rules,
Crushed stone for roads specified in JIS A5001 is 150 km /
After the collision at a speed of h, the peeling resistance by the adhesive tape was examined. In this case, the diameter of one test piece having a large peeling mark was measured from the largest one, and the average value was used as an index of the low-temperature chipping resistance of the test piece.

Table 2 shows the alloying temperature of each test piece,
The Al concentration, Fe concentration, appearance evaluation, powdering resistance evaluation, and low-temperature chipping resistance evaluation of the plating layer are shown together. As is clear from this, a hot-dip galvanized steel sheet made of steel that satisfies the chemical composition specified in the present invention in Table 1 is:
Compared with the case of steel having a chemical composition outside the present invention, the peel diameter, which is an index of low-temperature chipping resistance, is small, and the powdering resistance is also good, indicating excellent adhesion of the plating layer. .

[0042]

[Table 2]

When a steel sheet having a chemical composition outside the range defined by the present invention was used, insufficient alloying tended to occur. Therefore, Test Nos. 9, 11, 15, 16, 18 and 19 were conducted by increasing the alloying temperature. However, the powdering resistance was deteriorated, and the low-temperature chipping resistance was not improved. On the other hand, as shown in Test Nos. 23, 27, 29 , 31, and 34 , the steel having the chemical composition defined by the present invention has a slightly higher alloying temperature, so that alloying progresses and powdering resistance is improved. However, the low-temperature chipping resistance is improved. in this way,
It can be seen that the galvannealed steel sheet defined in the present invention has excellent adhesion of the plating layer.

[0044]

The plated steel sheet of the present invention is an alloyed hot-dip galvanized steel sheet whose base material is extremely low carbon steel and has excellent adhesion of the coating layer, and is required for automobiles and the like. It is an alloyed hot-dip galvanized steel sheet with excellent chipping properties and good powdering resistance and press workability during processing.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-4-301060 (JP, A) JP-A-4-41658 (JP, A) JP-A-4-297563 (JP, A) JP-A-4-297 263054 (JP, A) JP-A-4-157144 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C23C 2/06 C22C 38/00 C22C 38/32

Claims (1)

(57) [Claims] 1. The method of claim 1, wherein the chemical composition of the base material steel is C:
0.006% or less, Si: 0.05 to 0.25%, Mn: 0.06 to 0.48
%, P: not less than 0.015% and the following formula is satisfied, and the total of one or two of Ti and Nb: 0.005 to 0.2%, C
r: 0.3% or less, B: 0.003% or less, and sol.Al:
0.005 to 0.05%, the balance is composed of unavoidable impurities and Fe, and the plating layer has an average Fe concentration of 7 to 15% and A
l: 0.05 to 0.5%, with the balance being substantially Zn, low-temperature chipping resistance and powdering resistance
Excellent galvannealed steel sheet. P (%) ≦ [Si (%) / 20] +0.0175 ・ ・ ・ ・ ・
・