JP2002235160A - High tensile strength hot dip galvanized steel sheet and high tensile strength hot dip galvannealed steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile strength hot dip galvanized or galvannealed steel sheet which contains large quantities of Si and Mn, has excellent plating adhesion, mechanical properties and a balance of strength-ductility, and also has excellent corrosion resistance. SOLUTION: The steel sheet has a composition containing, by mass, 0.05 to 0.25% C, >0.50 to <2.00% Si, <=3.5% Mn and 0.01 to 1.0% Al. The mass ratio of Mn/S in the steel is >=2. The steel sheet also has a composite structure consisting of tempered martensite of >=20% by volume fraction and retained austenite of >=2%. The Al concentration (mass%) in a plated layer on the steel sheet satisfies the following inequality (1) in the case of a hot dip galvanized steel sheet and satisfies the following inequality (2) in the case of the hot dip galvannealed steel sheet, and the total of Si and Mn in the steel is >=3%: 0.67-1/50(Mn/Si)>=[the Al concentration in the plated layer]>=0.37-1/50(Mn/Si) (1), and 0.5-1/50(Mn/Si)>=[the Al concentration in the plated layer]>=0.2-1/50(Mn/Si) (2).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to high tensile strength (alloying).
Related to hot-dip galvanized steel sheet (including steel strip), especially high-strength (alloyed) hot-dip galvanized steel sheet with excellent strength-ductility balance and excellent plating adhesion that can withstand even press forming into complex shapes. And a high-tension (alloyed) hot-dip galvanized steel sheet having even better corrosion resistance.

[0002]

2. Description of the Related Art In recent years, from the viewpoint of preserving the global environment,
There is a demand for improved fuel efficiency of automobiles. Further, in order to protect occupants in the event of a collision, it is required to improve the safety of the vehicle body. Under such circumstances, weight reduction of the vehicle body and reinforcement of the vehicle body have been actively promoted. Particularly, in order to reduce the weight of an automobile body, it has been proposed to increase the strength of an automobile steel sheet such as a hot-rolled steel sheet and a cold-rolled steel sheet and to reduce the thickness of the steel sheet. On the other hand, since many automotive parts made of steel plate are formed by pressing,
Automotive steel sheets are required to have excellent press formability. Further, hot-dip galvanized steel sheets are excellent in rust resistance (corrosion resistance) and can be manufactured at low cost, and thus are frequently used as rust-resistant surface-treated steel sheets for automobile bodies.

In order to increase the strength of a steel sheet, it is necessary to add elements such as Si and Mn to strengthen the solid solution.
Since Mn is an easily oxidizable element, Si, Mn, and the like are concentrated on the surface during annealing, and the wettability of hot-dip galvanizing applied to the surface is deteriorated, and the plating adhesion is deteriorated. In order to solve the above problems, for example, in Japanese Patent Application Laid-Open Nos. 5-179356 and 5-51647, the addition amount of Si and Mn is reduced, quenching and quenching are performed at the time of hot rolling and winding, and the hot dip galvanizing line In addition, a method of plating after annealing in a two-phase region has been proposed. However, in practice, S
If even a small amount of i is added, the adhesion of the plating deteriorates and the plating easily peels off.
It has been virtually impossible to apply hot-dip galvanizing with good plating adhesion to a steel sheet having a large content.

[0004] In order to achieve both good elongation and strength, the final structure of the hot-dip galvanized steel sheet must be a composite structure containing tempered martensite and retained austenite, and the remainder composed of ferrite and a low-temperature transformation phase. is there. For this purpose, it is effective to add a large amount of Si and Mn in a complex manner. However, as described above, when a large amount of Si and Mn is contained, plating adhesion is deteriorated.

[0005]

DISCLOSURE OF THE INVENTION The present invention is intended to solve the above-mentioned problems, and is excellent in hot-dip galvanizing adhesion even if the base steel sheet contains a large amount of Si and Mn, and has good press formability. It is an object of the present invention to provide a high-tensile (alloyed) hot-dip galvanized steel sheet having excellent mechanical properties and strength-ductility balance. In addition, it is another object of the present invention to provide a high-tension (alloyed) hot-dip galvanized steel sheet having even better corrosion resistance.

[0006]

Means for Solving the Problems The present inventor of the present invention has found that, in a high-strength hot-dip galvanized steel sheet, the adhesion of hot-dip galvanized steel to the surface layer of the steel sheet while containing a large amount of Si and Mn and maintaining mechanical properties is reduced. As a result of intensive investigations on the conditions for preventing Mn / Si, if the Mn / Si mass ratio in steel is within a certain range and the relationship between the Mn / Si mass ratio in steel and the Al concentration in the plating layer is specified, It has been found that the above object can be achieved.

That is, the present invention relates to a method for preparing C:
0.05-0.25%, Si: more than 0.50% 2.00%
, Mn: 3.5% or less, and Al: 0.01 to
1.0%, with the balance being Fe and unavoidable impurities, the Mn / Si mass ratio in the steel being 2 or more, and consisting of tempered martensite, retained austenite, ferrite, and a low-temperature transformation phase. On a steel sheet having a composite structure in which the tempered martensite is 20% or more and the retained austenite is 2% or more, in terms of fraction, a hot-dip galvanized layer whose Al concentration in the plating layer satisfies the following formula (1). This is a high-strength hot-dip galvanized steel sheet excellent in strength-ductility balance and plating adhesion, characterized by having:

0.67-1 / 50 (Mn / Si) ≧ [Al concentration in plating layer (% by mass)] ≧ 0.37-1 / 50 (Mn / Si) (1) (Mn in formula (1)) / Si represents the mass ratio of Mn / Si in steel.)

A preferred present invention is a high-strength hot-dip galvanized steel sheet having excellent strength-ductility balance and plating adhesion containing at least one component selected from the following group in addition to the above steel sheet components. (First group) Cr: 1.0% or less, Mo: 1.
0% or less and B: at least one selected from the group consisting of 0.003% or less. (Second group) Ti: 0.1% or less, Nb: 0.
At least one selected from the group consisting of 1% or less and V: 0.1% or less. (Third group) Cu: 1.0% or less and Ni:
At least one selected from the group consisting of 1.0% or less
seed.

[0010] In a preferred aspect of the present invention, in the steel sheet composition, in addition to the strength-ductility balance and the plating adhesion, in which the total of Si and Mn in the steel is 3% or more by mass%, high-strength molten zinc excellent in corrosion resistance is provided. It is a plated steel sheet.

Further, the present invention relates to a method for producing C: 0.05% by mass.
~ 0.25%, Si: more than 0.50% and less than 2.00%, M
n: not more than 3.5% and Al: 0.01 to 1.0%, with the balance being Fe and unavoidable impurities,
Mn / Si mass ratio in steel is 2 or more, and it is composed of tempered martensite, retained austenite, ferrite and a low-temperature transformation phase, and the volume fraction of the tempered martensite is 20% or more and the retained austenite. On a steel sheet having a composite structure of 2% or more,
It is a high-strength hot-dip galvanized steel sheet excellent in balance between strength and ductility and plating adhesion, characterized by having a hot-dip galvanized layer having a l concentration satisfying the following formula (2).

0.5-1 / 50 (Mn / Si) ≧ [Al concentration in plating layer (mass%)] ≧ 0.2-1 / 50 (Mn / Si) (2) (Mn in formula (2) / Si represents the mass ratio of Mn / Si in steel.)

The preferred present invention is a high-strength hot-dip galvanized steel sheet excellent in strength-ductility balance and plating adhesion containing at least one component selected from the following group in addition to the above steel sheet components. (First group) Cr: 1.0% or less, Mo: 1.
0% or less and B: at least one selected from the group consisting of 0.003% or less. (Second group) Ti: 0.1% or less, Nb: 0.
At least one selected from the group consisting of 1% or less and V: 0.1% or less. (Third group) Cu: 1.0% or less and Ni:
At least one selected from the group consisting of 1.0% or less
seed.

[0014] In a preferred aspect of the present invention, in the steel sheet composition, in addition to the strength-ductility balance and the plating adhesion in which the total of Si and Mn in the steel is 3% or more by mass%, the high-tension molten zinc excellent in corrosion resistance is provided. It is a plated steel sheet.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention has been completed based on the following experimental facts. Hereinafter, the steel composition is represented by mass fraction. C: 0.10%, Si: 0.3 to 2%, Mn: 1.
A sheet bar having a thickness of 30 mm containing 0 to 3.5%, P: 0.01%, and Al: 0.04% is heated at 1250 ° C. to obtain a hot-rolled steel sheet having a thickness of 2.3 mm in five passes. Later, 62
Wound at 0 ° C. Then, the black scale is removed by pickling,
Cold rolled to 1.0 mm, 800 to 9 in an annealing furnace
After heating at 00 ° C, it was pickled with 5% hydrochloric acid at 60 ° C for 6 seconds. Then, it is annealed in a 5% H ₂ —N ₂ reducing atmosphere at 780 ° C. by a hot-dip galvanizing simulator, and hot-dip galvanized in a hot-dip galvanizing bath (0.08 to 0.25% Al—Zn) at 460 ° C. Then, gas wiping was performed so that the adhesion amount was 50 g / m ² on one side. Alloying in an electric furnace is 50
Performed at 0 ° C. The plating adhesion and mechanical properties of the obtained plated steel sheet were investigated.

Hereinafter, the alloyed hot-dip galvanized steel sheet is referred to as G
A, the unalloyed hot-dip galvanized steel sheet is represented by GI, the cold-rolled steel sheet is represented by CR, and the hot-rolled steel sheet is represented by Hot,
The relationship between the Mn / Si mass ratio in steel and the Al concentration in the plating layer will be described. From the above experimental results, FIG. 1 (relation between Mn / Si mass ratio in GI steel and Al concentration in plating layer) and FIG. 2 (Mn / Si mass ratio in GA steel and Al concentration in plating layer) Relationship with Mn in steel
If the / Si mass ratio is less than 2, mechanical properties and plating quality are poor, and if the Al concentration in the plating layer is too low or too high, the adhesion of For the first time, it was found that the formation delay or poor weldability occurred, and the above formula (1) or formula (2) was derived.

If the Mn / Si mass ratio in the steel and the Al concentration in the plating layer are adjusted so as to satisfy the above formula (1) or (2), surprisingly, Si, M
The present inventors have found that even when n exists, it is possible to manufacture GI and GA having good adhesion of hot-dip galvanizing to the surface thereof, and completed the present invention. FIG. 3 shows the effect of the Si content and the Mn content in the steel on various characteristics for G1 satisfying the above-described formula (1). 0.50 Si content in steel
% And less than 2.00% and a Mn / Si mass ratio of 2 or less, poor mechanical properties and poor plating adhesion do not occur, and the average level of corrosion resistance can be maintained.

FIG. 4 shows the effects of the Si content and the Mn content in steel on various properties of GA satisfying the above-mentioned formula (2). As in the case of GI, when the Si content in the steel exceeds 0.50% and less than 2.00% and the Mn / Si mass ratio satisfies any of 2 or less, poor mechanical properties and poor plating adhesion. Does not occur, and the average level of corrosion resistance can be maintained. Furthermore, in any case of GI and GA, it is found that when the total amount of the Si content and the Mn content is 3% or more, it is extremely advantageous for improvement of corrosion resistance.

In FIGS. 3 and 4, mechanical properties were evaluated as good if the tensile strength was 590 MPa or more and the elongation was 35% or more, and the others were poor. Regarding the plating adhesion, an adhesion test was performed using the criteria shown in Table 5 described later, and those having a rank of 3 or more were determined to be defective. For the evaluation of corrosion resistance, as for GI, as a result of the composite corrosion cycle test described
The hot-dip galvanized steel sheet of CC) was used as a comparative steel sheet, and the equivalent case was regarded as the average level of corrosion resistance, and those without pitting corrosion were evaluated as extremely good. GA was evaluated in the same manner as in the case of GI, with the comparative steel sheet being a galvannealed steel sheet of mild steel (SPCC).

In the present invention, the contents of the constituent components in the steel and the composite structure thereof are defined as follows. The content (%) of each element means% by mass. C: 0.05 to 0.25% C is an indispensable component for obtaining necessary strength and for obtaining a desired structure such as retained austenite, and at least 0.05% is required. %, The weldability deteriorates. Preferably 0.07
０．１0.18%.

Si: more than 0.50% and less than 2.00% Si is an indispensable component for solid solution strengthening and obtaining a desired structure, and it is possible to achieve high strength without deteriorating ductility. It is a component that does. The effect is not exhibited unless it is more than 0.50%. On the other hand, when the content is 2.00% or more, the plating adhesion deteriorates. Therefore, the above range was set. Preferably it is 0.6 to 1.6%.

Mn: 3.5% or less M is an indispensable component for obtaining the required strength in the same manner as C and for improving the hardenability of steel to obtain a desired structure.
When the content is 1% or more, the effect is sufficiently exerted, which is preferable. However, even if it exceeds 3.5%, the effect is saturated and the cost is increased, so the above range is set.

Al: 0.01-1.0% Al is important for obtaining necessary strength and for obtaining a desired structure, and as a result, the amount of Si added can be reduced, so that Al has the same tensile strength. It is more advantageous than Si-added steel for improving plating adhesion. To obtain the desired effect, a minimum of 0.01% is required. If the content exceeds 1.0%, the economic efficiency deteriorates, so the upper limit is set to 1.0%. Preferably 0.02
~ 0.5%.

Mn / Si mass ratio in steel: 2 or more In order to obtain an effect of improving plating adhesion, Mn / S in steel is required.
A higher i mass ratio is advantageous. This is because the surface condensate generated at the time of annealing just before plating,
This is because it changes to a Si—Mn-based composite oxide having good wettability with molten zinc. Further, when the steel sheet is heated in advance before passing it through a continuous hot-dip galvanizing facility (CGL), and then cooled, the surface is activated by pickling and then the Mn / Si in the steel is activated. The higher the mass ratio, the more the effect of improving the pickling property of the oxide film can be obtained. It is considered that this is because the oxide on the surface of the steel sheet changes from an oxide mainly composed of Si, which is difficult to pickle, to a Si-Mn-based composite oxide having good pickling properties.

In addition, the higher the Mn / Si mass ratio in the steel, the longer the alloying does not need to be delayed, so that the productivity in the case of alloying after hot-dip galvanizing is also improved. Mn / S in steel
When the i mass ratio is less than 2, the plating adhesion deteriorates, so the Mn / Si mass ratio in the steel was specified as 2 or more. However, if the Mn / Si mass ratio in the steel is higher than 7, the Si content becomes relatively small, and it may be difficult to obtain a desired micro composite structure.
The / Si mass ratio is preferably 7 or less. More preferred are 2.2 to 4.5.

The amount of (Si + Mn) in steel: 3% by mass or more Although zinc-coated steel sheets originally have good corrosion resistance, further improvement in corrosion resistance is required, for example, in recent years, there has been a demand for improved pitting corrosion resistance in automotive steel sheets. It has been desired that, in order to further improve the corrosion resistance, Mn content in steel and Si
The total content is preferably 3% by mass or more. The content of each steel component is within the above range, and (Si + Mn)
When the content exceeds 3% by mass, dense and inert SiO
_{(2) Since} the formation of the coating on the steel sheet surface is suppressed and oxygen diffusion into the steel is facilitated, the internal oxidation is promoted at the time of winding the hot-rolled steel sheet or at the time of heating in the primary processing step. Even if the (Si + Mn) content in the steel is 3% by mass or more, if the Mn / Si mass ratio in the steel is low and less than 2, the plating adhesion is poor and the corrosion resistance is not sufficient.

The mechanism by which the corrosion resistance is improved is not clear, but it is presumed that the internal oxidation further enhances the plating adhesion due to the anchor effect during hot-dip galvanization and contributes to the improvement of the corrosion resistance.

Tempered Martensite Since tempered martensite is softened by tempering and has a sufficient plastic deformability, it is effective in improving elongation characteristics. When the volume fraction is less than 20%, the effect of improving ductility is not recognized, so the content is set to 20% or more. However, if it exceeds 80%, it becomes difficult to increase the strength of the steel sheet.

Retained austenite Retained austenite undergoes strain-induced transformation into martensite during processing, widely dispersing locally applied processing strain,
It has the effect of improving the ductility of the steel sheet. If the volume fraction is less than 2%, remarkable improvement in ductility cannot be expected, so it is necessary to be 2% or more, and if it is 5% or more, the effect of improving ductility is more remarkable, so that it is preferable.

The production method for obtaining the above composite structure is not particularly limited. For example, a steel sheet is prepared in advance (Ac ₃ transformation point -5).
After heating to 0 ° C. or higher, the structure is quenched at a cooling rate of 10 ° C./sec or higher, and then heated between the Ac _{1 and} Ac ₃ transformation points by CGL, and at a cooling rate of 5 ° C./sec or higher. Obtained by cooling and tempering the tissue. However, this is 1
This is an example, and any high-strength steel sheet having a component and phase specified by the present invention may be obtained by any manufacturing method.

The low-temperature transformation phase in the present invention refers to martensite or bainite. Both martensite and bainite are hard phases and have the effect of increasing the strength of the steel sheet by strengthening the structure. Further, since transformation is accompanied by the occurrence of movable dislocations, it also has the effect of lowering the yield ratio of the steel sheet. In order to sufficiently obtain such an effect, the low-temperature transformation phase is preferably martensite. In the present invention, the amount of the low-temperature transformation phase is not particularly limited. What is necessary is just to distribute suitably according to the intensity | strength of a steel plate.

Al Concentration in Plating Layer In the present invention, the Al concentration in the plating layer is important. The inventor of the present invention has found that the steel sheet containing Mn / Si having a Mn / Si mass ratio of 2 or more as described above and containing Mn and Si has the above formula (1) for GI and the above formula (2) for GA. It has been found that good plating adhesion can be ensured by securing the Al concentration in the predetermined plating layer within the range. In the case of GI, since it is necessary to suppress the start of the alloying reaction due to reheating after plating, the Al concentration is made higher than in the case of GA. Further, when the Si content is large, local alloying reactions occur frequently. Therefore, in order to suppress the start of the alloying reaction based on this, the Al content is increased as the Mn / Si mass ratio is lower.

When the Al concentration is higher than the value on the left side of the above formula (1) or (2), in the case of GI, the weldability is deteriorated because the Fe—Al alloy layer generated at the beginning of plating is thick, In the case of GA, alloying is significantly delayed. If the Al concentration is lower than the value on the right side of the above equation (1) or (2), F
The formation of the e-Al alloy layer is suppressed, and a hard and brittle Γ phase is likely to be generated at the initial stage of plating, and the plating adhesion is deteriorated. Therefore, in order to ensure good plating adhesion, it is necessary to maintain a predetermined Al concentration within the range of the above formula (1) for GI and the above formula (2) for GA.

The method for adjusting the Al concentration in the plating layer to a predetermined amount is not particularly limited.
A method of increasing the amount of 1 or increasing the plating time to promote the reaction between Al and the base iron to increase the Al concentration in the plating layer is exemplified. Further, the steel sheet having a component in steel of the present invention may be heated in advance, lightly pickled after cooling, activated, and then passed through CGL.
The examples of these production methods do not limit the present invention.

Further, the steel sheet of the high-strength galvanized steel sheet or the high-strength galvannealed steel sheet of the present invention may contain one or more of the following elements in the following amounts (% by mass). Good. In that case, the following effects are further obtained. Cr: 1.0% or less Cr has an effect of improving the hardenability and accelerating the formation of a low-temperature transformation phase, so Cr is added as necessary. Preferably it is 0.05% or more, but if it exceeds 1.0%, the plating adhesion deteriorates, so the upper limit is preferably made 1.0%.

Mo: 1.0% or less Mo has the effect of improving the hardenability similarly to Cr and promoting the formation of a low-temperature transformation phase, so that Mo is added as necessary. Preferably, it is 0.05% or more, but if it exceeds 1.0%, the cost is increased, so the upper limit is preferably made 1.0%.

B: 0.003% or less B has an effect of improving the hardenability and is added as necessary. However, if the content exceeds 0.003%, the plating adhesion deteriorates. Therefore, the upper limit is preferably 0.003%.

Ti, Nb, V: 0.1% or less Ti, Nb and V form a carbonitride and have an effect of increasing the strength of the steel by precipitation strengthening. When adding these, it is preferable to add 0.01% or more, respectively. However, even if it exceeds 0.1%, the strength is excessively increased, and the ductility is rather deteriorated. Therefore, the upper limit is preferably set to 0.1%.

Cu: 1.0% or less Cu segregates in austenite and is important not only for obtaining necessary strength and for obtaining a desired structure, but also for improving plating adhesion, which is necessary. Add accordingly. The reason why the plating adhesion is improved is not clear at present, but 0.01% is required to obtain the desired effect.
% Or more is preferable. However, if the content exceeds 1.0%, the economic efficiency is deteriorated. Therefore, it is preferable to set the upper limit to 1.0%.

Ni: 1.0% or less Ni segregates in austenite like Cu, and is important not only for obtaining necessary strength and for obtaining a desired structure, but also for improving plating adhesion. Add as needed. Although the reason why the plating adhesion is improved is not clear at present, it is preferable to add 0.01% or more to obtain a desired effect. However,
If the content exceeds 1.0%, the economic efficiency deteriorates.
It is preferably set to 0%.

CGL (Hot Dip Galvanizing) Conditions The steel sheet for producing the hot dip galvanized steel sheet of the present invention has a C
The GL conditions are not particularly limited, and can be implemented by a conventional method. However, the CGL heating temperature (secondary heating temperature) is 650.
When the temperature is lower than ℃, the oxide film on the surface of the steel sheet cannot be reduced, and non-plating tends to occur. On the other hand, when the temperature is 850 ° C. or more, the surface concentration of Si and Mn is large during heating, so that non-plating is also likely to occur. Therefore, 650-850 degreeC is preferable.

The hot-dip galvanizing bath has an Al concentration of 0.08 to secure the adhesion of the plating layer after alloying.
% Is preferable. However, when the content exceeds 0.20%, alloying becomes difficult, or the weldability of the obtained hot-dip galvanized steel sheet may be deteriorated.
20% is preferred. As described above, in order to control the Al concentration in the plating layer within the range of the present invention, A
In addition to the l concentration, the temperature of the entering plate, the immersion time of the plating bath, and other operating conditions may be adjusted. If the bath temperature is 440 ° C. or lower, there is a possibility that a portion below the freezing point (420 ° C.) may appear due to the bath temperature fluctuation of the plating bath, resulting in lack of operational stability. On the other hand, if the temperature exceeds 480 ° C., the cost for heating and holding increases. Therefore, the bath temperature is preferably 440 to 480 ° C.

In the case of alloying, the alloying temperature is 450
If the temperature is lower than ℃, the ζ phase is likely to be generated, and not only the slidability of the GA may be lacking, but also the productivity is deteriorated due to the time required for alloying. On the other hand, when the temperature exceeds 600 ° C., the Γ phase is likely to be generated, and the plating adhesion of GA may be lacking. Therefore, the alloying temperature is preferably 450 to 600 ° C.

Furthermore, the degree of alloying is such that the Fe diffusion amount is 8 to 1
It preferably falls within the range of 3%. If it is less than 8%, the Δ phase remains, and the flaking resistance is liable to deteriorate.
% Or more, a Γ phase is generated, and the plating adhesion may be deteriorated. However, the conditions of these manufacturing methods are examples, and the present invention is not limited to a specific manufacturing method.

[0045]

EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples. (Examples 1 to 10, Comparative Examples 1 to 4) Chemical compositions (C, Si, Mn, P, S, Al, Cr, Cu, N) shown in Table 1
i, Mo, Ti, Nb, V and B) slabs (thickness 3
00mm) was heated at 1250 ° C, and hot-rolled to form a hot-rolled steel sheet having a thickness of 2.0 mm (steel A to H are examples; steels I to J are comparative examples), and then wound at 620 ° C. Next, after removing black scales by pickling, if cold rolling is to be performed as necessary, the rolls are rolled at a reduction rate of 50%, heated in a heating furnace (primary heating), passed through a CGL, and pickled. , Annealing, hot-dip galvanizing and alloying treatment. Cold rolling process,
Table 2 shows the primary heating temperature, the Al concentration in the plating bath, and the presence or absence of alloying. The cooling rate after the primary heating was 30 ° C. /
Second, the secondary heating temperature is 780 ° C., the cooling rate after the secondary heating is 10 ° C./sec, and the plating adhesion amount is 50 g / m ^{2 on} one side.
Was one by one. The alloying temperature was in the range of 450 to 600 ° C.

[0046]

[Table 1]

[0047]

[Table 2]

Table 1 shows the results of investigations on the Al concentration, the degree of alloying (Fe diffusion amount), the appearance after plating, the plating adhesion and the corrosion resistance of the obtained hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet. 3 is shown. In addition, Table 4 shows the results of investigation on the tempered martensite fraction, retained austenite fraction, ferrite fraction, low-temperature transformation phase, and mechanical properties (elongation, tensile strength) of the obtained plated steel sheet.

[0049]

[Table 3]

[0050]

[Table 4]

The Al concentration in the plating layer can be determined by dissolving the plating layer with an alkali such as NaOH or KOH to which an inhibitor has been added or an acid such as HCl or H ₂ SO ₄ , and dissolving the solution with a plasma emission spectrometer (ICP) or the like. It was determined by analytical quantification. The degree of alloying (the amount of Fe diffusion) is similarly determined by IC
It was measured by analyzing and quantifying Fe with P or the like. Regarding the appearance after plating, the area of the unplated portion of the steel sheet immediately after plating was quantified by image processing, and the area ratio of unplated was evaluated as good when less than 0.1% and poor when 0.1% or more.

The plating adhesion was measured by applying a cellophane tape to a plated steel sheet, bending the tape surface within 90 °, bending the tape back, and then measuring the amount of plating peeling per unit length when the tape was peeled off using a fluorescent X-ray. Measured as counts, Table 5
Good for ranks 1 and 2 according to the criteria of (○,
Δ) Three or more were evaluated as defective.

[0053]

The corrosion resistance was evaluated based on a galvanized steel sheet of mild steel sheet (SPCC) or a galvannealed steel sheet, and each specimen was subjected to a composite corrosion cycle test (0.5% salt water sprayed at 35 ° C. for 6 hours at 35 ° C.). After that, it was dried at 70 ° C. for 6 hours, and then dried at a temperature of 40 ° C. and a humidity of 90%.
After 2 cycles of 60 hours, that is, 60 days, the maximum corrosion depth was compared. After the corrosion test,
The maximum corrosion depth was determined by extreme value statistical processing for the corrosion depth (pitting corrosion). A steel plate having a depth substantially equal to the maximum corrosion depth of the standard steel sheet was evaluated as an average level, a steel plate having a half or less was evaluated as good, a steel plate having no pitting corrosion was evaluated as extremely good, and a steel plate having 1.5 times or more was evaluated as poor.

The volume fraction of the tempered martensite phase of the steel sheet was determined by polishing the steel sheet section embedded in the resin after polishing by 1 mass.
% Of sodium pyrosulfite in a picral solution (4 g picric acid / 100 cc ethanol), observed with an electron microscope at a magnification of 1000, and analyzed by image analysis, the area occupied by the martensite phase present in a 100 mm square area. The volume ratio of the martensite phase was determined.

The ferrite phase was obtained by polishing the cross section of a steel sheet embedded in resin, etching the structure with a nital solution ( ₃ vol% of a 69 mass% HNO ₃ solution, and 97 vol% of ethanol), and then observing it at a magnification of 250 × with an optical microscope. The photograph in the square area was subjected to image processing to determine the occupied area ratio of the ferrite phase, which was defined as the volume ratio of the ferrite phase.

The volume fraction of the retained austenite phase of the steel sheet was obtained by polishing a test piece taken from the steel sheet to the center plane in the thickness direction and measuring the X-ray intensity on the center plane of the thickness. That is, using MoKα radiation, the ferrite (200)
(211) The diffracted X-ray intensities of each surface and the austenitic (200) and (220) diffracted X-ray intensities of each surface were calculated, and the ferrite (including martensite) (200) (21) was obtained.
1) Integrated intensity and austenite (200) (220)
Was determined, and this was defined as the volume fraction of the austenite phase.

The mechanical properties were determined by measuring the yield strength (yield point) YP, the tensile strength TS, and the elongation El using a JIS No. 5 tensile test piece taken from the steel sheet with the direction perpendicular to the rolling direction as the tensile direction. Those having a tensile strength of 590 MPa or more and an elongation of 35% or more were evaluated as good.

When the steel composition, the steel structure and the plating layer were within the range of the present invention, the plating adhesion and the mechanical properties of the hot-dip galvanized steel sheet were all good. In particular, Example 3 was extremely excellent in corrosion resistance. On the other hand, those out of the range of the present invention were inferior in plating adhesion and mechanical properties.

[0060]

As described above, according to the present invention, a high-strength hot-dip galvanized steel sheet and a high-strength alloyed hot-dip steel sheet having excellent plating adhesion, excellent mechanical properties of a hot-dip galvanized steel sheet, and excellent corrosion resistance. A galvanized steel sheet is obtained. By applying the steel sheet of the present invention, it is possible to reduce the weight and fuel consumption of an automobile body, and also to greatly contribute to the improvement of the global environment.

[Brief description of the drawings]

FIG. 1 Mn / in the steel of hot-dip galvanized steel sheet (GI)
It is a figure showing the relationship between Si mass ratio and Al concentration in a plating layer.

FIG. 2 is a diagram illustrating a relationship between a Mn / Si mass ratio in steel of an alloyed hot-dip galvanized steel sheet (GA) and an Al concentration in a plating layer.

FIG. 3 Mn in steel of a galvanized steel sheet (GI)
It is a figure showing the relationship between Si content and corrosion resistance.

FIG. 4 is a view showing the relationship between the Mn and Si contents in steel of an alloyed hot-dip galvanized steel sheet (GA) and corrosion resistance.

──────────────────────────────────────────────────続 き Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI theme coat ゛ (reference) C23C 2/40 C23C 2/40 F term (reference) 4K027 AA02 AA05 AA23 AB02 AB05 AB07 AB44 AC12 AC52 AC73 AE02 AE23

Claims (6)

[Claims] 1. Mass%: C: 0.05 to 0.25%, Si: more than 0.50% and less than 2.00%, Mn: 3.5% or less, and Al: 0.01 to 1. 0%, the balance being Fe and unavoidable impurities, the Mn / Si mass ratio in the steel being 2 or more, and consisting of tempered martensite, retained austenite, ferrite, and a low-temperature transformation phase. Then, on a steel sheet having a composite structure in which the tempered martensite is 20% or more and the retained austenite is 2% or more, there is a hot-dip galvanized layer in which the Al concentration in the plated layer satisfies the following formula (1). A high-strength hot-dip galvanized steel sheet excellent in strength-ductility balance and plating adhesion. 0.67-1 / 50 (Mn / Si) ≧ [Al concentration in plating layer (mass%)] ≧ 0.37-1 / 50 (Mn / Si) (1) (Mn / Si of formula (1)) Represents the mass ratio of Mn / Si in steel.) 2. The high tensile galvanized steel sheet according to claim 1, further comprising at least one component selected from the following group in addition to the steel sheet component: . (Group 1) In mass%, Cr: 1.0% or less, Mo: 1.
0% or less and B: at least one selected from the group consisting of 0.003% or less. (Second group) Ti: 0.1% or less, Nb: 0.
At least one selected from the group consisting of 1% or less and V: 0.1% or less. (Third group) Cu: 1.0% or less and Ni:
At least one selected from the group consisting of 1.0% or less
seed. 3. The strength-ductility balance and plating adhesion according to claim 1 or 2, wherein in the steel sheet composition, the total of Si and Mn in the steel is 3% or more by mass%. High tensile galvanized steel sheet with excellent corrosion resistance. 4. In mass%, C: 0.05-0.25%, Si: more than 0.50% and less than 2.00%, Mn: 3.5% or less, and Al: 0.01-1. 0%, the balance being Fe and unavoidable impurities, the Mn / Si mass ratio in the steel being 2 or more, and consisting of tempered martensite, retained austenite, ferrite, and a low-temperature transformation phase. An alloyed hot-dip galvanized layer having a tempered martensite of 20% or more and a retained austenite having a composite structure of 2% or more, wherein the Al concentration in the plating layer satisfies the following formula (2). A high-tensile alloyed hot-dip galvanized steel sheet excellent in strength-ductility balance and plating adhesion, characterized by having: 0.5-1 / 50 (Mn / Si) ≧ [Al concentration in plating layer (% by mass)] ≧ 0.2-1 / 50 (Mn / Si) (2) (Mn / Si of formula (2)) Represents the mass ratio of Mn / Si in steel.) 5. A high-tensile alloyed hot-dip zinc alloy excellent in strength-ductility balance and plating adhesion according to claim 4, further comprising at least one component selected from the following group in addition to the steel sheet component. Plated steel sheet. (Group 1) In mass%, Cr: 1.0% or less, Mo: 1.
0% or less and B: at least one selected from the group consisting of 0.003% or less. (Second group) Ti: 0.1% or less, Nb: 0.
At least one selected from the group consisting of 1% or less and V: 0.1% or less. (Third group) Cu: 1.0% or less and Ni:
At least one selected from the group consisting of 1.0% or less
seed. 6. The strength-ductility balance and plating adhesion according to claim 4, wherein in the steel sheet composition, the total of Si and Mn in the steel is 3% or more by mass%. High tensile alloyed hot-dip galvanized steel sheet with excellent corrosion resistance.