JP2002003995A - Hot-dip galvanized steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high tensile steel sheet excellent in processability and BH property and used for automobile panel. SOLUTION: The steel sheet has a composition containing 0.0015-0.01% C, <=0.5% Si, 0.005-0.05% P, 0.0005-0.01% S, 0.01-0.1% sol.Al, <=0.01% N and 1.8-2.5%, in total, of one or more kinds selected from the group consisting of 1.5-2.5% Mn, 0.01-0.5% Cr and 0.01-0.5% Mo. Further, the composition can contain 0.003-0.0030% B and also can contain 0.003-0.15% Ti or 0.003-0.15% Nb.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet and a hot-dip galvanized steel sheet which are excellent in bake hardenability and formability and can satisfy the demand for reducing the weight of a vehicle body by reducing the thickness of an automobile.

[0002]

2. Description of the Related Art With the improvement of decarburizing ability in steelmaking, the production of ultra-low carbon steel sheets having good workability has been facilitated, and the demand thereof has been increasing. Among them, for example, JP-A-59-31827 and JP-A-59-3833.
The ultra-low carbon steel sheet to which Ti and Nb are added in combination is disclosed in Japanese Patent Publication No. 7 and the like, has extremely good workability, combines paint bake hardening (BH) properties, and has excellent hot-dip galvanizing properties. So it is occupying an important position. However, the bake hardening amount (BH amount) at that time is about 35 MPa in consideration of aging at room temperature.

On the other hand, many attempts have been made in the past to increase the strength while ensuring the formability. In particular, in the case of ultra-low carbon steel sheets of a class having a tensile strength of 340 to 490 MPa, P, Si and the like have been added to the steel, and high strength has been achieved by utilizing a solid solution strengthening mechanism by these elements.

For example, in JP-A-59-31827 and JP-A-59-38337,
There is disclosed a method of manufacturing a high-strength cold-rolled steel sheet in which mainly Si and P are added to an ultra-low carbon steel sheet to which Ti and Nb are added, and the tensile strength is up to 440 MPa class.

Conventionally, P,
Next, Si is frequently used. This is because P and Si have a very high solid-solution strengthening ability, can be increased in strength by adding a small amount, and have a small amount of deterioration in formability such as ductility and deep drawability and a small increase in cost. However, in fact, when trying to achieve an increase in strength using these elements, the yield strength as well as the strength increases significantly at the same time, and surface shape defects such as surface distortion occur. May be restricted. When hot-dip galvanizing is performed, non-plating occurs when a large amount of Si is added.
When a large amount of P is added, problems such as a delay in alloying processability are caused, and there is a practical limit to the amount of P added.

Further, Japanese Patent Application Laid-Open No. 2-111841 discloses that a very low carbon steel with Ti added is 1.5% or more and 3.5% or less.
Disclosed are cold rolled steel sheets and hot-dip galvanized steel sheets having excellent workability and having bake hardenability with less than Mn added.
By adding a large amount of Mn, the operation stability of hot rolling and the uniformity of the metal structure due to the decrease of the Ar ₃ transformation point are to be realized. It also discloses that Cr and V are added up to 0.2 to 1.0% for the purpose of further improving ductility.

However, also in the present invention, the BH amount does not deviate from a normal level (about 35 MPa).
It has not been possible to achieve both a higher BH content and a non-aging property at room temperature.

Further, a so-called dual phase steel (DP) having a low yield strength, a non-aging at room temperature and a high BH content and a mixture of a ferrite phase and a martensite phase are mixed.
Steel) is disclosed in JP-A-50-75113 and JP-A-51-39524. However, there is a disadvantage that the average r value is as low as about 1.0 and the deep drawability is poor.

A composite structure steel sheet (DP steel) made of these low carbon aluminum killed steels is disclosed in Japanese Patent Publication No. 3-222.
Japanese Patent Publication No. 4 and Japanese Patent Publication No. 3-21611 disclose a composite structure steel sheet made of extremely low carbon steel. These alloys contain a large amount of Nb, B, and Ti in an extremely low-carbon steel to form a structure after annealing to form a composite structure of a ferrite phase and a low-temperature transformation generation phase. It is intended to obtain a cold-rolled steel sheet having aging properties.

[0010] Further, Japanese Patent Publication No. 6-65684 discloses that high drawability, non-aging property at room temperature and 100 MPa
A steel sheet exhibiting such a high BH amount is disclosed. As described above, several proposals have been made for steel sheets that have high BH properties at room temperature and are not aged, and that exhibit excellent formability.
In order to obtain more stable characteristics in a line having a low cooling rate such as a CGL line for performing galvannealing, a required cooling rate for generating a low-temperature transformation phase and Ms
Control of the chemical composition needed to control the points is necessary.

[0011]

SUMMARY OF THE INVENTION It is an object of the present invention to provide an automobile panel having an r value of 1.30 or more and a BH amount of 50 MP.
a It is an object of the present invention to provide a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having excellent formability and BH characteristics.

More specifically, in order to promote a reduction in the weight of a vehicle body due to global environmental problems, it is soft at the time of molding, exhibits high bake hardening performance in a paint baking step after press molding, and has a non-bake hardening property at room temperature. A steel plate that is aged, and further provides a high-strength steel plate that has a high r-value performance, is excellent in deep drawing formability, and is capable of stably obtaining a composite structure steel even in an alloyed hot-dip galvanizing line with a slow cooling rate. That is.

[0013]

Means for Solving the Problems In order to achieve the above object, the present inventors have made intensive studies and found the following findings.

(I) In order to obtain a low-temperature transformation phase in a CGL line having a low cooling rate, Mn, Cr, Mo which affects the cooling rate and Ms point required for the low-temperature transformation phase formation
By adding at least one of the above and defining the amount in a specific range, a low-temperature transformation phase can be obtained even under such a condition of a low cooling rate.

(Ii) Further, higher r values can be obtained by adding Ti and Nb, and annealing at a higher temperature is necessary to obtain a composite structure steel by adding them in combination. A composite structure steel cannot be obtained for practical use.

That is, the present inventors have developed a composite structure steel which can be manufactured even in CGL having a low cooling rate in order to obtain a high-strength steel sheet having a high bake hardening amount and excellent workability in order to meet the need for further reduction in body weight. As a result of diligent studies on Mn, Mo, C, which increase the hardenability so that a low-temperature transformation phase can be formed in a line with a low cooling rate as a method for obtaining a composite structure steel having a low strength and a high r value,
It has been found that a high strength steel sheet having high bake hardenability and excellent deep drawability can be manufactured by adding appropriate amounts of r and B and adding Ti or Nb alone to obtain further excellent deep drawability. Thus, the present invention has been completed.

Therefore, the present invention is as follows. (1) Chemical composition in mass%, C: 0.0015 to 0.01%, Si:
0.50% or less, P: 0.05% or less, S: 0.01% or less, sol.A
l: 0.01 to 0.1%, N: 0.010% or less, and Mn: 1.5 to
One or more selected from the group consisting of 2.5%, Cr: 0.01 to 0.5%, and Mo: 0.01 to 0.5%, the total content of which satisfies 1.8 to 2.5%, with the balance being Fe and Alloyed hot-dip galvanized steel sheet consisting of unavoidable impurities.

(2) The chemical composition is represented by mass%, and B:
The galvannealed steel sheet according to the above (1), containing 0.0003 to 0.0030%. (3) The chemical composition is, in mass%, further Ti: 0.003 to 0.15
(1) or containing Nb: 0.003 to 0.15%
(2) The galvannealed steel sheet according to (2).

(4) When the chemical composition is expressed by mass%, C: 0.0015 to
0.01%, Si: 0.50% or less, P: 0.05% or less, S: 0.01%
Sol.Al: 0.01 to 0.1%, N: 0.010% or less, and
Mn: 1.5 to 2.5%, Cr: 0.01 to 0.5%, and Mo: 0.01
A hot-dip galvanized steel sheet comprising one or more selected from the group consisting of ~ 0.5%, the total content of which satisfies 1.8-2.5%, and the balance being Fe and unavoidable impurities.

(5) The chemical composition is represented by mass%, and B:
The galvanized steel sheet according to (4), containing 0.0003 to 0.0030%. (6) The chemical composition is, by mass%, further Ti: 0.003 to 0.15
% Or Nb: 0.003 to 0.15%, (4) or (5)
A hot-dip galvanized steel sheet according to the description.

[0021]

Next, the reason why the chemical composition of the steel sheet is limited as described above in the present invention will be described. In this specification, unless otherwise specified, "%"
Is% by mass.

C: C greatly affects the volume ratio of the low-temperature transformation phase and is an important element that determines the material properties. If the C content is less than 0.0015%, the grain boundary strength decreases and the decarburization cost increases, so the lower limit is made 0.0015%.

On the other hand, if the amount of C exceeds 0.01%, the strength becomes higher than the desired strength, so that the ductility decreases. Therefore, the content of C is set to 0.0015 to 0.01%. A preferred upper limit is 0.0090
%, A preferable lower limit is 0.0020%.

Si: Si is an element that promotes ferrite transformation, increases the C concentration in untransformed austenite, and facilitates formation of a composite structure. However, when hot-dip galvanizing is performed, if it is added in a large amount in order to degrade the wettability of the plating, uneven plating may occur and the plating quality may be degraded.

Therefore, the content of Si is set to 0.50% or less. Preferably it is 0.30% or less. P: P is an effective element as a solid solution strengthening element.
The addition of more than% suppresses the diffusion of iron from the iron interface during the galvannealing alloying treatment, thereby deteriorating the alloying treatment property. This necessitates an alloying treatment at a high temperature, and may cause peeling of the plating due to occurrence of powdering, chipping and the like. In addition, they segregate at the grain boundaries and deteriorate secondary work brittleness and weldability. Therefore, the P content is set to 0.05% or less. Preferably it is 0.03% or less. 0.005%
If the value is less than the above, the cost of removing P increases. Preferably, the P content is 0.005 to 0.05%.

S: S is preferably as low as possible because S impairs the hole expandability of the steel sheet. Therefore, it was set to 0.01% or less. Preferably it is 0.005%. On the other hand, the reason for making it less than 0.0005% is that
Since the cost for removing S is high, the content is preferably 0.0005 to 0.01%.

Sol. Al: Contained as a result of molten steel deoxidation. Further, it has an effect of forming fine precipitates of AlN by combining with N in the steel and suppressing the austenite crystal grains from becoming coarse. In order to have these effects, the content is made 0.01% or more. On the other hand, the addition of more than 0.1% increases the cost.

Therefore, the content of sol.Al is 0.01-0.1%.
And Preferably, it is 0.02 to 0.07%. N: The lower the N, the better. If the N is too large, it is necessary to add a large amount of a nitride-forming element. Therefore, the upper limit of the amount of N added is set to 0.010%.

Mn: Mn is an important element that affects the cooling rate and Ms point required for obtaining a composite structure, and is added at least one of Cr and Mo described later. When Mn is less than 1.5%, a large amount of Cr, Mo, or the like must be added instead of Mn in order to generate a low-temperature transformation phase in a heat cycle in a continuous hot-dip galvanizing line, resulting in an increase in cost. In addition, Mn is 2.5%
If the superaddition is excessive, the strength is excessively increased, the formability is deteriorated, and the rolling processability is lowered.

Therefore, the content of Mn is set to 1.5 to 2.5%. Cr: Like Cr, Cr affects the cooling rate and the Ms point for the formation of the low-temperature transformation phase. The present invention relates to Mn,
The cooling rate and the Ms point are controlled by the total content of Cr and Mo. However, considering the cost, the cost of Cr is higher than that of Mn.
%. Therefore, the content of Cr is set to 0.01 to 0.5%. A preferred lower limit is 0.1% and a preferred upper limit is 0.4%.

Mo: Mo, like Mn and Cr, is an important element in the present invention. It affects the cooling rate and the Ms point for the formation of the low-temperature transformation phase, and the effect is Mn, Cr
That is all. However, the addition of a large amount of Mo causes an increase in manufacturing cost.

Therefore, it is necessary to add in the range of 0.01 to 0.5%. A preferred lower limit is 0.1%, and a preferred upper limit is 0.
4%. Total content of Mn, Cr, Mo: Mn,
When the total content of Cr and Mo is less than 1.8%, the cooling rate is low, and in the production process of continuous hot-dip galvanizing to be immersed in a zinc pot, untransformed austenite is transformed into pearlite, and a low-temperature transformed phase is formed. It is not obtained and does not have the desired composite structure steel. Addition of more than 2.5% causes an excessive increase in strength, and degrades workability and shape freezing property.

Therefore, the content of Mn, Cr and Mo is 1.8 to 2.5.
%. B: B is an element that enhances hardenability and facilitates obtaining a composite structure steel such as the steel of the present invention. If the addition is less than 0.0003%, the above performance cannot be exhibited. Also, the addition of more than 0.0030% causes excessive strength increase,
Moldability deteriorates. Therefore, the content of B is 0.0003 to 0.00
30%.

Ti, Nb: Ti, Nb has the role of securing the workability and non-aging properties of ultra-low carbon steel by fixing all or part of C, S, and N. Further, the crystal grains of the hot-rolled steel sheet are refined to improve the workability of the product. T
If i and Nb are each less than 0.003%, the effect does not appear, so these are set as the lower limits of Ti and Nb.

On the other hand, if each exceeds 0.15%, a remarkable increase in alloy cost is caused. Therefore, the upper limit is set to 0.15%.
Thus, according to the present invention, a high-strength steel sheet having a TS of 340 MPa or more can be obtained.

The steel sheet according to the present invention can be manufactured as follows. First, as usual, melt the steel of the intended chemical composition in a converter, etc., make a slab by continuous casting, and then perform hot rolling while hot or once cooled to room temperature. After reheating, hot rolling is performed to obtain a hot-rolled steel sheet.

The hot rolling may be performed under the usual conditions. From the viewpoint of the load at the time of cold rolling and pickling properties, the winding temperature after hot rolling is from 300 ° C. to 750 ° C. It is recommended that

The wound hot-rolled coil is then pickled in a conventional manner and then subjected to cold rolling. There is no particular limitation on the cold rolling conditions, but the cold rolling rate is preferably 45% or more from the viewpoint of the sheet passing property during cold rolling.

The steel sheet obtained by cold rolling is subjected to recrystallization annealing before use. This method may employ the usual conditions applied to obtain a composite structure steel.

That is, in the recrystallization annealing, it is sufficient to heat the ferrite to the austenite transformation point or higher, and then cool it. Since the chemical composition of the steel sheet according to the present invention is controlled so that a composite structure can be obtained even at a low cooling rate, it is not particularly necessary to limit the cooling rate.

Further, as for the conditions of the hot dip galvanizing treatment from the viewpoint of ensuring the corrosion resistance, the steel sheet may be penetrated into the molten zinc bath, the basis weight is controlled, and the alloying treatment may be performed or cooled as usual.

[0042]

EXAMPLES The effects of the present invention will be described more specifically with reference to examples. After smelting steels of various chemical compositions in a converter and forming them into slabs by continuous casting, they were once cooled to room temperature and then heated again to 1250 ° C.
Hot rolling was performed at 30 ° C. to obtain a hot-rolled steel sheet having a thickness of 4.0 mm. Next, the obtained hot-rolled steel sheet was pickled and then cold-rolled to obtain a cold-rolled steel sheet having a thickness of 0.75 mm. Table 1 shows the chemical composition of the obtained steel sheet. Then, the cold-rolled steel sheet was passed through a continuous hot-dip galvanizing line and subjected to recrystallization annealing and plating.

The production conditions were as follows: a cold-rolled steel sheet was heated to 860 ° C., and then cooled and penetrated into a molten zinc bath.
/ M ² , and then an alloying treatment was performed. The tensile properties and BH properties of each of the alloyed hot-dip galvanized steel sheets thus obtained were investigated.

Further, in order to investigate the aging property, a heat treatment was performed at 50 ° C. for 3 days, and the performance change under accelerated aging conditions was examined. In order to investigate hole expandability, conduct a hole expansion test,
A powdering test was performed to investigate the plating performance.

Regarding the tensile properties, JIS5
No. 1 test piece was taken and subjected to a tensile test to yield strength (Y
S), tensile strength (TS), yield point elongation (YPE), elongation (El), and r value at 15% were measured.

Regarding the BH characteristics, after a 2% tensile pre-strain was given, a heat treatment was performed at 170 ° C. for 20 minutes, a treatment corresponding to the baking coating treatment after processing was performed, and a tensile test was performed to perform a tensile test. , The amount of change in yield strength (BH amount) was measured.

The aging property is determined by YS and YP before and after acceleration aging.
The changes in E and El (ΔYS, ΔYPE, ΔEl) were investigated. In the hole expansion test, a punched hole having a diameter of 10 mm was created with a punching clearance of 12%, formed using a 60 ° conical punch, and the critical hole expansion rate until a through crack in the plate thickness direction in the plate thickness direction was evaluated. . In the powdering test, after forming a cylinder by deep drawing with a drawing ratio of 1.8 and a diameter of 50 mm, the amount of plating peeling was measured by a gravimetric method.

The results are summarized in Tables 1 and 2. Table 1,
The following can be confirmed from Table 2. That is, the steel sheets A1 to A8 of the present invention are all good tensile properties,
It shows high bake hardenability and good aging performance.

The comparative steels B1 and B2 are unfavorable because the C content is out of the range of the present invention, and the r value indicating deep drawability is less than 1.30. B3 of the comparative steel has an Mn content out of the range of the present invention, has no low-temperature transformation phase structure, shows high yield strength, and has poor aging performance.

The steels B4 to B9 of the comparative steels have excessively high strength and poor workability because the chemical composition is out of the range of the present invention. Further, B5 and B6 have poor powdering properties because Si or P is out of the range of the present invention. For B7, the hole expandability is poor because S is out of the range of the present invention.

[0051]

[Table 1]

[0052]

[Table 2]

[0053]

As described above, according to the present invention, a high-strength steel sheet having not only good workability but also high baking hardenability, excellent deep drawability and excellent corrosion resistance is provided. Industrially useful effects can be obtained, such as being able to stably provide and sufficiently responding to the demand for reducing the weight of a vehicle body such as an automobile.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yozo Hirose 4-33, Kitahama, Chuo-ku, Osaka-shi, Osaka Prefecture Within Sumitomo Metal Industries, Ltd. (72) Inventor Tsuyoshi Ohno 4-chome, Kitahama, Chuo-ku, Osaka-shi, Osaka No. 33 Sumitomo Metal Industries Co., Ltd. (72) Inventor Yukichi Tokuda 4-33 Kitahama, Chuo-ku, Osaka City, Osaka Prefecture F-term in Sumitomo Metal Industries Co., Ltd. 4K027 AA02 AA23 AB02 AB28 AB42 AC73

Claims (6)

[Claims] 1. The chemical composition is represented by mass%, C: 0.0015 to 0.1%.
01%, Si: 0.50% or less, P: 0.05% or less, S: 0.01% or less, sol. Al: 0.01 to 0.1%, N: 0.010% or less, and M
n: 1.5 to 2.5%, Cr: 0.01 to 0.5%, and Mo: 0.01
An alloyed hot-dip galvanized steel sheet comprising one or more selected from the group consisting of 〜0.5%, the total content of which satisfies 1.8-2.5%, and the balance being Fe and unavoidable impurities. 2. The method according to claim 1, wherein the chemical composition is expressed in mass%, and B: 0.
The galvannealed steel sheet according to claim 1, containing from 0003 to 0.0030%. 3. The method according to claim 1, wherein the chemical composition comprises, in mass%, Ti:
The galvannealed steel sheet according to claim 1 or 2, which contains 0.003 to 0.15% or Nb: 0.003 to 0.15%. 4. The chemical composition, in mass%, C: 0.0015 to 0.1%.
01%, Si: 0.50% or less, P: 0.05% or less, S: 0.01% or less, sol. Al: 0.01 to 0.1%, N: 0.010% or less, and M
n: 1.5 to 2.5%, Cr: 0.01 to 0.5%, and Mo: 0.01
A hot-dip galvanized steel sheet comprising one or more selected from the group consisting of ~ 0.5%, the total content of which satisfies 1.8-2.5%, and the balance being Fe and unavoidable impurities. 5. The method according to claim 1, wherein the chemical composition is represented by mass%, and B: 0.
The hot-dip galvanized steel sheet according to claim 4, which contains 0003 to 0.0030%. 6. The composition according to claim 1, wherein the chemical composition further comprises Ti:
The hot-dip galvanized steel sheet according to claim 4 or 5, containing 0.003 to 0.15% or Nb: 0.003 to 0.15%.