JPH0762446A - Manufacture of thin steel sheet with high corrosion resistance of baking hardening type - Google Patents

Abstract

PURPOSE:To provide a thin steel sheet for automobile with high strength and excellent press formability together with excellent corrosion resistance. CONSTITUTION:A steel stock of baking hardening type having the composition consisting of, by weight, 0.01-0.15% P, and 0.18-0.65% Cu in the range where the value calculated by the formula (P(%)/31)+3X(Cu(%)/64) may be in the range of 0.01-0.03, and also in the range where Nb effective to form the texture necessary to improve the workability is contained so that the coefficient (K) in the formula related to the carbon content in view of the solid solution C necessary for the baking hardening property: Nb(%)=KX93X(C(%)-0.0005)/12 may be in the range of 0.7-1.5 is hot rolled, and coiled at the temperature of 750-500 deg.C. Then, the cold rolling is executed at the draft of >=50%, and the soaking is executed in the temperature range of >=850 deg.C up to Ac3 transformation point for one second or more, and the cooling to <=500 deg.C is executed at the cooling speed of 10-300 deg.C/s.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet for automobiles suitable for use as an outer panel of an automobile,
In particular, the present invention relates to the technical field of a method for producing a cold-rolled steel sheet for deep drawing, which has coating bake hardenability and has excellent press workability and corrosion resistance. The steel sheet of the present invention is also suitable as an original plate for a surface-treated steel sheet which has been subjected to a surface treatment such as galvanizing.

[0002]

2. Description of the Related Art In recent years, in order to reduce the weight of automobile bodies, it has been studied to increase the strength of automobile panel materials and use the increased strength to reduce the thickness of the steel sheet used. When the strength of a steel sheet for automobiles is increased as described above, the press formability of the steel sheet generally tends to deteriorate, and conventionally, development of a high-tensile steel sheet having excellent press formability has been desired.

A technique developed as a method for satisfying both the requirements for press formability and high strength is a paint bake hardening type steel sheet for automobiles. This is a technique for performing age hardening at the time of coating baking of a panel after press forming due to the effects of both dislocations introduced into the steel sheet during press forming and a very small amount of solute carbon in the steel.
The development of this paint bake hardening type steel sheet for automobiles has made it possible to improve the press formability of high strength steel sheets. However, if the thickness of the steel sheet is reduced by this technology,
A new problem arose from the viewpoint of corrosion resistance.

That is, there is a problem that the durability is relatively deteriorated due to the reduction in thickness when corrosion occurs due to the reduction in the thickness of the steel sheet used, and the guaranteed service life up to perforation is also a problem. In view of the fact that the demand for corrosion resistance of the steel sheet itself is becoming strict due to the demand for increased corrosion resistance, satisfactory corrosion resistance has not yet been obtained.

In order to improve the corrosion resistance, a thick coating surface treatment such as a galvanizing treatment is performed. However, when this surface treatment is performed, it is difficult to secure press formability. Occurs.

[0006] In short, in order to reduce the weight of an automobile body, (1) the strength is increased by imparting the baking paint curing property, (2) the corrosion resistance is improved by improving the corrosion resistance (for example, puncture resistance). ) (3) It is necessary to develop a steel sheet for automobiles that achieves the above two items and satisfies the three items of ensuring excellent press workability. On the other hand, in the prior art, it was difficult to balance these three factors, so there is a limit to the reduction of the steel plate thickness of the automobile panel, and the plate thickness is secured from the viewpoint of corrosion resistance reduction. There were parts that needed it.

[0007]

Therefore, in order to advantageously solve the above-mentioned problems, the present invention has both high strength and corrosion resistance as a thin steel sheet for automobiles, and also has excellent press workability, which in turn results in automobile bodywork. It is an object of the present invention to propose a method capable of advantageously manufacturing a thin steel sheet that can contribute to weight reduction of the steel sheet.

[0008]

[Means for Solving the Problems] The inventors of the present invention have conducted earnest studies for the purpose of establishing a technique that satisfies all of the above-mentioned problems, and as a result, have found that both the bake hardenability (BH property) and the corrosion resistance of the steel sheet themselves are improved. By developing a component design as a means to improve, and optimizing the manufacturing conditions in that component system,
We have newly established a technology that achieves excellent press formability.

That is, according to the present invention, C: more than 0.0005 wt% and 0.0050 wt% or less, Si: 1.0 wt% or less, Mn: 1.5 wt%
Including: P: 0.01-0.15wt% and Cu: 0.18-0.65wt
% In the range where the value calculated by (P (wt%) / 31) + 3 × (Cu (wt%) / 64) satisfies 0.01 to 0.03,
In addition, Nb is contained in a relational expression Nb (wt%) = K × 93 × (C (wt%)-0.0005) / 12 in which the coefficient K is 0.7 to 1.5, and Al is further contained. : 0.005-0.10 wt%, N: 0.0050 wt% or less, and S: 0.015 wt% or less, and the balance Fe and unavoidable impurities. After winding at a temperature of ℃, cold rolling with a reduction of 50% or more, soaking for 1 second or more in the temperature range of 850 ℃ or more and Ac ₃ transformation point or less, and then cooling to 500 ℃ or less A method for producing a bake-hardening type high corrosion-resistant thin steel sheet characterized by cooling at a rate of 10 to 300 ° C / s.

As a means for improving formability, in addition to the above basic components, Ti, in relation to N and S, is (48/14) × N (wt%) + (48/32) × S ( It is also effective to contain in the range of 1 time or more and 2 times or less of the value calculated by wt%).

Further, in order to adjust the strength level of the steel sheet, in addition to the above component system, B: 0.0001 to 0.003 wt%, N
i: 0.01 to 1.5 wt%, Cr: 0.05 to 1.5 wt% and Mo: 0.001
It is possible to contain one or more selected from 0.5 wt%.

[0012]

In the present invention, the composition range of the components is a new component system newly developed to secure the corrosion resistance of the steel sheet itself and to impart BH property and press formability. By subjecting the steel material to be subjected to the process of the manufacturing condition defined in the present invention, it becomes possible to have more excellent properties. The following will explain the reasons for each limitation of the present invention.

The present invention relates to the components of the steel sheet, and in particular,
The relationship between P and Cu and the quantitative relationship between C and Nb are important, and by controlling these relationships within a proper range, excellent balance between corrosion resistance and BH property can be achieved. Then, the appropriate range of the addition amount of each component is determined by the reason described below.

C: more than 0.0005 wt% and 0.0050 wt% or less In order to improve the total elongation (El.) And the Rankford value (r value), which are the most important as a steel sheet for working, the smaller the C content, the more advantageous. Yes, 0.005 wt% or less. However, in order to secure the BH property, a certain amount of solid solution C or more is necessary,
The lower limit of the amount added must exceed 0.0005 wt%.

Si: 1.0 wt% or less Si is an effective component added for increasing the strength of the deep drawing steel sheet. However, excessive addition exceeding 1.0 wt% causes deterioration of weldability and is not preferable, so the upper limit was made 1.0 wt%. From the viewpoint of workability, the addition amount of 0.1 wt% or less is preferable, and even when high tensile strength is desired, 0.5 wt% is required if surface treatment is also required to be considered.
% Or less is desirable.

Mn: 1.5 wt% or less Mn is contained just like Si for adjusting the strength level of the steel sheet. However, from the viewpoint of weldability, the upper limit of the addition amount is Mn.
1.5 wt% is appropriate.

P: 0.01 to 0.15 wt% In order to improve the corrosion resistance of a thin steel sheet and to secure its strength, P is
Add 0.01 wt% or more. This effect becomes more remarkable by adding Cu together, and the relationship between P and the amount of Cu added is expressed by the following formula (P (wt%) / 31) + 3 × (Cu (wt%) / 64). It is necessary to satisfy 0.01 to 0.03 (wt%). If the value of this formula is less than 0.01, it is difficult to secure the bare corrosion resistance of the steel sheet, while if it exceeds 0.03, there is a problem that workability deteriorates. The upper limit of the amount of P added satisfies the above formula in order to satisfy the secondary working brittleness resistance of the steel sheet, and
It should be 0.15 wt% or less.

Cu: 0.18 to 0.65 wt% By the combined addition of P to the steel sheet, the corrosion resistance of the thin steel sheet is remarkably improved. The lower limit of the added amount for that is 0.18 wt%, and the relation with the added amount of P is expressed by the above formula (P (wt%) / 31) + 3 × (Cu (wt%) / 64) = 0.01-0.03
It is necessary to satisfy (wt%). However, addition of more than 0.65 wt% is not appropriate because it adversely affects the press formability of the steel sheet.

Nb: Nb is a component that plays an important role in the present invention. That is, it contributes to fixing C in the hot rolling stage and forming a texture structure advantageous for deep drawability during subsequent recrystallization continuous annealing. For this purpose, it is necessary to control the addition amount in relation to the C amount.
The value must be in the range 0.7-1.5. Nb addition amount = {K × 93 × (C (wt%) − 0.0005) / 12} This range of K value makes it easy to secure solid solution C required for BH property during annealing. Is a parameter for controlling the ratio of fixing C in the stage before annealing, and NbC is added when the amount of Nb added is such that the K value is less than 0.7.
However, the amount of Nb added exceeding 1.5 is
It is excessive with respect to the amount of C, and not only adversely affects the elongation of the steel sheet, low YR and other materials, but is also not preferable from the viewpoint of securing the amount of BH and cost.

Al: 0.005-0.10 wt% Al is required to be at least 0.005 wt% in order to fix O in molten steel and improve the yield of additive elements such as Ti and Nb.
On the other hand, addition of more than 0.10 wt% not only provides no further effect but is also disadvantageous in terms of cost, so 0.01 wt%
Was set as the upper limit.

N: 0.0050 wt% or less, S: 0.015 wt% or less Secure the material of the thin steel sheet (non-aging at room temperature, ductility), predetermined BH
In order to secure the amount and the corrosion resistance, it is preferable that the amounts of N and S added be reduced. However, in consideration of steelmaking technology and cost, the amount of N is 0.0050wt%.
And 0.015 wt% of S as the upper limit of the allowable addition amount. Desirably, if the N content is 0.0035 wt% or less and the S content is 0.006 wt% or less, the effect of the present invention can be more effectively exhibited.

Further, when it is difficult to reduce the amount of N and S added, or when it is necessary to make the effect more harmless, it is effective to add Ti. As a matter of course, the addition amount needs to be an appropriate amount according to the N and S amounts, and the range thereof needs to satisfy the following formula. Ti addition amount: range of 1 to 2 times the value calculated by {(48/14) × N + (48/32) × S} The lower limit of the addition amount determined by the above formula is It is the minimum amount required for precipitation and fixation. The upper limit is 2
Up to twice. The reason for this is that addition in excess of this value leads to deterioration of the surface properties of the steel sheet, which is disadvantageous not only in terms of cost but also because it is difficult to secure BH property due to precipitation of TiC, which is not preferable.

In addition, depending on the desired strength level of the steel sheet, B: 0.0001 to 0.0030 wt%, Ni: 0.01 to 1.5 wt%, C
One or more selected from r: 0.05 to 1.5 wt% and Mo: 0.001 to 0.5 wt% can be contained.

B is a component for improving the grain boundary strength of the thin steel sheet to increase the strength, and in particular, the strength level of the steel sheet is 35
It is preferable to add B when a high tensile strength of kgf / mm ² or more is intended or when the plate thickness is 1 mm or more. In order to exert its effect, it is necessary to add 0.0001 wt% or more,
Addition of more than 0.0030 wt% is unsuitable because it deteriorates the material such as r-value and elongation of the thin steel sheet.

Ni, Cr, and Mo are components used for achieving high tensile strength of the steel sheet while minimizing deterioration of the material (r value, elongation, etc.), and these components are solid. The steel plate is made to have a high tensile strength by solution strengthening or precipitation strengthening. In order to obtain such an effect, the lower limit of each addition amount is Ni:
0.01 wt%, Cr: 0.05 wt%, Mo: 0.001 wt% must be set, while the upper limit (Ni: 1.5 wt%, Cr: 1.5 wt%, Mo:
Addition of more than 0.5 wt% is not preferable because not only the effect of increasing the tensile strength is saturated and no further effect is observed, but also the adverse effect on the material becomes apparent.

The same effects can be obtained by adding the above components individually or by adding a plurality of components in combination.

The steel sheet of the present invention can be obtained by using a steel material having a composition range limited by the above reasons and producing a thin steel sheet under the following production conditions. First, regarding the hot rolling conditions, the slab heating temperature before hot rolling is not particularly limited, but when fixing S and N with Ti, it is preferably 1280 ° C or lower, more preferably 1150 ° C or lower. Note that the same effect can be expected by so-called slab direct rolling, or by casting as a sheet bar having a thickness of about 30 mm and hot rolling as it is. The finishing temperature of hot rolling is preferably the Ar ₃ transformation point or higher, which is usually performed, but in the α range
Material deterioration is small even if the temperature is lowered to around 700 ° C.

The coiling temperature is in the range of 750 to 500 ° C.
If the temperature is lower than 500 ° C, it is difficult to obtain a good material. 53
The material is improved by high-temperature coiling at 600 ℃ or above, especially above 0 ℃. However, if the coiling temperature exceeds 750 ° C, not only the material improvement effect is saturated, but also the descaling property deteriorates significantly, so the upper limit is set to 750 ° C.

Regarding cold rolling conditions, the cold rolling rate after descaling needs to be 50% or more, and more preferably 70 to 90%, in order to improve drawability.

In the case of annealing subsequent to cold rolling, continuous annealing is preferable. The continuous annealing conditions include improvement of material properties such as drawability and ductility, and partial re-solution of NbC in steel.
In order to impart BH property, it is necessary to soak at a higher temperature than in the past, and the temperature should be 850 ° C or higher. On the other hand, when the annealing temperature is higher than the Ac ₃ transformation point, the material, particularly the drawability, is significantly deteriorated, so the annealing temperature is set to 850 ° C. or higher and the Ac ₃ transformation point or lower. If the soaking time is 1 second or longer, re-dissolution of the required amount of NbC is completed.

In the present invention, cooling after soaking and holding in continuous annealing is performed at a cooling rate of 10 to 300 ° C./s until at least 500 ° C. is reached. When the cooling rate in this temperature range is slower than 10 ° C / s, solid solution C is re-precipitated and BH
There is a great deal of happiness that damages sex. On the other hand, if it exceeds 300 ° C / s, the residual solid solution C becomes excessive and causes strain aging deterioration. Therefore, the cooling rate up to at least 500 ° C is limited to 10 to 300 ° C / s. Also, cooling at a cooling rate of 10 to 300 ° C / s is 50
If finished at a temperature higher than 0 ° C,
If the cooling rate between 500 ° C is slow, or if it is maintained, there is a problem that reprecipitation of NbC may occur.

The above description is of a method for producing a cold rolled steel sheet and a surface-treated original sheet by continuous annealing. However, even in the continuous hot-dip galvanizing line (CGL) step, cooling to the same heating temperature range and hot-dip coating temperature is carried out. By controlling the speed within the limited range of the present invention (5 to 300 ° C./s), it is possible to manufacture a hot-dip galvanized steel sheet that also achieves the effects of the present invention.

[0033]

EXAMPLES Steels (A) to (J) having the compositions shown in Table 1 were tapped to form slabs.

[0034]

[Table 1]

These slabs were then heated to 1150 ° C., then hot rolled to a 3.2 mm thickness at 900 ° C. and then wound at various temperatures shown in Table 2. Reduction rate after pickling
Cold rolling was performed at 75% to obtain a cold rolled sheet having a thickness of 0.8 mm. Then, the temperature is increased to a soaking temperature shown in Table 2 at a heating rate of 4 ° C./s, continuous annealing is performed at the soaking temperature for 20 seconds, and then 450
Cooling at various cooling rates also shown in Table 2 up to ° C,
Subsequently, it was cooled to room temperature at a cooling rate of 10 ° C / s. A sample is taken from the steel sheet obtained by subjecting the steel sheet after continuous annealing to temper rolling at a reduction rate of 0.5%, and tensile properties (YS, TS, El.
), Average rank Ford value, aging index (AI value) and B
The amount of H was investigated. The results are shown in Table 2.

[0036]

[Table 2]

In the same table, the CCT test (salt spray test; 35 ° C.-4 h, then drying treatment; 60 ° C.-2 h, then wet test; 50 ° C.-relative humidity 95% -2 h, one cycle) The results of investigations on the maximum pitting depth and the corrosion weight loss after 12 cycles are also shown. From the table, it can be seen that the conformity examples according to the present invention satisfy the three requirements of excellent press formability (tensile properties: high r value, high elongation) bake hardenability (BH property) and excellent bare corrosion resistance (CCT test results). I understand.

[0038]

EFFECTS OF THE INVENTION The next generation of the present invention, which is required for a thin steel sheet for press work such as an outer panel of a lightweight automobile body, satisfies the BH property and has a super deep drawing formability and an excellent corrosion resistance. Can manufacture thin steel sheets for automobiles, and the effect is extremely large.

Front page continuation (72) Inventor Masahiko Morita 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Kawasaki Steel Co., Ltd. Technical Research Division (72) Inventor Saiji Matsuoka 1 Kawasaki-cho, Chuo-ku, Chiba Prefecture Made in Kawasaki Iron Technology Co., Ltd. Technical Research Headquarters (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Co., Ltd. Technical Research Headquarters

Claims (3)

[Claims] 1. C: more than 0.0005 wt% and 0.0050 wt% or less, Si: 1.0 wt% or less, Mn: 1.5 wt% or less, P: 0.01 to 0.15 wt% and Cu: 0.18 to 0.65 wt%, (P (wt%) / 31) + 3 × (Cu (wt%) / 64) is contained in the range of 0.01 to 0.03.
In addition, Nb is contained in the relational expression Nb (wt%) = K × 93 × (C (wt%) − 0.0005) / 12 in a range in which the coefficient K is 0.7 to 1.5, and Al : 0.005-0.10 wt%, N: 0.0050 wt% or less, and S: 0.015 wt% or less, and the balance is Fe and unavoidable impurities. Winding at a temperature of ℃, then cold rolling with a reduction of 50% or more, and then 850 ° C or more,
After soaking for 1 second or more in the temperature range below the Ac ₃ transformation point, 500
A method for producing a bake hardening type high corrosion resistant thin steel sheet, which comprises cooling at a cooling rate of 10 to 300 ° C / s up to ℃. 2. In the steel material, in addition to the basic components, Ti is calculated as (48/14) × N (wt%) + (48/32) × S (wt%) in relation to N and S. 2. The method for producing a bake hardening type high corrosion-resistant thin steel sheet according to claim 1, wherein the content is in the range of 1 time or more and 2 times or less. 3. A steel material, in addition to the basic components, is selected from B: 0.0001 to 0.003 wt%, Ni: 0.01 to 1.5 wt%, Cr: 0.05 to 1.5 wt% and Mo: 0.001 to 0.5 wt% 1. The method for producing a bake hardening type high corrosion resistant thin steel sheet according to claim 1 or 2, containing at least one kind.