JP2812096B2 - High tensile strength steel sheet with tensile strength of 50 kgf / mm2 or more, excellent in corrosion resistance and secondary formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength thin steel sheet having excellent corrosion resistance and formability and a tensile strength of 50 kgf / mm ² or more.

[0002]

2. Description of the Related Art Recently, demands for high-strength steel sheets have been increasing at the same time as the thickness of constituent members for the purpose of reducing the weight of a vehicle body has been reduced due to social demands for reducing fuel consumption of automobiles.
However, on the other hand, it is required to achieve strict rust prevention targets such as 5 years for rust resistance and 10 years for perforated rust for rust prevention performance of automobiles. Therefore, the conversion from the conventional hot / cold rolled steel sheet to the rust-preventive steel sheet has been rapidly progressing, but the thinning of the constituent members has become a more severe condition, especially with respect to the perforated rust resistance.

[0003] In order to maintain the rust-preventive performance of a steel sheet, it is effective to increase the plating basis weight of the rust-proof steel sheet, but on the other hand, problems such as deterioration of weldability and deterioration of surface lubrication performance become apparent. . Then, in order to cope with such a problem, the situation which gives corrosion resistance to a base steel plate itself is increasing. In particular, there is a strong demand for future automotive components to satisfy the three requirements of strength, formability, and corrosion resistance.

[0004] Conventionally, as a method of imparting corrosion resistance to a steel sheet, P, Cu, Ni, or the like is added to the steel.
There is known a method in which a stable rust layer in which these elements are concentrated is formed on the surface of a steel sheet at an initial stage of a corrosion reaction, and the progress of subsequent corrosion is suppressed. Techniques that apply this method to thin steel sheets for automobiles include Japanese Patent Publication No. 57-14.
No. 748, JP-A-63-186850 and JP-A-4-141554 are disclosed. When the formability is emphasized, a so-called IF steel (Interst steel) is prepared by adding Nb, Ti or the like to the steel based on an ultra-low carbon steel.
It is possible to impart excellent deep drawability and corrosion resistance by continuous annealing by adding P, Cu, Ni, or the like to an italian free steel. Japanese Unexamined Patent Publication No. 4-141554 has achieved a high r value based on such a concept, but the strength level is limited to about 48 kgf / mm ² .

[0005] When strengthening the ultra-low carbon steel as a base, a method of solid solution strengthening with P, Si, Mn or the like is generally used. When a large amount of Si is added, the deterioration of the chemical conversion treatment is caused by Mn.
When a large amount is added, not only the problem of deterioration of corrosion resistance and the like occurs, but also to obtain a tensile strength of 50 kgf / mm ² or more, the formability must be significantly sacrificed.

As a method for increasing the tensile strength of the IF steel sheet to 50 kgf / mm ² or more without adding a large amount of solid solution strengthening element, Japanese Patent Publication Nos. 2-15609, 2-145726, and 2 The above technique is disclosed in, for example, Japanese Patent Application Laid-Open No. 170921/1990.

[0007] These techniques are based on the use of precipitation strengthening by ε-Cu, and a significant increase in strength can be expected by performing aging treatment at a temperature of about 500 ° C after annealing. However, these techniques only disclose proper components and manufacturing conditions only from the viewpoint of strength and deep drawing.

[0008] Accordingly, although the steel is Cu-added, Japanese Patent Publication No. 2-190443 and Japanese Patent Publication No. 3-111519.
As disclosed in Japanese Unexamined Patent Publication (Kokai) No. H10 (1995), a technique for improving the corrosion resistance of the steel sheet by galvanizing is disclosed, but from the viewpoint of improving the bare corrosion resistance of the steel sheet itself, P, Ni,
It is not a technique to disclose an appropriate component in consideration of the influence of elements such as Mn, Si and S. Recently, ε-Cu
There is also disclosed a technique intended to combine precipitation hardening and corrosion resistance, but Cr is also an essential element, which is not only unfavorable against seawater corrosion and the like, but also ε-Cu.
No appropriate conditions regarding the amount of precipitation of the compound were considered.

[0009]

SUMMARY OF THE INVENTION The present invention is expected to be applied to, for example, the above-mentioned materials for undercarriage of automobiles and other members, and is used for high-depth drawing excellent in corrosion resistance having a tensile strength exceeding 50 kgf / mm ² . For steel sheet.

[0010]

According to the present invention, there is provided a fuel cell system having a carbon content of 0,1.
It relates to a highly formable steel sheet based on ultra-low carbon steel of 04 wt% or less and having excellent corrosion resistance with a tensile strength exceeding 50 kgf / mm ² .
It aims at extremely excellent properties in terms of moldability. Accordingly, the present invention provides an ε-Cu
The basic technical idea is to add an appropriate amount of Cu from the viewpoint of strengthening precipitation and improving corrosion resistance. Further, P was added from the viewpoint of imparting corrosion resistance to the IF steel, and the component range was set to be appropriate for the problem of secondary working embrittlement caused by further increasing the tensile strength. The configuration of the present invention is as follows.

(1) A high tensile strength steel sheet having the following properties (the composition of the component is wt% or wtppm) and excellent in corrosion resistance and secondary formability and having a tensile strength of 50 kgf / mm ² or more. (A) C: 0.002 to 0.04%, Mn: 0.05 to 1%, S
i: 0.3% or less, P: 0.05 to 0.12%, S: 0.01% or less,
sol.Al: 0.02-0.06% or less, N: 0.004% or less,
Nb: 0.002 to 0.02%, Cu: 0.6 to 1.8%, Ni: 0.2 to
1%, the balance being Fe and unavoidable impurities, and (b) between the component compositions, ｛1.2 × P / 31− (C−12 × Nb
/ 93)｝ is 3 to 15 ppm of B when the value is positive, and 2 ppm or less (0p) when the value is zero or negative.
pm), and (c) a part of Cu in the steel is ε.
-Cu is precipitated in the following range. −140 × log [Cu] + 50 ≦ Rp ≦ 100 −10 × [Cu] ^-1 (Rp is the precipitation% with respect to the total amount of Cu as ε-Cu)

(2) The following properties (component composition is wt% or
Is wtppm) and has excellent corrosion resistance and secondary formability
Tensile strength 50kgf / mm²High tensile thin steel sheet above. (A) C: 0.002 to 0.04%, Mn: 0.05 to 1%, Si: 0.3% or less, (a) C: 0.002 to 0.04%, Mn: 0.05 -1%, P: 0.05-0.12%, S: 0.01% or less, sol. Al: 0.02 to 0.06%, N: 0.004% or less, Nb: 0.002 to 0.02%, Ti: 0.005 to 0.05%, Cu: 0.6 to 1.8 %, Ni: 0.2 to 1%, the balance being iron and unavoidable impurities. (B) Regarding the above-mentioned component composition, × Ti^*/48)｝ (However, Ti^*= Ti-48 x S / 32-48 x N / 14) If the value is positive, B is contained in 3 to 15 ppm, and if the value is zero or negative, B is 2 ppm or less (including Oppm). (C) Part of Cu in the steel is precipitated as ε-Cu in the following range. −140 × log [Cu] + 50 ≦ Rp ≦ 100-10 [Cu]^-1  (Rp is the precipitation% based on the total Cu amount as ε-Cu.)

(3) A high tensile strength steel sheet having the following properties (the composition of the component is wt% or wtppm) and excellent in corrosion resistance and secondary formability and having a tensile strength of 50 kgf / mm ² or more. (A) C: 0.002 to 0.006%, Mn: 0.05 to 0.5%,
Si: 0.3% or less, P: 0.05 to 0.12%, S: 0.01%,
sol.Al: 0.02 to 0.06% N: 0.004% or less,
Nb: 0.002-0.02%, Cu: 0.6-1.8%, Ni: 0.2-1%
B: 3 to 15 ppm, the balance consisting of iron and unavoidable impurities, and (b) 0.8 ≦ (12 × Nb
/93)/C≦1.4, and (c) a part of the Cu is precipitated as ε-Cu within the range of the following formula. −140 × log [Cu] + 50 ≦ Rp ≦ 100 −10 [Cu] ⁻¹ (Rp is the precipitation% relative to the total Cu as ε-Cu.)

(4) A high tensile strength steel sheet having the following properties (the composition of the component is wt% or wtppm) and excellent in corrosion resistance and secondary formability and having a tensile strength of 50 kgf / mm ² or more. (A) C: 0.002 to 0.006%, Mn: 0.05 to 0.5%,
Si: 0.3% or less, P: 0.05 to 0.12%, S ≦ 0.01%,
sol.Al: 0.02 to 0.06% N: 0.004% or less,
Nb: 0.002-0.02%, Ti: 0.005-0.05% Cu: 0.6-1.8
%, Ni: 0.2 to 1%, B: 3 to 15 ppm, balance consisting of Fe and unavoidable impurities, (b) 0.8 ≦ (12 × Nb / 93 + 12 × Ti ^* / 48) /C≦1.4 (where Ti ^* = Ti−48 × S / 32−48 × n /
14), (c) Part of the Cu in the steel is precipitated as ε-Cu in the following range. −140 × log [Cu] + 50 ≦ Rp ≦ 100 −10 [Cu] ^-1 (Rp is the precipitation% based on the total Cu amount as ε-Cu)

(5) 50 kg of the corrosion resistance and workability described in the above (1) to (4), which contains 0.03 to 0.4% of Mo.
High tensile strength thin steel sheet having a tensile strength of f / mm ² or more.

[0016]

In the following description, the component composition is expressed as wt% or wt.
ppm. The most important components in the present invention are:
Mn: 0.05 to 1.0%, Si: 0.3% or less, S:
Based on steel in a range regulated to 0.01% or less, P: 0.05 to 0.12%, Cu: 0.6 to 1.8.
%, Ni: added in the range of 0.2 to 1%, and a part of Cu is finely precipitated as ε-Cu.

This will be described below. First, a hot rolled steel sheet (1.4 mm to 2.0 mm thick) and a cold rolled steel sheet (0.6 mm to 1.4 mm thick) in which a part of Cu in steel is finely precipitated as ε-Cu, which are basic technologies of the present invention. ), The relationship between the tensile strength, the chemical conversion property of the original sheet (the density of the film crystal was evaluated in five levels) and the maximum pitting corrosion depth was compared with a steel sheet reinforced with Si and Mn based on a similar steel. The results are shown in FIG. Here, the maximum pit depth is 1
% Salt water for 4 hours, followed by drying at 50 ° C. and 20% humidity for 2 hours, followed by 50 ° C. and 95% humidity at 18%.
The evaluation was performed when 40 cycles had elapsed under the time condition. In the case of strengthening with Mn, the chemical conversion property deterioration is relatively small with respect to the increase in tensile strength, but the increase in the maximum corrosion depth is remarkable. On the other hand, when strengthening with Si, the influence on the maximum corrosion depth is relatively small, but the chemical conversion treatment property is extremely poor. On the other hand, in the steel sheet strengthened by the fine precipitation of ε-Cu, both the maximum corrosion depth and the chemical conversion treatment property are maintained at good levels, and the effect of the present invention is proved.

Next, the reasons for limiting the components in the steel of the present invention will be described below. C: 0.002 to 0.04%. C is desirably small from the viewpoint of the formability of the hot-cold rolled steel sheet. However, from the viewpoint of corrosion resistance, the content up to 0.04% poses no practical problem. In particular, at 0.04% or more,
Not only is the deterioration in ductility remarkable, but the formation of a dense stable rust layer is hindered by the presence of the second phase carbide (pearlite), so the upper limit is 0.04%. On the other hand, in the case of cold-rolled steel sheet, in consideration of improvement in deep drawability not strength performance only, preferably it is less than 0.006%, r _m value in this case 1.8 or more is obtained. The lower limit is set to 0.002% as an amount at which no further reduction effect is recognized from the viewpoint of stable production in terms of steelmaking technology and formability.

Si: 0.3% or less. Since Si is not used as a strengthening element like C, a smaller amount is desirable. In particular, as shown in FIG. 1, the upper limit is restricted to 0.3% in order to degrade the chemical conversion property of the steel sheet.

Mn: 0.05-1%. Mn deteriorates the corrosion resistance of the steel sheet as shown in FIG.
FIG. 2 shows the effect of Mn alone on the maximum corrosion depth. When more than 1% is added, the corrosion resistance deteriorates. Therefore, the upper limit is restricted to 1% from the viewpoint of corrosion resistance. In the case of extremely low carbon steel, the formability is further improved by setting the upper limit to 0.5%. Further, the lower limit is set to 0.05% as an amount sufficient to precipitate S in steel as MnS from the problem of hot embrittlement.

P: 0.05 to 0.12%. P is an extremely harmful element for secondary working embrittlement in high-strength steel sheets based on IF steel. However, the present invention is positively added from the viewpoint of improving corrosion resistance. FIG.
Although the effect of P alone on corrosion weight loss was examined, an effect was observed when 0.05% or more of P was added. However, when strengthening the grain boundary by solid solution C and B as a countermeasure for secondary working embrittlement described later, a large amount of solid solution C and B
Must be present, and the deep drawability is significantly deteriorated. Therefore, the upper limit is set to 0.12%.

Cu: 0.6-1.8%. Cu is one of the most important elements in the present invention,
An appropriate amount is added from the viewpoint of strengthening precipitation by ε-Cu and improving corrosion resistance. In particular, when achieving both precipitation strengthening and corrosion resistance, the total addition amount and the precipitation rate as ε-Cu are important factors. FIG. 4 shows that in steels in which the amounts of Mn, Si and P are the lower limits of the component composition of the present invention, 50 kgf / mm.
^The range in which a tensile strength of ² or more and a corrosion weight loss reduction effect of 20% or more and 30% or more can be obtained is determined by the amount of Cu added and ε-C
This is arranged in relation to the amount of u precipitation (the hardness before aging is Hv _O , and the highest attained hardness after aging is Hv _{F, which} is a percentage of Hv / (Hv _F −Hv _O )). As is clear from FIG. 4, the strength and the 20% were within the range of the following equation (1).
The above-described corrosion weight loss reduction effect is obtained, and in the equation (2), both strength and a corrosion weight loss reduction effect of 30% or more are compatible. On the other hand, the upper limit of Cu is set to 1.8% or less because the frequency of occurrence of surface flaws called Cu flaws increases. −140 × log [Cu] + 50 ≦ Rp ≦ 100 −10 [Cu] ⁻¹ (1) −140 × log [Cu] + 50 ≦ Rp ≦ 100 −30 [Cu] ⁻¹ (2) (Rp is ε −Cu is the precipitation% based on the total amount of Cu.)

Ni: 0.2 to 1%. Ni not only improves corrosion resistance by being added together with P and Cu, but also effectively acts to reduce Cu flaws. In the present invention, the lower limit is set to 0.2% as an amount at which an effect on corrosion resistance is recognized. The upper limit is set to 1% or less from the viewpoint of saturating the effect on corrosion resistance and alloy cost.

S: 0.01% or less. S is preferably as low as possible from the viewpoint of corrosion resistance, and further from the viewpoint of reducing the required amount of Mn as a measure against hot embrittlement,
The upper limit is set to 0.01%. sol. Al: 0.02
To 0.06%. sol. Al may be in the general range of Al-killed steel, and from the viewpoint of stable operation, the above range is set.

N: 0.004% or less. N is desirably low for the material of the IF steel. In particular, the range is set to the above range in order to reduce Ti ^* by forming a nitride with Ti. Here, Ti ^* = Ti−48 × S / 32−48 × N / 14
It is.

B: 2 ppm or less (including 0 ppm), or 3 to 15 ppm.

When C in steel is precipitated and fixed with Nb or Ti, a small amount of B needs to be added from the viewpoint of preventing embrittlement in secondary working. However, as shown in FIG.
Relational expression between the amount and the amount of residual solid solution C, Δ1.2 × P / 31−
When (C-12 × Nb / 93-12Ti ^* / 48)｝ is zero or negative, it is expected that grain boundary strengthening by the amount of solid solution C can be expected. Therefore, the content is set to 2 ppm or less (including 0 ppm). on the other hand,
When the above expression is positive, the above range is set. The lower limit is 3 ppm at which the effect of B substantially occurs, and the upper limit is
It is 15 ppm or less as a critical amount at which deterioration of deep drawability becomes remarkable in a cold-rolled steel sheet. Note that Ti ^* is as described above. Here, the above expression is an explanation of the condition described in claim 2, but when Ti is not included, the above expression matches the condition described in claim 1, and also explains the condition described in claim 1. Is what you do.

Nb, Ti: Nb is 0.002 to 0.02
%, Ti is 0.005 to 0.05%, and 0.8 ≦
(12Nb / 93 + 12Ti ^* / 48 ⁾ /C≦1.4 or less. This condition is a condition for precipitating and fixing C in the steel as carbide. Below the lower limit, the microstructure is refined in the case of a hot-rolled steel sheet, and excellent deep drawability and non-aging property are obtained in the case of a cold-rolled steel sheet. I can't get it. On the other hand, above the upper limit, deep drawability deteriorates in the case of Nb, and pitting corrosion resistance deteriorates in the case of Ti. Here, the case where Ti is not included is the invention of claim 3, and the case where both elements of Ti and Nb are included is the invention of claim 4.

Mo: 0.03 to 0.4%. The addition of Mo in the above range to the inventions of claims 1 to 4 further improves the pitting corrosion resistance, and thus constitutes the invention of claim 5.
That is, by satisfying the above component conditions, 50 k
not only it has a gf / mm ² or more tensile strength and excellent corrosion resistance, when the C is less than 0.006% or the very high 1.8 or more r _m values as a cold-rolled steel sheet as the intensity level Have.

Regarding the control of the precipitation rate of ε-Cu, which is an important component in the present invention, the aging treatment may be performed after continuous annealing or by optimizing the cooling conditions in box annealing. If necessary, ε-Cu may be precipitated by applying a slight pressure before the treatment, and a tensile strength of 50 kgf / mm ² and excellent corrosion resistance can be obtained by any of the methods. Further, the steel sheet is a method of adding a zinc-based electroplating process and an organic composite coating process after controlling the precipitation rate of ε-Cu, or ε after hot dip galvanizing process.
The corrosion resistance can be further improved by the method of controlling the precipitation of Cu.

[0031]

【Example】

Embodiment 1 This embodiment is an embodiment to which continuous annealing is applied. The slabs of the components shown in Table 1 were heated to 1200 ° C., and then hot-rolled into a 3.6 mm-thick steel sheet.
Each was wound at 0 ° C. 1.4 mm after pickling the steel sheet
After cold rolling to 850 ° C., continuous annealing was performed. Thereafter, the results of examining the strength, the average corrosion weight loss by the above method, and the maximum pitting depth for the steel sheets in which the precipitation rate of ε-Cu was changed by aging treatment under various conditions are shown in Table 2. Table 2 shows that the steel of the present invention has a TS of 50 kgf.
/ Mm ² or more, and the average corrosion weight loss is about 80 mg /
cm ² or less, the maximum pit depth is about 0.6 mm or less,
It showed excellent corrosion resistance.

[0032]

[Table 1]

[0033]

[Table 2]

Embodiment 2 In this embodiment, an embodiment in which box annealing is applied will be described. A part of the steel shown in Table 1 was heated to 1200 ° C., then hot-rolled to a hot-rolled steel sheet having a thickness of 3.6 mm, and then wound at 450 ° C. After cold-rolling the steel sheet to 1 mm after pickling, performing box annealing in the range of 750 to 850 ° C., and then performing cooling under various conditions to change the precipitation rate of ε-Cu. Table 3 shows the results of an investigation on the strength, strength and corrosion resistance evaluated under the same conditions as in Example-1. Even when box annealing was performed, a thin steel sheet having excellent corrosion resistance was obtained as in Example 1.

[0035]

[Table 3]

Example 3 An example of a hot-rolled steel sheet having the composition of the present invention will be described. For some of the steels shown in Table 1, slabs were 1200 ° C
After cold-rolling, a cold-rolled steel sheet having a thickness of 1.4 to 2.3 mm was rolled at 450 ° C. or lower. After pickling the hot-rolled steel sheet, 450-550 to precipitate ε-Cu.
Table 4 shows the results of examining the strength and corrosion resistance of the steel sheet subjected to the aging treatment in the range of ° C. In the hot-rolled steel sheet of the present invention, a hot-rolled steel sheet having excellent strength and corrosion resistance was obtained as in the case of the thin steel sheet.

[0037]

[Table 4]

Example 4 The results of examining the properties of the cold-rolled steel sheet of the present invention after electrogalvanizing are shown. 2. For a part of the steel shown in Table 1, the slab was heated to 1200 ° C. and then hot-rolled.
After a hot-rolled steel sheet having a thickness of 6 mm was formed, it was wound at 450 ° C.
After cold-rolling the steel sheet to 1.2 mm after pickling, 85
Continuous annealing at 0 ° C., further subjected to electrogalvanizing steel plate that has been subjected to aging treatment in the range of 450 ~ 550 ° C.,
Table 5 shows the results of examining the strength and corrosion resistance. Even when the thin steel sheet of the present invention was electrogalvanized, a thin steel sheet having more excellent corrosion resistance and a predetermined strength was obtained.

[0039]

[Table 5]

[0040]

As described above, the thin steel sheet according to the present invention has a tensile strength of 50 kgf / mm, which is excellent in corrosion resistance and workability.
It is a thin steel plate having a thickness of ² mm2 or more and, for example, when used for a member of an automobile, can greatly contribute to reducing the weight of the vehicle body and extending the life.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between tensile strength, chemical conversion treatment of an original sheet (evaluation of the densities of film crystals in five levels), and maximum pit depth after a predetermined corrosion resistance test.

FIG. 2 is a view showing the influence of Mn and S on the maximum erosion depth of a thin steel sheet.

FIG. 3 is a view showing the influence of P and Cu on average corrosion weight loss.

FIG. 4 is a range in which a steel having a composition in which the amounts of Mn, Si and P are the lower limits of the present invention can provide a tensile strength of 50 kgf / mm ² or more and a corrosion weight loss reduction effect of 20% or more and 30% or more. FIG. 3 is a diagram showing the relationship between the amount of Cu added and the amount of ε-Cu precipitated.

FIG. 5 is a diagram showing, in the relationship between the amount of P in steel and the amount of residual solid solution C, critical conditions for secondary work embrittlement and the effect of secondary work embrittlement suppression by the addition of B.

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Tomoyoshi Ohkita 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-2-197545 (JP, A) JP-A 4-141554 (JP, A) (58) Fields investigated (Int. Cl. ⁶ , DB name) C22C 38/00-38/60

Claims (5)

(57) [Claims] 1. The following characteristics (component composition is wt.% Or wt.
high tensile strength steel sheet having a tensile strength of 50 kgf / mm ² or more and excellent in corrosion resistance and secondary formability. (A) C: 0.002 to 0.04%, Mn: 0.05 to 1%, S
i: 0.3% or less, P: 0.05 to 0.12%, S: 0.01% or less, sol.Al:
0.02-0.06%, N: 0.004% or less, Nb: 0.002-0.02%, Cu: 0.
6 to 1.8%, Ni: 0.2 to 1%, the balance being Fe and unavoidable impurities. (B) Between the above component compositions, ｛1.2 × P / 31− (C
−12 × Nb / 93) When the value of｝ is positive, B is further increased to 3 to
15 ppm, and when the value is zero or negative, B is 2 ppm or less (0 ppm
(C) Part of Cu in the steel is precipitated as ε-Cu in the following range. −140 × log [Cu] + 50 ≤ Rp ≤ 100 -10 × [Cu] ^-1 (Rp is the precipitation% relative to the total amount of Cu as ε-Cu) 2. The following properties (component composition is wt.% Or wt.
ppm) and excellent in corrosion resistance and secondary formability
Tensile strength 50kgf / mm²High tensile thin steel sheet above. (A) C: 0.002 to 0.04%, Mn: 0.05 to 1%, Si: 0.3% or less, P: 0.05 to 0.12%, S: 0.01% or less, sol. Al: 0.02 to 0.06%, N: 0.004% or less, Nb: 0.002 to 0.02%, Ti: 0.005 to 0.05%, Cu: 0.6 to 1.8 %, Ni: 0.2 to 1%, the balance being iron and unavoidable impurities. (B) Regarding the above-mentioned component composition, × Ti^*/48)｝ (However, Ti^*= Ti-48 x S / 32-48 x N / 14) when the value is positive, B is contained in 3 to 15 ppm, and when the value is zero or negative, B is 2 ppm or less (including Oppm). (C) Part of Cu in the steel is precipitated as ε-Cu in the following range. −140 × log [Cu] + 50 ≦ Rp ≦ 100-10 [Cu]^-1  (Rp is the precipitation% based on the total Cu amount as ε-Cu.) 3. The following properties (component composition is wt% or pp
high tensile strength steel sheet having high tensile strength of 50 kgf / mm ² or more and excellent in corrosion resistance and secondary formability. (A) C: 0.002 to 0.006%, Mn: 0.05 to 0.5%,
Si: 0.3% or less, P: 0.05 to 0.12%, S: 0.01%, sol.A
l: 0.02 to 0.06% N: 0.004% or less, Nb: 0.002 to 0.02%, Cu: 0.6
~ 1.8%, Ni: 0.2 ~ 1%, B: 3 ~ 15ppm, balance consists of iron and unavoidable impurities. (B) 0.8 ≦ (12 × Nb / 93) / C ≦ 1.
(C) Part of the Cu is precipitated as ε-Cu within the range of the following equation. −140 × log [Cu] + 50 ≦ Rp ≦ 100 −10 [Cu] ⁻¹ (Rp is the precipitation% relative to the total Cu as ε-Cu.) 4. The following properties (component composition is wt% or wt.
high tensile strength steel sheet having a tensile strength of 50 kgf / mm ² or more and excellent in corrosion resistance and secondary formability. (A) C: 0.002 to 0.006%, Mn: 0.05 to 0.5%,
Si: 0.3% or less, P: 0.05-0.12%, S ≦ 0.01%, sol.A
l: 0.02 to 0.06% N: 0.004% or less, Nb: 0.002 to 0.02%, Ti: 0.0
05 ~ 0.05% Cu: 0.6 ~ 1.8%, Ni: 0.2 ~ 1%, B: 3 ~
15 ppm, the balance being Fe and unavoidable impurities, (b) 0.8 ≦ (12 × Nb / 93 + 12 × Ti ^*)
/48)/C≦1.4 (where Ti ^* = Ti−48 ×
S / 32-48 × N / 14), and (c) some of the Cu in the steel is ε-
Precipitated as Cu. −140 × log [Cu] + 50 ≤ Rp ≤ 100 -10 [Cu] ^-1 (Rp is ε-Cu and the precipitation% based on the total amount of Cu) 5. The method according to claim 1, wherein Mo is contained in an amount of 0.03 to 0.4%.
^2. A high-tensile thin steel sheet having a tensile strength of 50 kgf / mm ² or more, which is excellent in corrosion resistance and workability described in 1.