JP3261760B2 - High corrosion resistant cold rolled steel sheet excellent in workability and method for producing the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold rolled steel sheet for automobiles having good press formability and excellent corrosion resistance, and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, it has been considered to reduce the thickness of a vehicle from the viewpoint of reducing the weight and cost of an automobile. However, when the sheet thickness is reduced, a margin after corrosion is reduced, so that there is a problem that strength after corrosion is reduced. If the basis weight of galvanizing is increased to prevent this, there arises a problem that weldability deteriorates. Further, a steel sheet used for such an application is required to have excellent formability such as deep drawability and to be inexpensive. A steel sheet that meets such conditions is
Many proposals have been made, but satisfactory characteristics have not been obtained so far.

For example, Japanese Patent Application Laid-Open No. 3-253541 discloses a method of reducing the amount of C in a Cu-P system, adding a small amount of S,
It discloses that by adding a certain amount of i and Ti, it exhibits excellent corrosion resistance in an environment where wet and dry are repeated. Japanese Patent Application Laid-Open No. 3-150315 discloses Cu-P
A method for producing a steel sheet which is excellent in corrosion resistance and formability with respect to steel in which the amount of C is reduced and a small amount of Ni is added is disclosed. Japanese Patent Application Laid-Open No. 4-141554 discloses a cold-rolled steel sheet having high strength and excellent corrosion resistance and a method for producing the same. Japanese Patent Application Laid-Open No. 4-168246 discloses a cold-rolled steel sheet containing P, Ti, Nb and the like and having excellent formability and corrosion resistance .

[0004]

However, Japanese Patent Laid-Open Publication No. Hei 3-2
The steel sheet disclosed in Japanese Patent No. 53541 is a Ti-killed steel, so that surface defects are likely to occur, and nozzle clogging is likely to occur when a slab is manufactured by continuous casting. Further, in the method disclosed in JP-A-3-150315, in order to improve moldability,
Although it is stipulated that box annealing is used for recrystallization annealing, box annealing is disadvantageous not only in terms of cost but also in that P is easily segregated, steel becomes brittle, and workability is deteriorated. .

Further, the steel sheet disclosed in Japanese Patent Laid-Open No. 4-141554, the elongation (El) is less than 40%, Lankford value (r _m value) is insufficient 2.0 below and press formability There is a disadvantage that there is. Cu, P and C
Steel to which r is added has a disadvantage of being inferior in pitting corrosion resistance.
Furthermore, the cold-rolled steel sheet containing P, Ti, Nb and the like disclosed in Japanese Patent Application Laid-Open No. 4-168246 has a drawback that the corrosion resistance is poor because NbC is generated.

[0006]

SUMMARY OF THE INVENTION The present invention has been studied to solve the technical problems of the prior art as described above, and has made it possible to minimize C in a Cu-P system.
By combining Ti, Nb, and B, a steel having both corrosion resistance and formability was invented, and a preferable production method thereof was also invented, as follows.

(1) The following composition (composition is wt%)
It is a highly corrosion-resistant cold-rolled steel sheet having excellent workability. (A) C: 0.001 to less than 0.006%, Si: less than 0.35%,
Mn: 0.05-0.5%, P: 0.03-0.08%, S: less than 0.01%, sol.A
l: 0.01 to 0.1%, N: 0.0035% or less, Cu: 0.1 to 0.5%, Ni: 0.1
~ 0.5%, Ti: 0.01 ~ 0.06%, Nb: 0.003 ~ 0.015%, B: 0.000
2 to 0.002%, the balance being Fe and unavoidable impurities. (B) Further, the component composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S, and the relationship of 0.004 ≦ Nb × (10 × P + 2 × Cu + Ni) is satisfied.

(2) A cold-rolled steel sheet having the composition described in (1) above, wherein ten points of the surface have an average roughness Rz (μm).
Is 1 to 8 and Rz × S / (10 × P + 2 × Cu + Ni) ≦ 0.025.

(3) A highly corrosion-resistant cold-rolled steel sheet having excellent workability and comprising the following steps and a method for producing the same (composition is wt%). (A) C: 0.001 to less than 0.006%, Si: less than 0.35%,
Mn: 0.05 to 0.5%, P: 0.03 to 0.08%, S: 0.01
%, Sol. Al: 0.01 to 0.1%, N: 0.0035% or less, Cu: 0.1 to 0.5%, Ni: 0.1 to 0.5%, Ti:
0.01 to 0.06%, Nb: 0.003 to 0.015%, B: 0.0002
0.000.002%, the balance being Fe and unavoidable impurities, and the component composition was (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) ) × S, and a slab having a component composition satisfying the relationship of 0.004 ≦ Nb × (10 × P + 2 × Cu + Ni) is prepared. (B) After heating the slab at 1100 ° C. or more, the slab exceeds the Ar ₃ point. The hot-rolled sheet obtained is rolled at a temperature of 550 ° C. ≦ CT ≦ (650 + 200 × (200 × B (%) − P
(%)). (C) The cold rolled hot rolled sheet is subjected to continuous annealing at a temperature of 750 ° C. or more and 900 ° C. or less.

[0010]

The reasons for limiting the composition of the present invention (which is wt%) are as follows. C: 0.001 to less than 0.006%. The smaller the C, the better the deep drawability. Therefore, the carbon content is 0.1.
006% or less. However, even if the carbon content is less than 0.001%, the deep drawability does not change much,
Since the manufacturing cost increases, the lower limit is made 0.001%.

Si: Less than 0.35%. If the amount of Si is large, the moldability is deteriorated and the plating property is also impaired.
It shall be less than 5%.

Mn: 0.05-0.5%. Mn is an element necessary to prevent red hot embrittlement due to inevitably contained S, but if added over 0.5%, it is disadvantageous in terms of cost, so Mn is set to 0.05 to 0.5%.

P: 0.03 to 0.08%. P is an element that significantly improves the corrosion resistance by being added in combination with Cu, but a large amount of P makes the steel brittle and reduces the workability. Therefore, the range is set to 0.03% or more.
08% or less.

S: Less than 0.01%. S is desirably small so as to deteriorate the steel material and deteriorate the corrosion resistance, and is set to less than 0.01%.

Sol. Al: 0.01 to 0.1%. Al is an element necessary for deoxidation, and 0.01%
Although it is necessary as described above, if it is added in excess of 0.1%, alumina inclusions increase to deteriorate the surface properties.
10% or less.

N: 0.0035% or less. It is preferable that N is small because the deep drawability is improved.
If it is 5% or less, it does not adversely affect the deep drawability, so the content is made 0.0035% or less.

Cu: 0.1-0.5%. Cu is P
However, if the amount is not more than 0.1%, the effect is small, and if it exceeds 0.5%, hot brittleness is liable to occur, and Since the drawability is also impaired, the content is set to 0.1% or more and 0.5% or less.

Ni: 0.1-0.5%. Ni is C
It is an element effective for preventing hot brittleness due to u and for improving corrosion resistance. However, if it is less than 0.1%, such an effect cannot be expected. Even if it is contained, the effect saturates, and it is only disadvantageous in cost. Therefore, Ni is set to 0.1% or more and 0.5% or less. Ti: 0.01 to 0.06%.

Ti is an element necessary for preventing deterioration of the material due to solid solution carbon and solid solution nitrogen. For that purpose, it is necessary to add 0.01% or more. Further, if the addition amount exceeds 0.06%, further effects cannot be obtained and the cost becomes disadvantageous. Therefore, the content is set to 0.01% or more and 0.06% or less. Here, in order to completely precipitate and fix solid solution carbon and solid solution nitrogen in steel with Ti, the following conditions are required. 4 × C <Ti− (48/14) × N− (48/32) ×
S Nb: 0.003 to 0.015%, and 0.004 ≦ Nb × (10 × P + 2 × Cu + Ni).

[0020] Nb is Cu, by combined addition is P, aid the formation of passive film, on top of improving pitting corrosion resistance, also has the effect of reducing the anisotropy of the r _m values. This effect is ineffective when Nb is less than 0.003%, and when it exceeds 0.015%, the effect is saturated, the recrystallization temperature of steel increases, and the cost is reduced. Is also disadvantageous. Therefore, Nb is 0.003% or more and 0.015 or more.
% Or less. This effect is ineffective when Nb exists as a precipitate. That is, Nb needs to be dissolved in steel. In the steel of the present invention, Ti
Is combined with C, N and S, so that Nb is completely dissolved in the steel.

Further, since the passivation film is weak when the amounts of P, Cu and Ni are small, it is necessary to increase the amount of Nb for pitting corrosion resistance. That is, 0.004 ≦ Nb × (10 ×
P + 2 × Cu + Ni) is required.

The effect of Nb on pitting corrosion resistance was determined by the same test method as in Example 1 described below (corrosion cycle was from 10 to 180). 4%, P0.05%, Ni0.2%
And a base corrosion-resistant steel (comparative steel)
FIG. 1 shows a comparison of the steel of the present invention with 0% added. Furthermore, the pitting corrosion resistance based on the ratio of maximum erosion depth / corrosion weight loss is expressed by C
FIG. 2 shows a comparison between the corrosion-resistant steel (comparative steel) based on u0.4%, P0.05%, and Ni0.2% and the steel of the present invention in which the amount of Nb added was changed. From FIGS. 1 and 2, it can be seen that the corrosion-resistant steel plate without Nb has the same pitting corrosion resistance as the ordinary steel plate (SPCC), whereas the corrosion-resistant steel plate with solid solution Nb has much better pitting corrosion resistance. Understand.

B: 0.0002 to 0.002%, and (P / 200) <B. B has an effect on secondary work brittleness, and has a large effect on steel containing P and easily subject to secondary work brittleness, such as the steel of the present invention. This effect is ineffective when B is less than 0.0002%,
Addition of more than 0.002% hardens the steel, so it is as described above. Also, (P / 200) <B
This is because P embrittles the steel and reduces its effect.

The above is the cold-rolled steel sheet according to the first aspect of the present invention. When the surface roughness of the steel sheet satisfies the following conditions, the corrosion resistance is further improved and the invention of the second aspect is constituted. . That is, the surface roughness Rz of the steel sheet is: Rz (μm): 1 to 8, and Rz × S / (10 × P + 2 × Cu + Ni) ≦ 0.025
And

As the surface roughness increases, the corrosion resistance deteriorates. Therefore, Rz ≦ 8 μm. However, if Rz is 1μ
Even if it is less than m, it costs only and does not affect the corrosion resistance. Therefore, it is desirable that Rz ≧ 1 μm. The effect of Rz on corrosion resistance varies depending on the steel composition, and Rz × S /
When (10 × P + 2 × Cu + Ni) ≦ 0.025, the corrosion resistance is further improved.

Rz × S / (10 × P + 2 × Cu + Ni)
FIG. 3 shows the relationship between the weight loss and corrosion. From FIG. 3, Rz × S /
When (10 × P + 2 × Cu + Ni)> 0.025, it can be seen that the corrosion resistance is poor. Further, the steel 15 to which Nb is not added has slightly lower corrosion resistance than the steel of the present invention.

Next, preferable production conditions of the cold rolled steel sheet as described above will be described. A steel having the above-described components is formed into a slab by, for example, a continuous casting method or an ingot-making method, and then manufactured under the following conditions. The slab heating temperature may be a temperature at which precipitates in the slab are re-dissolved. For steel having the above-described components, the heating temperature is desirably 1100 ° C. or higher.

The finishing temperature for degradation deep drawability in the following _three points Ar, had better be rolled at Ar ₃ point or more. If the winding temperature is 550 ° C. or higher, ferrite grains are large and workability is good. However, if the temperature is too high, segregation of P at the grain boundary is promoted, and the brittleness in secondary processing is deteriorated. The temperature is likely to occur when the amount of P is large and is unlikely to occur when the amount of B is large. That is, it is desirable that the winding temperature be (650 + 200 × (200 × BP) ° C. or less).

Although the cold rolling conditions are not particularly limited, it is desirable that the cold pressure ratio is 50% or more in order to have excellent deep drawability. Since the higher the annealing temperature for recrystallization, the better the workability such as deep drawability and the like, the annealing temperature is preferably 750 ° C. or more, preferably 820 ° C. or more. But 90
Exceeding 0 ° C. is not preferable in terms of cost. In addition, recrystallization annealing must be performed by continuous annealing. The reason for this is that in the method using box annealing, P segregates at the grain boundaries during slow cooling after annealing, which deteriorates workability and corrosion resistance.

Rz is controlled by grinding with a grindstone such as a rolling roll and a pressure-regulating roll after annealing, and Cr after grinding with a grindstone.
Alternatively, Ni plating, shot blasting, electric discharge machining, laser machining, etching, EBT machining, etc. are performed. In short, it is necessary to control the Rz of the roll to be lower than usual.

[0031]

【Example】

(Embodiment 1) A specific embodiment of the present invention will be described as follows. The steel of the present invention and the comparative steel having the component compositions as shown in Table 1 were melted to form slabs. This was heated at 1250 ° C., hot-rolled at 900 ° C. to obtain a sheet thickness of 2.8 mm, and then wound up at 620 ° C. to obtain a hot-rolled sheet. After pickling, cold pressure down to 0.7mm (cold pressure ratio: 75%)
Then, a cold-rolled steel sheet was obtained. It was recrystallized and annealed at 850 ° C., and the pressure was adjusted to 0.5% to obtain a cold-rolled steel sheet. In the remarks in Table 1, X = Ti− (48/14) × N− (48/32) × S−4 × C,
Y = Nb × (10 × P + 2 × Cu + Ni) −0.004, and X ≧
0 indicates that the Ti content is equal to or greater than carbon, nitrogen and sulfur.

Table 2 shows the results of measuring the mechanical properties of the obtained steel sheet. The tensile test was performed on a JIS No. 5 test piece. For the r _m values, r _m value _{= (r 0 + 2 × r} 45 + r
₉₀ ) / 4 and Δr = (r ₀ −2 × r ₄₅ + r ₉₀ ) / 2. Further, the fracture surface transition temperature is a temperature at which a conical punch is pushed in from the end of the cup after a cup is formed at a drawing ratio of 2.1 and brittle fracture does not occur. did. For Rz, the surface roughness of the steel is 3
The measurements were repeated and the average was determined. Also, Z = Rz × S /
(10 × P + 2 × Cu + Ni).

The corrosion resistance test was performed at 70 × 15
0% sample was sprayed with 1% NaCl salt water at 35 ° C for 4 hours, then forced dried at 50 ° C at 20% humidity
At 50 ° C. and a humidity of 95% or more for 18 hours.
A combined cycle test in which exposure was carried out for a period of time was performed, and the maximum erosion depth after 60 cycles, the average erosion depth (15 points were selected from the deepest erosion depths, and averaged), and the corrosion loss was evaluated (each). Each of the test pieces was measured at three points, and the average value was evaluated) . In Table 2, there were cases where holes were formed in the sample. In that case, the plate thickness was calculated as the erosion depth.

As is evident from Table 2, the steel types 1 and 2 have low P, and therefore have poor corrosion resistance. Steel 9 and 1
In the case of 0, since P is too large, the moldability is poor. Steels 11 and 12 have too little Cu and therefore have poor corrosion resistance, and steels 13 and 14 have too much Cu and have poor formability. Further, it can be seen that since steel 15 does not contain Nb, Δr is large and pitting is likely to occur. Steel 15 and 1
In No. 6, since B was not added, secondary working embrittlement was caused. Steel 19 has a large amount of C, Si, and Nb, so that the formability is inferior, and since Ni is not added, the corrosion resistance is not so good. Further, steels satisfying X <0 (steel 10, 1
Regarding 5, 18, and 19), since solid solution carbon or solid solution nitrogen is not completely fixed, workability such as deep drawability is inferior. Further, it can be seen that when Y <0 (Steel 2, 15 and 21), the pitting corrosion resistance is poor. Further, it can be seen that steel (Steel 20) having a surface roughness R _Z > 8 has poor corrosion resistance. In addition, steel of Z> 0.025 (steel 10, 11, 1
2, 15, 17, 19, and 20) also have poor corrosion resistance.

[0035]

[Table 1]

[0036]

[Table 2]

(Embodiment 2) In this embodiment, an embodiment of a manufacturing method will be described. Steels having the component compositions shown in Steel 7 and Steel 15 in Table 1 were manufactured under the conditions shown in Table 3, and the results of evaluation of mechanical properties and corrosion resistance are shown in Table 4.

As shown in Table 4, under the condition of steel 7A, the workability is deteriorated because the winding temperature is too low. In addition, it can be seen that, under the condition of steel 7D, since the winding temperature is too high, pitting corrosion is likely to occur. In any of the steels 15, Nb is not added under any conditions, so that pitting is likely to occur. Under the condition A, the winding temperature is low, and under the condition E, the annealing temperature is low, so that the formability is deteriorated. Further, since steel 15 does not contain B, secondary working embrittlement is likely to occur, and steel 7
It can be seen that the steel 15 subjected to box annealing has undergone secondary working embrittlement due to the P segregation. Further, the results of measuring the mechanical properties of the steel 7 subjected to the alloying hot-dip galvanizing under the C condition (35 g / m ^{2 on} one side) are shown as 7 C ′. It can be seen that the mechanical properties of the above are maintained.

[0039]

[Table 3]

[0040]

[Table 4]

[0041]

As described above, the steel according to the present invention is:
It has excellent workability such as deep drawability and also has corrosion resistance. In addition, since the continuous annealing method is used, it can be manufactured industrially advantageously, and has remarkably great merits from the viewpoint of cost and energy saving. The steel according to the present invention also has a characteristic that even if a surface treatment is performed to further increase the rust-preventive force, the material does not deteriorate so much.

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of Nb addition on pitting corrosion resistance in steel of the present invention.

FIG. 2 is a graph showing the effect of Nb addition on pitting corrosion resistance in the steel of the present invention.

FIG. 3 shows the effect of Rz × S / (10 × P
FIG. 9 is a diagram showing the effect of (+ 2 × Cu + Ni).

──────────────────────────────────────────────────続 き Continuation of the front page (72) Inventor Tomoyoshi Ohkita 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nippon Kokan Co., Ltd. (56) References JP-A-4-2211025 (JP, A) JP-A-Hei JP-A-2-173213 (JP, A) JP-A-3-253541 (JP, A) JP-A-3-18403 (JP, A) JP-A-4-120217 (JP, A) JP-A-2-267242 (JP, A A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02 C21D 9/46

Claims (3)

(57) [Claims] 1. A highly corrosion-resistant cold-rolled steel sheet having the following composition (composition is wt%) and excellent workability. (A) C: 0.001 to less than 0.006%, Si: less than 0.35%,
Mn: 0.05-0.5%, P: 0.03-0.08%, S: less than 0.01%, sol.A
l: 0.01 to 0.1%, N: 0.0035% or less, Cu: 0.1 to 0.5%, Ni: 0.1
~ 0.5%, Ti: 0.01 ~ 0.06%, Nb: 0.003 ~ 0.015%, B: 0.000
2 to 0.002%, the balance being Fe and unavoidable impurities. (B) Further, the component composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S, and 0.004 ≦ Nb × (10 × P + 2 × Cu + Ni). 2. An average roughness Rz (μm) of ten points on the surface is 1
The high corrosion-resistant cold-rolled steel sheet excellent in workability according to claim 1, wherein Rz × S / (10 × P + 2 × Cu + Ni) ≦ 0.025. 3. A method for producing a high corrosion-resistant cold-rolled steel sheet having excellent workability comprising the following steps (composition is wt%). (A) C: 0.001 to less than 0.006%, Si: less than 0.35%, Mn: 0.05 to 0.5% , P: 0.03 to 0.08%, S: less than 0.01%, sol. Al:
0.01 to 0.1% , N: 0.0035% or less, Cu: 0.1 to 0.5%, Ni: 0.1
~ 0.5%, Ti: 0.01 ~ 0.06%, Nb: 0.003 ~ 0.015%, B: 0.00
02-0.002%, with the balance being Fe and unavoidable impurities, and the component composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48 / 32) × S, and, providing a slab having a chemical composition satisfying 0.004 ≦ Nb × (10 × P + 2 × Cu + Ni) the relationship, the (b) after the slab was heated at 1100 ° C. or more, Ar ₃ point Hot-rolled at a temperature higher than
(° C) is 550 ° C ≦ CT ≦ (650 + 200 × (200 × B (%) − P
(%) Winding in the range), the hot-rolled sheet taken the winding (c), after cold rolling, performing a continuous annealing at a temperature of 900 ° C. 750 ° C. or higher.