JP3146839B2 - 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 having excellent corrosion resistance, workability, weldability, chemical conversion property and the like which are optimal for a steel sheet for automobiles.

[0002]

2. Description of the Related Art For cold rolled steel sheets and the like currently used for automobiles, it is considered to reduce the thickness of automobiles from the viewpoint of reducing the weight and cost of the automobile itself. However, when the plate pressure is reduced, the margin after corrosion is reduced.
The problem of strength reduction after corrosion arises. In general, to increase the corrosion resistance of automotive steel sheets, the simplest method is to increase the basis weight of galvanizing, but increasing the amount of coating leads to an increase in cost and the peeling of the coating layer when processing the steel sheet. A problem arises, and the base is exposed and corrodes easily. In addition, spot welding is used when assembling automotive parts, and the amount of plating applied greatly affects the welding. That is, it is recognized that the weldability deteriorates with an increase in the amount of adhesion. Also, steel sheets used for automobiles are required to have excellent formability such as deep drawability and be inexpensive. Many steel plates satisfying such conditions have been proposed, but satisfactory characteristics have not been obtained so far.

For example, Japanese Unexamined Patent Publication (Kokai) No. 3-253541 discloses a method of reducing the amount of C in a Cu-P system, adding a small amount of S, and adding a certain amount of Si and Ti to provide excellent corrosion resistance in an environment where wet and dry are repeated. Is disclosed. Also, in Japanese Patent Application Laid-Open No. 3-150315, the amount of C in the Cu-P
A method for producing a steel sheet which is excellent in corrosion resistance and formability with respect to steel containing a small amount of Ni is disclosed. JP 4-1415
No. 54 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]

SUMMARY OF THE INVENTION However, Japanese Patent Application Laid-Open No. 3-253541
Since the steel sheet disclosed in the above publication is a Ti-killed steel, surface defects are likely to occur, and nozzle clogging is likely to occur when a slab is manufactured by continuous casting. In addition, the method disclosed in Japanese Patent Application Laid-Open No. 3-150315 stipulates that recrystallization annealing is performed by box annealing in order to improve formability. However, box annealing is only disadvantageous in terms of cost. However, there is a disadvantage that P tends to segregate, the steel becomes brittle, and the workability deteriorates.

The steel sheet disclosed in Japanese Patent Application Laid-Open No. 4-141554 has an elongation (El) of less than 40% and a Rankford value (rm).
Value) is less than 2.0, there is a disadvantage that press formability is insufficient. Also, there is a blind spot that steel with Cu, P and Cr added has poor pitting resistance. Further, a cold-rolled steel sheet containing P, Ti, Nb and the like disclosed in JP-A-4-168246 is
Since NbC is formed, it has the disadvantage of poor corrosion resistance.

[0006] The present invention provides a lower cost, more corrosion resistant,
An object of the present invention is to provide a surface-treated steel sheet having excellent workability, weldability, and chemical conversion properties.

[0007]

Means for Solving the Problems As a result of repeated studies on solving the above-mentioned problems in the prior art, the present inventors have made it possible to reduce C as much as possible and reduce Ti, Nb, and B in a Cu-P system. On at least one surface of the composite-added steel sheet,
Forming a Ni-P-based alloy plating layer or a Ni-P-based alloy plating layer containing one or more of W, Mo, Cr, and Cu;
By performing heat treatment in a non-oxidizing atmosphere to form a diffusion alloy region containing Fe-Ni-P as the main component at the interface between the steel sheet base and the plating layer, it is possible to sufficiently secure extremely high corrosion resistance and formability. Successful.

That is, the present invention provides: 1) C: 0.001 to 0.006%, Si: less than 0.35%, M
n: 0.05 to 0.5%, P: 0.03 to 0.08%, S: less than 0.01%, s
ol. 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 to
0.002%, the balance consists of Fe and unavoidable impurities, and the composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S , And a diffusion alloy layer containing Fe-Ni-P as a main component on at least one surface of the steel sheet that satisfies the relationship of 0.004 ≤ Nb x (10 x P + 2 x Cu + Ni). Excellent corrosion resistance and workability. Cold rolled steel sheet.

2) C: 0.001 to 0.006% by weight, Si: 0.
Less than 35%, Mn: 0.05 to 0.5%, P: 0.03 to 0.08%, S: 0.
Less than 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 to
0.002%, the balance consists of Fe and unavoidable impurities, and the composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S , And 0.004 ≤ Nb x (10 x P + 2 x Cu + Ni) On at least one surface of the steel sheet, one or more of W, Mo, Cr, Cu containing Fe-Ni-P as a main component Cold rolled steel sheet with excellent corrosion resistance and workability, having a diffusion alloy layer containing

3) The average roughness Rz (μm) of the ten points on the surface is
1 to 8 and Rz × S / (10 × P + 2 × Cu + Ni) ≦ 0.0
25. The cold-rolled steel sheet according to claim 1, which is excellent in corrosion resistance and workability.

4) C: 0.001 to 0.006% by weight, Si: 0.
Less than 35%, Mn: 0.05 to 0.5%, P: 0.03 to 0.08%, S: 0.
Less than 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 to
0.002%, the balance consists of Fe and unavoidable impurities, and the composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S , And 0.004 ≤ Nb x (10 x P + 2 x Cu + Ni) A steel sheet satisfying the relationship of
Before annealing or after pickling and cold rolling and before annealing,
At least one surface of the steel sheet is plated with a Ni-P alloy containing 8 to 18% by weight of P by electroplating or electroless plating, and immediately in a non-oxidizing atmosphere for 750 to 90%.
Diffusion heat treatment at 0 ° C to form a diffusion alloy region containing Fe-Ni-P as the main component on the surface of the steel sheet.
A method for manufacturing cold-rolled steel sheets with excellent corrosion resistance and workability.

5) C: 0.001 to 0.006% by weight, Si: 0.
Less than 35%, Mn: 0.05 to 0.5%, P: 0.03 to 0.08%, S: 0.
Less than 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 to
0.002%, the balance consists of Fe and unavoidable impurities, and the composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S , And 0.004 ≤ Nb x (10 x P + 2 x Cu + Ni) A steel sheet satisfying the relationship of
Before annealing or after pickling and cold rolling and before annealing,
Ni containing at least one surface of the steel sheet in a range of 8 to 18% by weight by electroplating or electroless plating and 15% by weight or less of one or more of W, Mo, Cr and Cu.
-P-based alloy plating, immediately 750 in non-oxidizing atmosphere
Diffusion heat treatment at ~ 900 ° C, Fe-Ni-P
A method for producing a cold-rolled steel sheet having excellent corrosion resistance and workability, characterized by forming a diffusion alloy region mainly composed of:

6) When the slab is hot-rolled to prepare a steel sheet having the above composition, the slab is heated at a temperature of 1100 ° C. or higher, and then hot-rolled at a temperature exceeding the Ar3 point. When the winding temperature CT (℃) is 550 ℃ ≦ CT ≦ (650 + 200 × (200 × B
(%)-P (%)).

7) When a steel sheet having the above composition is heat-treated in a non-oxidizing atmosphere, a diffusion alloy region containing Fe-Ni-P as a main component is formed on the steel sheet surface by heating in a continuous annealing furnace. Method of manufacturing cold rolled steel sheet with excellent workability.

[0015]

The present invention will be described below in detail.

The reasons for limiting the steel components as described above (all components are by weight in the following) are as follows.

C: 0.001 to 0.006% C is preferably small in order to maintain excellent mechanical properties. Therefore, the upper limit is 0.006% as long as the effect of the present invention is not impaired.
Limited to. The lower limit is that the workability is not significantly improved even if the carbon content is excessively low, and it is necessary to add other elements in order to make the carbon content extremely low. I do.

Si: Less than 0.35%.

Si does not deteriorate press formability,
It contributes to the strengthening of steel sheets as a solid solution strengthening element. However, if there is a large amount of Si, the formability will deteriorate and the plating property will also be impaired.

Mn: 0.05 to 0.5%.

Since Mn is an element necessary to fix inevitably contained S and prevent red-hot brittleness, the lower limit is 0.05%.
And If the content exceeds 0.5%, the Rankford value is remarkably deteriorated, and the cost is disadvantageous. Therefore, the upper limit is set to 0.5%.

P: 0.03 to 0.08%.

P is an element that can strengthen steel at the lowest cost and is an element that improves the corrosion resistance of the steel sheet itself. IF
If the content is more than 0.1% based on steel, the strength is consequently increased and segregation at the grain boundaries is liable to occur, and the problem of secondary processing deterioration becomes apparent. On the other hand, in order to contribute to corrosion resistance, 0.03% addition is necessary, and this was set as the lower limit.

S: 0.01% or less.

If S exceeds 0.01%, the ductility of the steel is degraded and the corrosion resistance is adversely affected. Preferably it is 0.007% or less.

Sol.Al: 0.01 to 0.1%.

Al is necessary for deoxidation and fixation of N. However, if added in a large amount, it will increase the cost and increase the amount of alumina inclusions and deteriorate the surface properties. At least 0.1%.

N: 0.0035% or less.

N is preferably as small as possible in order to obtain a high Rankford value, but the upper limit thereof is set to 0.0035% or less as long as the effect of the present invention is not impaired.

Cu: 0.1-0.5%.

Cu is an element that improves the corrosion resistance of the steel sheet itself when added in combination with P, and its effect is obtained at 0.1% or more. Excessive addition not only deteriorates deep drawability, but also due to surface defects during hot rolling or coexistence with Sn,
Because hot cracking is likely to occur during hot rolling,
0.5%.

Ni: 0.1 to 0.5%.

Ni is an element effective for reducing surface flaws when Cu is added and further improving corrosion resistance. However, excessive addition causes deterioration of the deep drawability and raises the cost. Therefore, the lower limit is set to 0.1% and the upper limit is set to 0.5%. 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. 0 for that.
It is necessary to add more than 01%. 0.06%
If the value exceeds, no further effect is obtained and the cost is disadvantageous. Therefore, the content is set to 0.01% or more and 0.06% or less. Here, the following conditions are required to completely precipitate and fix solid solution carbon and solid solution nitrogen in steel with Ti.

4 × C <Ti− (48/14) × N− (48/32) × S Nb: 0.003 to 0.015%, and 0.004 ≦ Nb × (10 × P
+ 2 × Cu + Ni).

Nb is combined with Cu and P by adding
In addition to helping passivation film growth and improving pitting resistance,
It also has the effect of reducing the rm value anisotropy. This effect is due to Nb
If the content is less than 0.003%, there is no effect, and if it exceeds 0.015%, the effect is saturated, the recrystallization temperature of the steel increases, and the cost is disadvantageous. Therefore, Nb is 0.
003% or more and 0.015% or less. This effect is ineffective if Nb exists as a precipitate. In other words, Nb must be dissolved in steel. In the steel of the present invention, the total amount of Nb is dissolved in the steel because Ti combines with C, N and S.

Further, since the passivation film is weak when the contents of P, Cu and Ni are small, the amount of Nb for pitting corrosion resistance needs to be increased accordingly. That is, 0.004 ≤ Nb x (10 x P + 2 x Cu + N
i) is required.

The influence of Nb on the pitting corrosion resistance was determined by the test method shown in the examples described below, and the maximum erosion depth and corrosion weight loss were compared with those of ordinary steel sheets with Cu 0.4%, P 0.05%, and Ni 0.2%. Fig. 1 shows a comparison between the corrosion-resistant steel (comparative steel) with the base steel of the present invention and the steel of the present invention containing 0.010% of Nb. Furthermore, the maximum erosion depth
/ The pitting corrosion resistance according to the ratio of corrosion weight loss is as follows: Cu 0.4%, P0.05%, Ni
Figure 3 shows a comparison between the corrosion-resistant steel (comparative steel) based on 0.2% and the steel of the present invention in which the amount of Nb added was changed. From Figures 1 and 2,
Corrosion-resistant steel sheets without Nb have a pitting corrosion resistance of ordinary steel sheets (SPC
In contrast to C), it can be seen that the corrosion-resistant steel sheet in which solute Nb is present has much better pitting resistance.

B: 0.0002 to 0.002%, and (P / 200) <
B.

B has an effect on secondary work embrittlement, and has a large effect on steels containing P and easily subject to secondary work embrittlement, such as the steel of the present invention. The effect is that B is 0.00
If it is less than 02%, there is no effect, and if it exceeds 0.002%, it hardens the steel. The reason for (P / 200) <B is to reduce the effect of P, which makes the steel brittle.

Although the steel composition as described above has sufficient corrosion resistance to the steel sheet itself, it is still insufficient for a steel sheet for automobiles used in a severe environment.

According to the present invention, an Fe—Ni—P diffusion alloy plating layer is formed on the steel sheet in order to impart further corrosion resistance and mechanical properties to the steel sheet. Ni-P alloy plating containing 8 to 18% by weight of P has an almost amorphous structure, and when a steel sheet having such a coating layer is heat-treated, a uniform diffusion alloy is obtained compared to a general crystalline plating film. A layer is formed in a short time. Such a diffusion layer protects the underlying steel from corrosion and once the underlying Fe
The corrosion product of Fe formed after the initiation of corrosion of iron is quickly and densely formed. As a result, it is possible to obtain excellent corrosion resistance that cannot be obtained by the conventional technology.

If P is less than 8% by weight, the Ni-P alloy plating is crystalline and the distribution of P is not uniform. For this reason, the composition of the diffusion alloy region formed when subjected to the heat treatment is not uniform, and the contribution to the denseness of the base formed rust is not sufficient.
Excellent corrosion resistance cannot be obtained. On the other hand, P is 18% by weight
If it is too large, the Ni-P alloy plating becomes brittle and its adhesion decreases.

For this reason, peeling of the plating is likely to occur in the process of heat treatment or the like. For this reason, the P content of the plating layer formed on the steel sheet in the present invention was set to 8 to 18% by weight. A desirable range is 8 to 15% by weight, and a more desirable range is 10 to 13% by weight.

Also, Ni-P can be used as a Ni-P-based alloy plating in which one or more of W, Mo, Cr, and Cu are combined in a range of 15% by weight or less. W, Mo, Cr, and Cu all have an inhibitory role against steel corrosion, and also have a synergistic effect with Ni and P to further improve the denseness and stability of initial rust. The total content of W, Mo, Cr, and Cu is desirably 15% by weight or less. Corrosion resistance improves with an increase in the total content of W, Mo, Cr, and Cu. However, if the total value exceeds 15% by weight, the adhesion decreases, and plating peeling is likely to occur in the subsequent process. Therefore, the contents of W, Mo, Cr, and Cu were set to 15% by weight in total. In order to exhibit the effect of containing W, Mo, Cr and Cu, the lower limit is desirably 0.5% by weight or more.

The plating amount of the Ni—P alloy plating layer is not particularly limited, but is preferably in the range of 0.1 to 8 g / m ² . If it is less than 0.1 g / m ² , the effect of improving corrosion resistance is not sufficient, and if it exceeds 8 g / m ² , the workability of the plating layer is reduced and it is easy to peel off, and the line speed must be reduced to increase the plating amount. This is disadvantageous in production efficiency.

When the surface roughness of the steel sheet satisfies the following conditions, the corrosion resistance is further improved.

Rz (μm): in the range of 1 to 8 and Rz ×
S / (10 × P + 2 × Cu + Ni) ≤ 0.025.

As the surface roughness increases, the corrosion resistance deteriorates. Therefore, Rz ≤ 8 µm. However, even if Rz is less than 1 μm, the cost is high and the corrosion resistance is not affected. Therefore, it is desirable that Rz ≤ 1 μm. The effect of Rz on corrosion resistance depends on the steel composition, and Rz × S / (10 × P + 2
× Cu + Ni) ≦ 0.025, the corrosion resistance is further improved.

FIG. 3 shows the relationship between Rz × S / (10 × P + 2 × Cu + Ni) and corrosion loss. From Fig. 3, Rz × S / (10 × P + 2 ×
If Cu + Ni)> 0.025, the corrosion resistance is poor. In addition, steel 15 to which Nb was not added had slightly lower corrosion resistance than the steel of the present invention.

Next, preferable production conditions for the cold-rolled steel sheet as described above will be described. A slab of the steel having the above-mentioned components is produced, for example, by 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-mentioned components, the heating temperature is desirably 1100 ° C. or higher.

When the finishing temperature is lower than the Ar3 point, the deep drawability deteriorates. Therefore, it is preferable to perform rolling at the Ar3 point or higher. 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 secondary work brittleness deteriorates. The temperature is likely to occur when the amount of P is large, B
It is unlikely to occur when the amount is large. That is, the winding temperature is (650
It is desirable that the temperature be + 200 × (200 × B−P) ° C. or less.

The step of performing Ni-P plating is performed before annealing, but is performed either before pickling at the outlet of the pickling line and before cold rolling, or by plating after pickling and cooling. You can go. In particular, when this plating is performed before cold rolling, it is advantageous because cleaning before plating and pickling as an activation treatment before plating are not required.

Although the cold rolling conditions are not particularly defined, it is preferable that the cold pressure ratio is 50% or more in order to have excellent deep drawability.

There are various methods for forming the Ni-P-based alloy plating layer. Electroplating or electroless plating (chemical plating) is desirable in terms of simplicity and the quality of the obtained film.

Next, the steel sheet provided with the Ni-P alloy plating layer is heat-treated in a non-oxidizing atmosphere to form a diffusion alloy region containing Fe-Ni-P as a main component at the interface between the steel sheet base and the plating layer. . The heat treatment for diffusion can be performed by ordinary annealing equipment after cold rolling. In particular, a method using continuous annealing with high productivity is desirable. The continuous annealing described herein can be a general continuous annealing facility for cold-rolled steel sheets or an annealing facility as a pretreatment facility for a hot-dip galvanizing line.
At this time, the maximum temperature is reached. However, the higher the temperature, the better the workability such as deep drawability.
It is good that it is more than ° C. If the temperature is lower than 750 ° C, the diffusion layer between the Ni-P alloy plating layer and the steel surface is not sufficiently formed, and the formation of dense rust during the corrosion process is not sufficient, so that the effect of improving corrosion resistance is small. On the other hand, if the temperature exceeds 900 ° C., pick-up of the plated metal to the roll in the heat treatment furnace is apt to occur, and as a result, it is likely to cause surface flaws. Further, if annealing is performed at a temperature exceeding 900 ° C., roughening of the ferrite grains tends to occur after press forming due to coarsening of the ferrite grains. The holding time at the highest plate temperature varies depending on the temperature.
Seconds are preferred. If the length is too short, a sufficient diffusion region is not formed, so that the effect of improving corrosion resistance does not appear. If it is longer than 120 seconds, the interface layer becomes brittle due to excessive diffusion alloying.
The adhesion and workability of the plating layer are reduced. During the heat treatment, overaging treatment may be performed at a temperature of about 300 to 400 ° C. for about several minutes. A preferred diffusion region formed by the heat treatment has a depth of about 0.1 to 20 μm. For the recrystallization annealing, a manufacturing method using continuous annealing as described in claim 7 is desirable. The reason for this is that in the box annealing method, P segregates at the grain boundaries during slow cooling after annealing, which degrades workability and corrosion resistance.

When performing the heat treatment, the roll pickup in the furnace can be avoided by heating at a heating rate of 50 ° C./sec or more by a direct heating furnace.

When the Ni-P-based alloy plating layer is heat-treated, a part thereof forms a diffusion alloy layer to form a steel sheet / diffusion alloy region / Ni-P-based alloy plating layer. May form a diffusion alloy layer to form a steel plate / diffusion alloy zone, but the present invention includes both.

After the heat treatment, temper rolling is performed under appropriate conditions as necessary.

The control of Rz is performed by grinding with a grindstone such as a rolling roll and a pressure adjusting roll after annealing, Cr or Ni plating after grinding stone polishing, shot blasting, electric discharge machining, laser machining, etching, EBT machining, etc. Performed by
In short, it is necessary to control the Rz of the roll lower than usual.

A cold rolled steel sheet having both such corrosion resistance and workability is an extremely useful steel sheet as a material for automobiles.

[0063]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Specific embodiments of the present invention will be described below, but it goes without saying that the present invention is not limited to these embodiments.

The evaluation of corrosion resistance and plating adhesion was carried out in each of the examples by evaluating the prepared test materials by the following methods.

(1) Corrosion resistance: A corrosion test was performed once a day in a corrosive environment in which salt spray was combined with dry and wet cycles, and the corrosion depth after the test was measured and evaluated according to the following criteria.

○ The maximum erosion depth is 0.2 mm or less △ The maximum erosion depth is more than 0.2 mm and 0.4 mm or less × The maximum erosion depth is more than 0.4 mm (2) Workability: plating at the tip of bending in 180 degree bending test The damage of the coating was observed and evaluated according to the following criteria.

○ Zero damage or extent to which fine cracks are generated △ Large cracks are generated or plating pieces are partially peeled off × Exfoliation of plating is observed over a wide range Example 1 A specific example of the present invention will be described. , As follows.

A steel of the present invention and a comparative steel having the component compositions shown in Table 1 were melted and made into slabs. After heating at 1250 ° C, hot-rolling at 900 ° C to a sheet thickness of 2.8 mm,
It was wound at 620 ° C to obtain a hot rolled sheet. After pickling, cool down to 0.7 mm (cooling rate: 75%), P content 12% by weight, adhesion amount
1g / m2 Ni-P plating was performed. This was heat-treated 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 Ti
Indicates that the amount is at least equivalent to that of 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. r
For the _m value, r _m value _{= (r 0 +2 × r 45} + r 90) / 4,
Δr = (r ₀ −2 × r ₄₅ + r ₉₀ ) / 2 The fracture surface transition temperature is defined as the cup formation at a drawing ratio of 2.1.
A conical punch was pushed in from the end of the cup, and the temperature at which brittle fracture did not occur was indicated.

Regarding Rz, the surface roughness of the steel was measured three times, and the average was determined. Also, Z = Rz × S / (10 × P + 2 ×
Cu + Ni).

As is clear from Table 2, in steel types 1 and 2,
It can be seen that the corrosion resistance is inferior because P is small. Steel 9 and 10
Is too poor in formability because of too much P. And steel 11 and 12
Corrosion resistance is inferior because Cu is too low, and formability is inferior in Steels 13 and 14 because Cu is too high. Further, it can be seen that since steel 15 does not contain Nb, Δr is large and pitting is likely to occur. Since steels 15 and 16 do not contain B, secondary work embrittlement occurs. Steel 19 is composed of C, Si, Nb
In many cases, the moldability is poor, and because Ni is not added, the corrosion resistance is not very good. In addition, for steels with X <0 (steel 10, 15, 18, 19), workability such as deep drawability is inferior because solid solution carbon or solid solution nitrogen is not completely fixed. Also, it can be seen that the pitting corrosion resistance is inferior when Y <0 (steel 2, 15, 21). Furthermore, surface roughness Rz>
0.025 steel (steel 10, 11, 12, 15, 17, 19, 20) also has poor corrosion resistance.

Example 2 Steels of the present invention (steel numbers: 3,5) shown in Table 1 were melted and made into slabs. After heating at 1250 ° C, hot rolling at 900 ° C
After making the sheet thickness 2.8 mm, it was rolled up at 620 ° C to obtain a hot-rolled sheet. After pickling, cool down to 0.7 mm (cooling ratio: 75%),
Test specimens were prepared by Ni-P plating in the range shown in 3 and continuous annealing at a temperature of 850 ℃ C, followed by temper rolling of 0.5%. The results are shown in Table 4. The steel of the present invention has excellent corrosion resistance,
This shows plating adhesion.

Example 3 Steels of the present invention (steel numbers: 3,5) shown in Table 1 were melted and made into slabs. After heating at 1250 ° C, hot rolling at 900 ° C
After making the sheet thickness 2.8 mm, it was rolled up at 620 ° C to obtain a hot-rolled sheet. After pickling, Ni-P plating was performed within the range shown in Table 3 and
The specimens were cold-pressed to 7 mm (cold-pressure ratio: 75%), continuously annealed at a temperature of 850 ° C, and temper-rolled to 0.5% to prepare test specimens. Table 5 shows the results. The steel of the present invention has excellent corrosion resistance and plating adhesion.

[0074]

As described above, the cold rolled steel sheet according to the present invention is excellent in workability such as deep drawability and also has corrosion resistance.

[0075]

[Table 1]

[0076]

[Table 2]

[0077]

[Table 3]

[0078]

[Table 4]

[Brief description of the drawings]

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

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

Fig. 3 Effect of Rz × S / (10 × P + 2 ×) on corrosion weight loss of steel sheet
The figure which shows the influence of Cu + Ni).

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasuhiro Matsuki 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Toyofumi Watanabe 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Inventor Yukimitsu Shiobara 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-5-65595 (JP, A) JP-A-6-93472 (JP) , A) JP-A-63-79996 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C22C 38/00-38/60 C21D 8/02-8/04 C21D 9/46 -9/48 C23C 18/50 C25D 5/26

Claims (7)

(57) [Claims] (1) C: 0.001 to 0.006% by weight, Si: 0.35% by weight%
Less, Mn: 0.05-0.5%, P: 0.03-0.08%, S: 0.01%
Less, 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 to
0.002%, the balance consists of Fe and unavoidable impurities, and the composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S , And a diffusion alloy layer containing Fe-Ni-P as a main component on at least one surface of the steel sheet that satisfies the relationship of 0.004 ≤ Nb x (10 x P + 2 x Cu + Ni). Excellent corrosion resistance and workability. Cold rolled steel sheet. 2. C: 0.001 to 0.006%, Si: 0.35% by weight%
Less, Mn: 0.05-0.5%, P: 0.03-0.08%, S: 0.01%
Less, 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 to
0.002%, the balance consists of Fe and unavoidable impurities, and the composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S , And 0.004 ≤ Nb x (10 x P + 2 x Cu + Ni) On at least one surface of the steel sheet, one or more of W, Mo, Cr, Cu containing Fe-Ni-P as a main component Cold rolled steel sheet with excellent corrosion resistance and workability, having a diffusion alloy layer containing 3. An average roughness Rz (μm) of ten points on the surface is 1 to 3.
8 and Rz × S / (10 × P + 2 × Cu + Ni) ≤ 0.025
The cold-rolled steel sheet having excellent corrosion resistance and workability according to claim 1 or 2. 4. C: 0.001 to 0.006% by weight, Si: 0.35% by weight
Less, Mn: 0.05-0.5%, P: 0.03-0.08%, S: 0.01%
Less, 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 to
0.002%, the balance consists of Fe and unavoidable impurities, and the composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S , And 0.004 ≤ Nb x (10 x P + 2 x Cu + Ni) A steel sheet satisfying the relationship of
Before annealing or after pickling and cold rolling and before annealing,
At least one surface of the steel sheet is plated with a Ni-P alloy containing 8 to 18% by weight of P by electroplating or electroless plating, and immediately 750 to 90% in a non-oxidizing atmosphere.
Diffusion heat treatment at 0 ° C to form a diffusion alloy region containing Fe-Ni-P as the main component on the surface of the steel sheet.
A method for manufacturing cold-rolled steel sheets with excellent corrosion resistance and workability. 5. C: 0.001 to 0.006% by weight, Si: 0.35% by weight%
Less, Mn: 0.05-0.5%, P: 0.03-0.08%, S: 0.01%
Less, 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 to
0.002%, the balance consists of Fe and unavoidable impurities, and the composition is (P / 200) <B, 4 × C <Ti− (48/14) × N− (48/32) × S , And 0.004 ≤ Nb x (10 x P + 2 x Cu + Ni) A steel sheet satisfying the relationship of
Before annealing or after pickling and cold rolling and before annealing,
Ni containing at least one surface of the steel sheet in a range of 8 to 18% by weight by electroplating or electroless plating and 15% by weight or less of one or more of W, Mo, Cr and Cu.
-P-based alloy plating, immediately 750 in non-oxidizing atmosphere
Diffusion heat treatment at ~ 900 ° C, Fe-Ni-P
A method for producing a cold-rolled steel sheet having excellent corrosion resistance and workability, characterized by forming a diffusion alloy region mainly composed of: 6. The slab is hot-rolled to prepare the steel sheet according to claim 4 or 5, wherein the slab is heated at 1100 ° C. or more and then hot-rolled at a temperature exceeding the Ar3 point. The winding temperature of the hot-rolled sheet CT (° C) is 550 ° C ≤ CT ≤ (650 + 200 × (200
× B (%)-P (%)).
Or the method for producing a cold-rolled steel sheet excellent in corrosion resistance and workability described in 5. 7. When the steel sheet according to claim 4 or 5 is heat-treated in a non-oxidizing atmosphere, the diffusion alloy region mainly composed of Fe-Ni-P is formed on the surface of the steel sheet by heating in a continuous annealing furnace. A method for manufacturing cold-rolled steel sheets that are formed and have excellent corrosion resistance and workability.