JP3303529B2 - Electroplated steel sheet and its manufacturing method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electroplated steel sheet which is excellent in secondary work brittleness, excellent in formability and fatigue resistance and is suitable for press parts for automobiles and the like, and a method for producing the same.

[0002]

2. Description of the Related Art Conventionally, an ultra-low carbon steel sheet to which a carbonitride forming element has been added has been developed as a deep drawing steel sheet to be subjected to continuous annealing, but such a steel sheet is subjected to secondary working after severe deep drawing. And brittle fracture. In particular
When obtaining a high-strength steel sheet by adding Si, Mn and P,
Since Si and P have a strong property of embrittlement of a steel sheet, they greatly degrade secondary work brittleness resistance.

Therefore, in order to remedy this drawback, means for adding B has been used by utilizing the fact that B in steel has the same grain boundary strengthening action as C. However, the deterioration of the workability due to the addition of B is well known, and therefore, the B content has been suppressed to an extremely small amount, and sufficient secondary work embrittlement resistance has not been obtained.

[0004] On the other hand, in the production process of these steel sheets, various methods for controlling hot rolling, cold rolling, annealing conditions and the like within a specific range have been disclosed. Ar ₃ transformation point or more temperature from the viewpoint of propensity, also the coiling temperature is, formability (ductility, etc.), the temperature range of 650 to 750 ° C. from the viewpoint of deep drawability improved, also, the annealing temperature is again Relatively low temperatures which are energetically favorable above the crystallization temperature have been used.

Until now, as a method for producing such a thin steel sheet, for example, Japanese Patent Application Laid-Open No. 62-278232 discloses a method for producing a cold-rolled steel sheet for non-aging deep drawing by a direct rolling method.
No. 7321 discloses a method for producing a cold-rolled steel sheet having excellent deep drawability, and JP-A-2-200730 discloses a method for producing a cold-rolled steel sheet having excellent press formability. In any of the above, B is added for improving secondary work brittleness resistance, but there is no specific disclosure about secondary work brittleness.

Japanese Patent Application Laid-Open No. 63-241122 discloses a method for producing a hot-dip galvanized steel sheet for ultra-deep drawing. However, the B content is as small as 0.0010 mass% or less, and the Although there is an improvement, it is still insufficient.

Further, Japanese Patent Application Laid-Open No. 62-40318 discloses a method for producing a cold-rolled steel sheet having excellent deep drawability.
In each of the publications, a method for producing a cold-rolled steel sheet having excellent press formability is disclosed. However, in both of these methods, since the annealing temperature is in the range of not less than the recrystallization temperature of the ordinary method and not more than 800 ° C., it is not expected that the secondary work brittleness resistance and the formability are sufficiently improved.

JP-A-61-13323 discloses a method for producing a thin steel sheet having excellent formability, and JP-A-62-205231 discloses a method for producing a high-strength steel sheet. These are mainly intended to produce a slab thinner than a normal thickness, and to reduce or simplify the rolling process using the slab.In the former method, the annealing conditions after cold rolling are used. Although it is not specifically shown, although a specific example relating to secondary working brittleness resistance is shown, its effect is insufficient, and in the latter method, a specific example of an annealing temperature of 775 ° C. or less is shown. Under these conditions, sufficient resistance to 2
Improvement in secondary working brittleness cannot be expected.

As a method for solving the above problems, Japanese Patent Application Laid-Open No. 5-117758 discloses a method for producing a cold-rolled steel sheet having excellent secondary work brittleness resistance and low in-plane anisotropy. However, low temperature winding and B of 0.001 to 0.004 mass
%, The secondary work brittleness resistance is excellent, but the formability is insufficient.

In addition, these prior arts do not mention fatigue resistance at all, and do not specifically disclose secondary work brittleness after electroplating, formability and fatigue resistance.

[0011]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to develop a technique which advantageously solves the above-mentioned problems in the prior art, and specifically, has an excellent secondary resistance after plating. An object of the present invention is to provide an electroplated steel sheet exhibiting both work brittleness and fatigue resistance and exhibiting excellent formability, and a method for producing the same.

Further, an object of the present invention is to provide a method for electroplating without causing deterioration of ductility due to the addition of B and deterioration of brittleness resistance during secondary processing due to the absence of B, which are problems in the prior art. An object of the present invention is to provide an electroplated steel sheet having excellent secondary work brittleness resistance, fatigue resistance and formability by controlling the tension, and a method for producing the same.

[0013]

Means for Solving the Problems The inventors of the present invention have found that when the secondary working brittleness resistance is improved by adding B and low-temperature winding as in the prior art, deterioration of the formability is observed, and the tension during electroplating is reduced. Control and accumulate the residual stress of compression to improve the secondary brittleness resistance and the formability. In this case, the addition of B, the deterioration of the formability due to low temperature winding is small, and the excellent formability and The present inventors have found that they exhibit excellent secondary processing embrittlement resistance and that they can impart extremely excellent fatigue resistance by controlling the tension during electroplating, and thus completed the present invention.

In this regard, in the prior art, as described in, for example, JP-A-56-142892, efforts have been made to minimize the stress during electrodeposition as much as possible. is there.

That is, the gist of the present invention is that, in mass%, C: 0.0030% or less, Si: 0.015% or less, Mn: 0.15% or less, P: 0.015% or less,
S: 0.020% or less, sol.Al: 0.060% or less, N: 0.006
0% or less, Ti: 0.020 to 0.080%, Nb: 0.003 to 0.02
A slab having a steel composition of 0%, balance iron and unavoidable impurities, is hot-rolled, rolled, hot-rolled steel strip and then
Cold rolled at a rolling rate of 50% or more, and 840 in continuous annealing
The steel is subjected to recrystallization annealing at a temperature of at least 100 ° C., and is subjected to plating while applying a tension of 30 to 80 N / mm ² or more in an electroplating line. This is a method for producing an electroplated steel sheet having excellent fatigue properties.

According to a preferred embodiment of the present invention, the tension applied to the steel sheet during electroplating is preferably 30 to 60 N
a / mm ^2. According to still another preferred embodiment, the steel composition is C: 0.0020% or less, P: 0.010% or less by mass%, and is subjected to electroplating while applying a tension of 30 to 80 N / mm ^2. Is also good.

From another aspect, the present invention provides a method for preparing C:
0.0030% or less, Si: 0.015% or less, Mn: 0.15
%, P: 0.015% or less, S: 0.020% or less, s
ol.Al: 0.060% or less, N: 0.0060% or less, Ti: 0.02
0-0.080%, Nb: 0.003-0.020%, with steel composition consisting of balance iron and unavoidable impurities, 30-80 N / m on surface
An electroplated steel sheet excellent in secondary work brittleness resistance, formability and fatigue resistance, comprising an electroplated layer having a residual compressive stress of m ² .

By performing electroplating under tension, the obtained electroplated steel sheet has a compressive stress, that is, a residual compressive stress, after the plating is completed, and the presence thereof significantly reduces the fatigue resistance of the electroplated steel sheet. It will be improved.

[0019]

The operation of the present invention will now be specifically described below. First, the reason why the composition range of the present invention is limited as described above will be described.

C: 0.0030 mass% or less C is advantageous in terms of material as the content thereof is small, and when the content is large, the amount of C which forms a solid solution increases, and Ti and Nb for fixing C which forms a solid solution are used. And the amount of composite precipitates increases, resulting in deterioration of the material. If the content exceeds 0.0030 mass%, the material starts to deteriorate significantly.
mass%. Preferably, in order to ensure moldability,
0.0020 mass% or less.

Si: 0.015 mass% or less Since Si deteriorates the resistance to secondary working brittleness, the upper limit of the content is set to 0.015 mass%. Preferably it is 0.010 mass%.

Mn: 0.15% by mass or less Mn is an effective component for obtaining an appropriate strength. However, the upper limit of the content is limited in view of the effect of deteriorating drawability by solid solution strengthening and cost. 0.15 mass%. Preferably, it is 0.12 mass% or less.

P: 0.015 mass% or less When P is contained in a large amount, the segregation amount at the grain boundary increases to cause grain boundary embrittlement, resulting in secondary working embrittlement resistance and deterioration in formability. However, if the content is 0.015 mass% or less, the allowable range is sufficiently high, so the upper limit is set to 0.015 mass%. Preferably, it is 0.01 mass% or less.

S: not more than 0.020 mass% If S is contained in a large amount, it is considered that MnS is formed and the punching property is improved. .

Sol. Al: 0.060 mass% or less Al is useful as a nitride. That is, Ti, Nb
By forming composite precipitates presumed to be (Ti, Nb) C and (Ti, Al) N due to the coexistence of, C and N are fixed and formability is improved. However, an excessive amount leads to a high cost, so that unnecessary addition is unnecessary. For these reasons, the upper limit of the content is set to 0.060 mass%. Preferably, it is at most 0.040 mass%.

N: 0.0060 mass% or less N deteriorates formability and aging resistance similarly to C.
Therefore, it is desirable to reduce as much as possible. Its content is
The allowable upper limit is set to 0.0060 mass% or less. Preferably, it is 0.0030 mass% or less.

Ti: 0.020 to 0.080 mass% From the viewpoint of moldability, Ti needs to have an amount sufficient to sufficiently fix C, S and N. However, when contained in large amounts, FeTiP
The range of Ti is set to 0.020 to 0.080 mass% because it is considered that the formability is degraded in order to form Ti. Preferably, 0.030 to
0.060 mass%.

Nb: 0.003 to 0.020 mass% Nb is effective for forming carbonitrides and improving formability similarly to Ti, and its effect is remarkable when the content is 0.003 mass% or more. However, if the content is too large, the recrystallization temperature is increased, and the ductility is deteriorated. Therefore, it is necessary to add the content within a range that does not cause these problems. Therefore, the upper limit of the content is set to 0.02 mass%. Preferably, 0.
008 to 0.015 mass%.

Next, the reasons for limiting the manufacturing process conditions in the present invention will be described. First, the steelmaking method may be performed in a converter or the like according to a conventional method, and there is no need to limit the conditions. Also, the method for producing a billet may be an ordinary continuous casting and ingot-making method, and the present invention is not particularly limited.

Hot rolling : The steel material having the steel composition according to the present invention is hot-rolled. The hot rolling conditions in this case are not limited to specific conditions, but are preferably set under the following conditions. Cold rolling is performed.

(A) Finishing temperature: 800 to 930 ° C. If the finishing temperature is less than 800 ° C., the average r due to residual strain is obtained.
Values and elongation, while exceeding 930 ° C. causes coarsening of crystal grains. Therefore, the finishing temperature is preferably set at 800 to 930 ° C. More preferably, the finishing temperature is 880
~ 910 ° C.

(B) Winding temperature: 650 to 750 ° C. The winding temperature is low at a low temperature of less than 650 ° C., which impedes the coarsening of precipitates, causes the ductility to deteriorate, and has an excellent average r value. Can't get it. Winding at a temperature higher than 750 ° C is difficult for stable production, so the upper limit is set to 750 ° C. Preferably, it is 680-720 ° C.

Cold Rolling : With a cold-rolled steel sheet, as the cold-rolling rate increases, the texture of the (111) plane develops, and the r value, which is an index of deep drawability, increases. Therefore, in order to obtain excellent formability, rolling is performed at a cold rolling reduction of 50% or more, preferably 50 to 90%.

Recrystallization annealing temperature : The cold-rolled steel sheet is then subjected to continuous annealing, and the properties are improved as the annealing temperature increases. Therefore, in order to obtain excellent formability, high-temperature annealing is necessary, and the annealing temperature is set to 840 ° C or higher.

Electroplating : In the present invention, in order to improve the resistance to secondary working brittleness in electroplating, the tension at the time of plating is controlled to accumulate an appropriate compressive residual stress without deteriorating ductility. The mechanism of compressive residual stress accumulation observed in that case will be described as follows. First, since the steel sheet to which the plating layer adheres during plating is under tension, that is, tensile force, if the plating layer is adhered in this state, the tension is released after plating, and the steel sheet returns to its original state. A force acts so as to compress the solder. At this time, a compressive force acts on the plating layer by being attracted to it, and compressive stress accumulates inside.

That is, the present invention is characterized in that, in the upper electroplating layer, after the tension is released, the inward movement of the steel sheet is restricted, so that compressive stress is accumulated in the steel sheet. The unit tension at the time of electroplating in the present invention is
30.0 to 80.0 N / mm ² , preferably 30 to 60 N / mm ² .

When the tension in the plating cell is less than 30.0 N / mm ^{2, the} secondary compressive brittleness resistance is improved because the accumulated residual stress of compression is not sufficient, but the effect of improvement in fatigue resistance is remarkable. , The lower limit is 30.0 N / mm ² . Further, when the unit tension is more than 80.0 N / mm ² , the ductility is extremely deteriorated, so the preferable upper limit is 80.0 N / mm ² . More preferably, it is 60 N / mm ² . Incidentally, the conventional electroplating is usually performed under substantially no tension or under a tension of less than 30 N / mm ^{2 in} order to secure the sheet passing property.

[0038] Conventionally, as described above, there is no idea of accumulating the residual stress in the steel sheet by limiting the inward movement of the steel sheet after the tension is released by the upper electroplating layer. . Fatigue is governed by sheet thickness and residual stress, but the effect of residual stress on fatigue is negatively correlated with fatigue resistance when residual stress is tensile, and positive when compressive stress is applied. There is a correlation. Secondary working brittleness has the same relationship as fatigue resistance, and the presence of compressive residual stress improves secondary working brittleness.

[0039]

EXAMPLE In this example, a steel slab having a steel composition (mass%) shown in Table 1 was used as a raw material, hot-rolled and cold-rolled under the following conditions, and Zn-Ni electroplating was performed after continuous annealing. To produce an electroplated steel sheet.

Hot rolling finish temperature: 890 to 930 ° C. (standard condition: 900 ° C.) Hot rolling winding temperature: 650 to 680 ° C. (standard condition: 660 ° C.) Cold rolling reduction: 80% Continuous annealing: 850 to 870 ° C. (Standard conditions: 860 ° C) Skin pass pass reduction: 0.3% Unit tension: 25.0-90.0 N / mm ² (Standard conditions: 34.3 N / mm ² ) (Tension in plating cell / cross-sectional area) Diameter: 90mm, die hole diameter: 51.8mm, punch diameter: 50mm, formed into a cylindrical shape with a drawing ratio of 1.4 to 2.2 (standard condition: drawing ratio 1.2) The temperature at which cracks occurred was measured and determined.

The fatigue characteristics were measured by a plane bending swing test and evaluated by the fatigue limit. The unit tension was adjusted by changing the tension in the plating cell.
First, except that the unit tension was changed for the test steel A in Table 1, hot rolling, cold rolling, continuous annealing,
Then, Zn-Ni plating is continuously performed, and the relationship between brittle transition temperature and elongation and unit tension at that time is shown in FIG. 1, and the relationship between brittle transition temperature and drawing ratio is shown in a graph in FIG.

Similarly, the respective characteristics that affect the finish rolling temperature, the winding temperature, and the continuous annealing temperature are also shown in graphs in FIGS. From Fig. 3, the hot rolling finish temperature is 8
It turns out that 00-930 ° C is preferable. Similarly, FIG. 4 shows that the winding temperature is preferably 650 to 750 ° C.

FIG. 5 is a graph showing the correlation between the recrystallization annealing temperature and each of the mechanical characteristics. It can be seen that the characteristics are improved by setting the recrystallization annealing temperature to 840 ° C. or higher. Next, the tensile properties, average r value, secondary work brittleness (brittle transition temperature), fatigue resistance, etc., of the electroplated steel sheet according to the present invention manufactured in the above-described standard condition for each test steel in Table 1 were investigated. did. These results are collectively shown in Table 2 above.

The tensile tests were conducted according to the JIS No. 5 test, and the average r values were r _L (rolling direction), r _C (45 ° direction in rolling direction), and r _T (90 ° direction in rolling direction). And r = (r _L + 2r _C + r _T ) / 4. The test procedure in this example is as follows.

Secondary work brittleness resistance (low temperature brittle crack resistance after forming): After performing cylindrical drawing, a drop test was performed at a temperature lower than room temperature to determine a ductile-brittle transition temperature. .

Fatigue Property (Fatigue Strength of Material): The number of repetitions of the force until a fracture occurs when a material having a certain magnitude is repeatedly applied to cause a fatigue crack when the force is repeatedly applied.

The fatigue limit (threshold stress causes fatigue failure): drawing ratio: cylindrical aperture diameter of the molding before the implementation of the disc D _o, an aperture ratio diameter after cylindrical molded and D is as follows.

Aperture ratio (DR) = D _o / D During a normal test, the aperture ratio is defined as follows. Diameter of cylindrical punch: Dp Thickness of test piece: t Diameter of test piece: D Drawing ratio (DR) = D / (Dp + t)

[0049]

[Table 1]

[0050]

[Table 2]

As is evident from Table 2, all of the examples of the present invention exhibited excellent moldability with El> 48.0%, average r value ≧ 2.2, and brittle transition temperature ≦ −70 ° C. and durability ratio.
(Fatigue limit / strength) Above 75%, practically no secondary work brittleness and no fatigue fracture. In particular, unit tension 30
The durability ratio, workability, and secondary work brittleness resistance of the plated steel sheet of the present invention under the conditions of 6060 N / mm ² are remarkably improved. In FIG. 2 and Table 2, the relationship between the drawing ratio and the brittle transition temperature can be explained as follows.

Generally, there are two types of brittle cracks: grain boundary fracture and intragranular fracture. The former greatly depends on the grain boundary strength. For example, steel containing a large amount of P has Is considered to increase the possibility of grain boundary fracture. In addition, the latter intragranular fracture is considered to be largely caused by residual stress during molding.

From the relationship between the drawing ratio and the brittle transition temperature, the effect of residual stress on the temperature at which vertical cracks occur can be understood. As the drawing ratio increases, the compressive strain in the circumferential direction caused by forming increases. Therefore, tensile residual stress accumulated during molding also increases. Thereby, the secondary working brittleness is degraded.

That is, the present invention accumulates residual compressive stress before molding, thereby reducing tensile residual stress accumulated after molding and improving secondary work brittleness resistance.

On the other hand, as in Run Nos. 11 to 13 and 15, even when the unit tension at the time of electroplating is suitable, the comparative examples in which the component composition is out of the limited range of the present invention have a brittle transition temperature ≧ −. Moldability is inferior at 50 ° C and El <48%. As in Run No. 14, not only the component composition but also the unit tension exceeds 80 N / mm ² , the moldability is also poor.

In Comparative Examples, such as Run No. 5, in which the component tension is out of the range of the present invention even if the component compositions are compatible, no remarkable improvement in fatigue property is observed. However, as in Run No. 9, the unit tension is> 80 N / mm
^In the case of ² , the deterioration of ductility is large. As described above, only the electroplated steel sheet which satisfies the production conditions as well as the component composition limited by the present invention exhibits excellent secondary work brittleness resistance, fatigue resistance and formability.

[0057]

According to the present invention, an ultra-low carbon steel sheet containing proper amounts of Ti and Nb is used as a material, and the hot rolling conditions, the cold rolling conditions, the electroplating conditions and the like are optimized to obtain excellent resistance. Electroplating steel sheets with excellent secondary work brittleness, fatigue resistance and formability can be easily manufactured.
The electroplated steel sheet obtained according to the present invention can be advantageously applied to applications in which press working is performed, such as for automobiles.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between unit tension and brittle transition temperature.

FIG. 2 is a graph showing a relationship between a drawing ratio and a brittle transition temperature in cylindrical drawing.

FIG. 3 is a graph showing a relationship between a hot rolling finish temperature and an r value.

FIG. 4 is a graph showing the effect of winding temperature on elongation (El) after cold rolling.

FIG. 5 is a graph showing the effect of annealing temperature on mechanical properties after cold rolling.

──────────────────────────────────────────────────続 き Continuation of front page (56) References JP-A-5-65676 (JP, A) JP-A-4-247850 (JP, A) JP-A-6-240367 (JP, A) JP-A-56- 142892 (JP, A) JP-A-4-259396 (JP, A) WO 92/14854 (WO, A1) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25D 5/00-7 / 12

Claims (2)

(57) [Claims] [Claim 1] In mass%, C: 0.0030% or less, Si: 0.015% or less, Mn:
0.15% or less, P: 0.015% or less, S: 0.020% or less, sol.Al:
0.060% or less, N: 0.0060% or less, Ti: 0.020 to 0.080%, Nb:
A slab having a steel composition of 0.003 to 0.020%, with the balance being iron and unavoidable impurities, is hot-rolled, rolled and formed into a hot-rolled steel strip, and then cold-rolled at a rolling rate of 50% or more, and continuously annealed. Recrystallization annealing at 840 ° C or higher at
A method for producing an electroplated steel sheet having excellent secondary work brittleness, formability and fatigue resistance, wherein plating is performed while applying a tension of up to 80 N / mm ² . 2. In% by mass, C: 0.0030% or less, Si: 0.015% or less, Mn:
0.15% or less, P: 0.015% or less, S: 0.020% or less, sol.Al:
0.060% or less, N: 0.0060% or less, Ti: 0.020 to 0.080%, Nb:
From 0.003 to 0.020%, having a steel composition consisting of the balance of iron and unavoidable impurities, characterized by comprising an electroplated layer leaving a 30 to 80 N / mm ² of compressive stress on the surface, resistance to secondary Electroplated steel sheet with excellent processing brittleness, formability and fatigue resistance.