JP3386657B2 - High strength cold rolled steel sheet - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to relates to a high strength cold rolled steel sheet, high strength, especially, are used in an automobile or the like vehicle, and if necessary, electric plating, Kasei treatment, chromate treatment is applied Related to cold rolled steel sheet.

[0002]

2. Description of the Related Art In recent years, weight reduction of automobile bodies has been demanded from the viewpoint of exhaust gas regulation. There is a method of reducing the plate thickness as one of the effective means for reducing the weight of the vehicle body.However, when using this method, the thickness of the plate itself is adjusted so as to be commensurate with the reduced thickness in terms of ensuring safety. It is necessary to increase strength. Therefore, conventionally, in steel, Si, Mn, Cr, P
Or so-called solid solution strengthening elements such as C,
Mn is mainly added to increase the strength of the steel sheet.

By the way, the above high strength steel sheet is annealed at a high temperature of 800 ° C. or more after cold rolling in the manufacturing process in order to secure an excellent material. Further, in order to impart corrosion resistance, plating or chemical conversion treatment may be performed after the annealing. The annealing is usually performed in an N ₂ —H ₂ atmosphere. This atmosphere is a reducing atmosphere for Fe, but an oxidizing atmosphere for Si, Mn, Cr, P, etc. Is selectively oxidized to form an oxide and is concentrated on the surface of the steel sheet. When this steel sheet is subjected to, for example, hot dip galvanizing, the above oxide reduces the wettability between the hot dip zinc and the steel sheet, often causing a so-called "non-plating" phenomenon on the steel sheet surface. Furthermore, even if degreasing or pickling is performed after annealing, these oxides cannot be completely removed from the steel sheet, so when performing electroplating or chemical conversion treatment, the "non-plating" phenomenon or "repellency" of chemical conversion treatment liquid Often causes. That is, platability,
It was difficult to obtain a high-strength cold-rolled steel sheet excellent in chemical conversion treatability.

Among these problems, in order to improve the hot-dip galvanizing property, Japanese Examined Patent Publication No. 61-9386 proposes a method of subjecting the surface of a steel sheet to undercoating with Ni prior to hot-dip coating. However, in this method, a steel sheet containing 0.1 wt% or more and 3.0 wt% or less of Si, or 0.03 to 0.10 wt% of C and 0.001 of Si.
~ 0.10 wt%, Mn 0.5-2.0 wt%,
0.01 to 0.10 wt% P, 0.50 wt% Mo
In the case of targeting a steel sheet containing each of the following, it is necessary to perform Ni plating with an adhesion amount of 10 g / m ² or more, which causes a large increase in manufacturing cost. When such a large amount of Ni plating is applied, the wettability between the hot dip galvanized steel sheet and the steel sheet is certainly improved, but another problem is that defects caused by Si and Ni frequently occur on the plating surface during the alloying process. There was a problem.

Further, Japanese Patent Laid-Open No. 57-70268 discloses that
The method of performing the base plating of Fe on the surface of the steel sheet is disclosed instead of the above Ni plating. However, with this method,
Although it was possible to prevent "non-plating" of Si-added steel,
Therefore, it is necessary to plate Fe of 5 g / m ² or more, which is extremely uneconomical as in the case of Ni. further,
JP-A-55-122865 and JP-A-4-2545
Japanese Patent No. 31 discloses a method in which after cold rolling, a steel sheet is previously oxidized to form an iron oxide film on the surface layer, and then reduction annealing is performed to suppress the formation of an oxide film of an alloy element and then perform desired plating. Disclosed. However, since this method is a method of controlling the thickness of the iron oxide film remaining before plating by reduction annealing to be a certain value or less, it is excessively reduced by reduction annealing, and the alloying elements are concentrated on the surface and rather plated. There is a problem that the property becomes poor, that is, the balance between the oxide film and the reduction amount is lost. In addition, in order to prevent this over-reduction, an enormous amount of iron oxide is required, so the iron oxide film is peeled off by rolls during rolling, and selective oxidation of alloy elements occurs during subsequent reduction annealing. Therefore, there is a problem that the plating property is hindered or the peeled iron oxide film is scattered in the annealing furnace, which adversely affects the operation.

On the other hand, one of the properties particularly required for steel sheets for automobiles is pretreatment for coating, that is, chemical conversion treatment. If the chemical conversion treatment is not good, there is a problem that the subsequent baking coating will not be successful. However, in order to increase the strength of the steel sheet, as described above, C, Si, Mn, P
If a large amount of such a strengthening component as described above is contained, chemical conversion treatability is markedly deteriorated due to the generation of oxides on the surface described above. Also,
When it is used for other purposes, mainly as a steel sheet for home appliances, it may be subjected to chromate treatment. However, also in this case, as in the case of the chemical conversion treatability, since a large amount of reinforcing components such as C, Si, Mn, and P are contained for the purpose of increasing the strength of the steel sheet, the chromate treatability does not work well. There was a problem.

[0007] As described above, even in high-strength steel sheet with a currently attractive as a high strength material for automobile, which, electrostatic <br/> vapor plating, chemical conversion treatment, the corrosion resistance after performing chromate treatment In order to use it as a good surface-treated steel sheet,
The reality is that there is still room for improvement.

[0008]

[0008] The present invention has been made in view of such circumstances, and ingenuity to the base material of the steel plate, electrical plating properties,
It is an object of the present invention to provide a high-strength cold-rolled steel sheet having excellent chemical conversion treatability and chromate treatability.

[0009]

As mentioned above, Si,
When various surface treatments such as plating, chemical conversion treatment, and chromate treatment are performed on a high-strength cold-rolled steel sheet to which a solid solution strengthening element such as Mn is added, these elements are concentrated on the surface to form a film, "Non-plating" occurs because it impedes the wettability of molten steel and the chemical conversion treatment liquid with the steel sheet. Further, it is already known that there is a correlation between the amount of surface concentration, the plating property, and the alloying speed, and the smaller the amount of surface concentration, the better the plating property and the faster the alloying speed.

Therefore, the inventor diligently studied in detail the surface layer structure of the steel sheet for suppressing the surface thickening. As a result, SiO ₂ , MnO, FeSiO ₃ , Fe ₂ Si is previously formed in the crystal grain boundary of the high-strength cold-rolled steel sheet surface containing a specified component, or in the grain boundary, or in the crystal grain boundary and the grain.
When _{O 4, MnSiO 3, Mn 2} SiO 4, allowed to produce a P ₂ O ₅ oxide such, electric plating of the high-strength steel sheet, Kasei processability, drastically improve the chromate treatment of Then, they have completed the present invention.

That is, the present invention relates to electroplating or
Cold rolled steel for chemical conversion treatment or chromate treatment
A plate, high strength cold rolled steel sheet containing Si less 0.1 wt% or more 3.0 wt%, a steel plate surface of 0.1 to 10
Grain boundaries to a depth of 0μm and / or within the grain, electrostatic
Vapor plating resistance, reduction conversion treatment and effectiveness oxide to improve the chromate treatment _{of, SiO 2, MnO, FeSiO 3} , Fe 2
It is a high-strength cold-rolled steel sheet characterized by being formed of one or more selected from SiO ₄ , MnSiO ₃ , Mn ₂ SiO _4, and P ₂ O ₅ . The present invention also has electroplating
Cold rolling for chemical conversion or chromate treatment
A steel sheet , wherein C: 0.03 to 0.10 wt% Si: 0.001 to less than 0.10 wt% Mn: 0.5 to 2.0 wt% P: 0.01 to 0. 10 wt% Mo: containing less 0.50 wt% of high-strength cold-rolled steel sheet, 0.1 to 1 from the steel sheet surface layer
Within a grain boundary and / or grain to a depth of 00 μm ,
Electroplating resistance, reduction conversion treatment and effectiveness oxide to improve the chromate treatment _{of, SiO 2, MnO, FeSiO 3} , Fe
A high-strength cold-rolled steel sheet, characterized in that it is formed by forming at least one selected from ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ and P ₂ O ₅ .

[0012]

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below. The crystal grain boundaries in the surface layer of the high-strength cold-rolled steel sheet according to the present invention, or in the grains, or the oxides present in the grain boundaries and grains are generated during the hot rolling in the previous stage of cold rolling the steel sheet. I will let you. In particular, when the coil winding temperature after hot rolling is high and the subsequent cooling rate is slow, these oxides grow. The appearance of the oxide formed at the grain boundaries is shown in FIG. 1, and it is observed just below the so-called black skin. The results of EPMA analysis of this oxide are shown in FIG. From FIG. 2, S
Since peaks of i, Mn, P, and O are seen, it is understood that oxides of these elements are generated. Further, FIG. 3 shows the results of observing a replica of the surface layer of the hot rolled steel sheet with a TEM (transmission electron microscope). From FIG. 3, not only the grain boundaries but also the precipitates are confirmed in the crystal grains of the surface layer.
Table 1 shows the result of elemental analysis of this by EDX. From Table 1, it is possible to confirm Fe, Mn, Si, etc. in the grain boundaries, and Mn, Si, etc. in the grains. Further, in view of the fact that they can be exfoliated by a replica and the existence form of these elements in steel, it is considered that these elements form an oxide.

[0014]

[Table 1]

Further, the oxides present immediately below the surface layer of the steel sheet are the oxides immediately below the black skin formed in the hot rolling stage.
It remains even after the subsequent steps such as pickling, cold rolling and annealing. FIG. 4 shows the results of elemental analysis of the steel sheet according to the present invention after cold rolling and annealing by an emission analysis method by glow discharge (GDS) from the surface layer to a depth of about 10 μm.
According to this, the peaks of Si, Mn, and P that appear at a depth of about 0.5 to 3 μm from the surface layer correspond to the above oxides.

Usually, in reduction annealing with CGL, Si, M
n, etc. are concentrated surface is selectively oxidized, but since these elements are less likely to move from the bulk of the steel sheet surface near to the surface, the movement of oxygen into the interior of is promoted conversely, generates an internal oxide layer However, surface concentration on the outermost surface is suppressed. In other words, the surface oxide layer changes the outward diffusion of metal elements (surface concentration) to the inward diffusion of oxygen (internal oxidation).
Therefore, the surface of the steel sheet does not have an oxide coating such as Si or Mn that deteriorates the plating adhesion and the chemical conversion treatment property, and the performance of the steel sheet is good. Therefore, according to the present invention, Si,
Various surface treatment of high-strength steel sheet alloying elements such as Mn are added in combination, specifically, electric plating resistance, Kasei processability, such as chromate treatment is to dramatically improved.

[0017]

[0018]

Further, the high-strength cold-rolled steel sheet according to the present invention does not cause "non-plating", "uneven amount of adhesion", etc. for the same reason as described above even if it is subjected to electroplating, chemical conversion treatment and chromate treatment. . In this case, Si is 0.1 wt% or more or M
A steel plate containing 0.5 wt% or more of n may be used. Furthermore, the high-strength cold-rolled steel sheet according to the present invention exhibits excellent performance even when the treatment target is electrogalvanizing, electrogalvanizing-nickel plating, electrogalvanizing-iron plating, or the like. This is Si or Mn
Since the concentration of oxides such as iron on the surface is suppressed, the decrease in the electrodeposition reactivity between the iron on the steel plate surface and the metal ions in the electroplating bath is suppressed. This is because "unevenness" does not occur. The chemical conversion treatment and the chromate treatment are improved for the same reason.

[0020] it follows, Si, Mn, Cr, in order to dramatically improve electrical plating of reinforced high-strength cold-rolled steel sheet element is added, of forming processability, a chromate treatment of such as P, After all, it is most effective and appropriate to remarkably suppress the surface concentration of Si, Mn and the like.

Next, the reasons for limiting the components used in the present invention and other conditions will be described. In the first invention, the lower limit of the Si amount is set to 0.1.
wt%, to that of the lower limit of the Mn content and 0.5 wt% in the second invention, an oxide of the steel plate surface layer, 0.1 wt content of Si
% Less than Der Ru steel plate, or the content of Mn 0.5 wt
The% less than Der Ru steel plate, degreasing of the steel sheet after annealing, because it can be removed sufficiently during pickling, basic sulfate bath using conventional plating tank, or can be electroplated with hydrochloric acid bath method Because there is. Furthermore, the same applies to the chemical conversion treatment and the chromate treatment, and the chemical conversion treatment and the chromate treatment can be similarly performed without changing the conventional equipment or the conventional method .

In the first invention, the upper limit of the amount of Si is 3.0 wt%,
In the second invention, the upper limit of the amount of Mn is set to 2.0 wt% is that S
i amount exceeds 3.0 wt% or Mn amount is 2.0
This is because if it exceeds wt%, an oxide film is formed on the surface of the steel sheet, and the adhesion to the plating bath, the chemical conversion treatment liquid, and the chromate treatment liquid is significantly reduced. In the second invention, Si is 0.1 w
Even when it is less than t%, Mn is 2.0 to 0.
A high-strength cold-rolled steel sheet having the characteristics of the present application is obtained only by limiting the content to 5 wt%, and the lower limit of Si is 0.00
1 wt% is an unavoidable content.

Similarly, 0.1 wt% or more of Cr in steel,
Even in a high-strength steel sheet containing 2.0 wt% or less, Cr in the steel is selectively oxidized by heating the steel sheet surface in the annealing process and diffused to the steel sheet surface, so these oxides are concentrated and the steel sheet surface To form a film. Further, this oxide layer is sufficiently will not be removed by pickling, the steel sheet
Electroplating, chemical conversion treatment, and chromate treatment are markedly inhibited, and " non-plating", "repelling", and "uneven amount of adhesion" occur. However, in the present invention, the crystal grain boundaries of the surface layer of the high-strength steel sheet, or within the grains, or within the crystal grain boundaries and grains,
When oxides are generated, surface enrichment of Si, Mn, P, Cr, etc. is suppressed, and these oxides do not form on the surface of the steel sheet, so these "non-plating", "repelling", and "uneven amount of adhesion" Does not happen. Therefore, 0.1 w of Cr in steel
The present invention is also effective for a high-strength cold-rolled steel sheet containing t% or more and 2.0 wt% or less.

P is an essential element for high-strength steel sheets because P has a large degree of deterioration in deep drawability and can harden steel sheets and B has a great effect on the secondary work embrittlement of steel. These are selectively oxidized by heating the surface of the steel sheet in the annealing process and diffused into the surface layer of the steel sheet, but do not significantly impair the wettability with molten zinc. In addition, even if degreasing and pickling after annealing is not sufficient and remains on the surface layer, "non-plating"
It is also unlikely to cause "repellent" or "uneven chemical conversion treatment". However, if P is contained in a large amount, alloying delay may occur, so 0.10 wt%
The content is as follows, but the content of B is not particularly limited.

In addition, the addition of Mo has the effect of improving the mechanical properties of the steel sheet. Therefore, in the present invention, the content is 0.5 wt% or less. On the other hand, the reason why the thickness of the oxide layer is limited to 0.1 or more and 100 μm or less is 0.1
This is because if the thickness is less than or equal to μm, the amount of oxide itself according to the present invention is too small to suppress the surface thickening, and if the thickness is greater than or equal to 100 μm, the oxide is brittle, and the machine of the steel sheet itself is poor. This is because there is a possibility that the physical characteristics may deteriorate.

[0026]

EXAMPLES The contents of the present invention will be supplemented below with reference to examples. The high-strength steel sheets having the compositions shown in Table 2 were hot-rolled, then pickled and cold-rolled. Then, the above-mentioned various surface treatments were performed to manufacture surface-treated steel sheets as listed in Tables 3 and 4. The condition is 860 to 910 ° C. after performing slab heating at 1200 to 1250 ° C. and then hot rolling.
Then, it was finish-rolled and wound into a coil at a winding temperature of 450 to 740 ° C. Next, the black skin of the hot rolled steel sheet was removed by pickling, and then cold rolling, reduction annealing and various surface treatments were performed. No. No. 1 is 850
° C , No. 2 was performed at 860 ° C. Moreover, in Tables 3 and 4, "thickness" represents the thickness of the range in which the oxide is distributed from the steel plate surface layer .

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030] The surface treatment method, the appearance of the steel sheet subjected to various surface treatment method such as electroplating or chemical convertibility, whether good in terms of the visually observed, "non-coating"
It was judged whether or not there was any occurrence, and whether there was "uneven amount of adhesion" . Na us, the presence observation of the oxide immediately below the steel sheet surface layer was performed etched by 1% nital solution after cross polishing.

The results of various surface treatments produced as described above are shown in Tables 3, 4, and 5 above. Various defects such as "non-plating" and "uneven amount of adhesion" when the outside of the claims of the present invention (comparative example) and deterioration of mechanical properties of the steel sheet itself when the oxide layer is as thick as 150 μm occur. did. However, within the scope of the claims of the present invention (Example),
Not to mention "non-plating" and "uneven amount of adhesion", there were no problems such as deterioration of mechanical properties of the steel sheet itself.

[0032]

[Table 5]

[0033]

As described above, the high-strength cold-rolled steel sheet according to the present invention has a crystal grain boundary of the steel sheet surface layer, or within the crystal grain, or an oxide is formed at the crystal grain boundary and within the grain. Even if it contains Si, Mn, Cr, etc., it is possible to perform various surface treatments that were difficult in the past as efficiently as ordinary steel, and it is extremely effective for the production of highly functional surface-treated steel sheets. is there.

[Brief description of drawings]

FIG. 1 is a diagram showing an oxide layer at a crystal grain boundary of a surface layer of a hot rolled steel sheet.

FIG. 2 is a diagram showing a sectional analysis result of the oxide layer by EPMA.

FIG. 3 is a view showing a cross section of a high-strength cold-rolled steel sheet according to the present invention, taken by an electron microscope.

FIG. 4 is a diagram showing a result of a depth direction analysis of a steel plate cross section by glow discharge.

[Explanation of symbols]

1 grain boundary Within 2 grains 3 black skin 4 Steel plate surface 5 Oxide layer

─────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-276060 (JP, A) JP-A-8-291367 (JP, A) JP-A-5-125485 (JP, A) JP-A-7- 118796 (JP, A) Patent 2978096 (JP, B2) Patent 3126911 (JP, B2) Patent 2972124 (JP, B2) (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00-38 / 60

Claims (2)

(57) [Claims] 1. Electroplating or chemical conversion treatment, or
Is a cold-rolled steel sheet to be subjected to chromate treatment, and is a grain boundary of a high-strength cold-rolled steel sheet containing Si in an amount of 0.1 wt% to 3.0 wt% inclusive, from the steel plate surface layer to a depth of 0.1 to 100 μm. and / or in the crystal grains, electroplating properties,
Kasei process and effectiveness oxide to improved chromate treatment _{resistance, SiO 2, MnO, FeSiO 3} , Fe 2 SiO 4, M
A high-strength cold-rolled steel sheet, which is formed by forming at least one selected from nSiO ₃ , Mn ₂ SiO _4, and P ₂ O ₅ . 2. Electroplating or chemical conversion treatment, or
Is a cold-rolled steel sheet subjected to chromate treatment, and as a composition element of the steel sheet, C: 0.03 to 0.10 wt% Si: 0.001 to less than 0.10 wt% Mn: 0.5 to 2.0 wt % P: 0.01-0.10 wt% Mo: 0.50 wt% 0.1-1 from the steel plate surface layer of the high-strength cold-rolled steel plate
Within a grain boundary and / or grain to a depth of 00 μm ,
Electroplating resistance, reduction conversion treatment and effectiveness oxide to improve the chromate treatment _{of, SiO 2, MnO, FeSiO 3} , Fe
A high-strength cold-rolled steel sheet formed by forming at least one selected from ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO _4, and P ₂ O ₅ .