JP2003147486A - High-strength cold rolled steel sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high-strength cold rolled steel sheet free from the occurrence of 'bare spots' at hot dipping and 'nonuniform alloying' with alloying after plating and having excellent hot-dip platability and alloying ability after plating by devising the base material of a steel sheet. SOLUTION: The high-strength cold rolled steel sheet has oxides effective for the improvement of the above hot-dip platability, etc., in the grain boundary and/or crystal grain of the surface layer of the high-strength cold rolled steel sheet containing 0.1 to 3.0 wt.% Si.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength cold-rolled steel sheet, and particularly to a high-strength cold-rolled steel sheet used for automobile bodies and the like, and optionally subjected to hot dipping, alloying treatment after plating, and the like. Related to steel plate.

[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, a method has been proposed in which the surface of the steel sheet is subjected to undercoat plating with Ni prior to hot-dip plating (see Patent Document 1). However, in this method, 0.1 wt% of Si is used.
% Or more and 3.0 wt% or less, or 0.03 to 0.10 wt% C and 0.001 to 0.1 Si
Less than 0 wt%, Mn 0.5-2.0 wt%, P 0.
When a steel plate containing 01 to 0.10 wt% and 0.50 wt% or less of Mo is targeted, the adhesion amount is 10 g.
It was necessary to apply Ni plating of not less than / m ² , resulting in a significant 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, there is a method of applying Fe undercoat on the surface of the steel sheet instead of the above Ni plating (see Patent Document 2). However, this method was able to prevent "non-plating" of Si-added steel, but for that reason 5 g /
It was necessary to plate Fe of min ² or more, which was extremely uneconomical as in the case of Ni.

[0006] Furthermore, after cold rolling, the steel sheet is pre-oxidized to form an iron oxide film on the surface, and then reduction annealing is performed to suppress the formation of an oxide film of an alloy element and then perform desired plating. Has been disclosed (see Patent Documents 3 and 4). However, since these methods are methods of controlling the thickness of the iron oxide film remaining before plating by reduction annealing to a certain value or less, they are excessively reduced by reduction annealing, and the alloy elements are concentrated on the surface. There is a problem that the plating property becomes poor, that is, the balance between the oxide film and the reduction amount is lost. In addition, to prevent this overreduction,
Since an enormous amount of iron oxide is required, the iron oxide film is peeled off by rolls etc. during rolling, and selective oxidation of alloying elements occurs during subsequent reduction annealing, which hinders platability or peels off. There is also a problem that the iron oxide film is scattered in the annealing furnace and adversely affects the operation.

As described above, even a high-strength steel sheet, which is currently attractive as a high-strength material for automobiles, is still unsuitable for use as a surface-treated steel sheet having good corrosion resistance after being subjected to hot dipping or the like. The reality is that there is room for improvement.

[0008]

[Patent Document 1] Japanese Examined Patent Publication No. 61-9386 (page 3, left column, lines 13 to 22)

[Patent Document 2] JP-A-57-70268 (page 3,
(Line 17 in the upper left column to Line 6 in the upper right column)

[Patent Document 3] JP-A-55-122865 (2
(Page, lines 6-18 in the upper right column)

[Patent Document 4] Japanese Unexamined Patent Publication No. 4-254531 (page 2,
Paragraph [0008] in the right column)

[0009]

In view of the above situation, the present invention has devised a base material of a steel sheet so that "non-plating" during hot dipping and "uneven alloying" during alloying treatment after plating. It is an object of the present invention to provide a high-strength cold-rolled steel sheet which is excellent in hot dip galvanizing property and alloying processability after plating without causing the above phenomenon.

[0010]

As mentioned above, Si,
When high-strength cold-rolled steel sheet to which solid solution strengthening elements such as Mn are added together is hot-dipped, these elements concentrate on the surface to form a film and wet the hot-dip zinc or chemical conversion solution with the steel sheet. "Non-plating" occurs because it inhibits the property. 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, and FeSi are previously formed in the crystal grain boundaries of the high-strength cold-rolled steel sheet surface containing the specified component, or in the grain boundaries, or in the crystal grain boundaries and the grains.
O ₃ , Fe ₂ SiO ₄ , MnSiO ₃ , Mn ₂ SiO
The inventors have found that the generation of oxides such as ₄ , P ₂ O _{5 and the} like dramatically improves the hot-dip galvanizing properties of the high-strength steel sheet and the alloying processability after plating, and completed the present invention.

That is, the present invention is a high-strength cold-rolled steel sheet used for hot dip coating, wherein the high-strength cold-rolled steel sheet containing Si in an amount of 0.1 wt% or more and 3.0 wt% or less is 0. SiO ₂ , MnO, FeSiO ₃ , Fe ₂ are present in the crystal grain boundaries and / or the crystal grains to a depth of 1 to 100 μm.
A high-strength cold-rolled steel sheet, characterized in that one or more kinds of oxides selected from SiO ₄ , MnSiO ₃ , Mn ₂ SiO ₄ and P ₂ O ₅ are formed.

Further, the present invention is a high-strength cold-rolled steel sheet used for hot dip coating, wherein C: 0.03 to 0.10 wt% Si: 0.001 to 0.10 wt% as a composition element of the steel sheet. Less than Mn: 0.5 to 2.0 wt% P: 0.01 to 0.10 wt% Mo: 0.5 to 1 wt% from the steel plate surface layer of the cold rolled steel plate containing 0.50 wt% or less.
Within a grain boundary and / or grain to a depth of 00 μm,
SiO ₂ , MnO, FeSiO ₃ , Fe ₂ SiO ₄ , MnS
A high-strength cold-rolled steel sheet, characterized in that one or more oxides selected from iO ₃ , Mn ₂ SiO ₄ and P ₂ O ₅ are formed.

According to the present invention, the hot-dip galvanizing property and the alloying processability after plating can be obtained without causing "non-plating" during hot-dip plating and "uneven alloying" during alloying process after plating. It becomes possible to manufacture an excellent high-strength cold-rolled steel sheet.

[0016]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.

Oxides existing in the grain boundaries in the surface layer of the high-strength cold-rolled steel sheet according to the present invention, or in the grains, or in the grain boundaries and in the grains are hot-rolled before the cold rolling of the steel sheet. It is generated during rolling. 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.

[0018]

[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
Although n and the like are selectively oxidized and surface-concentrated, in the high-strength cold-rolled steel sheet according to the present invention, it becomes difficult for these elements to move from the bulk in the vicinity of the steel sheet surface to the surface, and conversely the movement of oxygen to the inside is promoted. As a result, an internal oxide layer is generated, and surface concentration on the re-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 film of Si, Mn, etc. 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, various surface treatability of the high-strength steel sheet to which alloy elements such as Si and Mn are added, specifically, the hot dip galvanizability and the alloying treatability after plating are dramatically improved. To do.

To give an example of the hot dip galvanizing property, the steel contains 0.1 wt% to 3.0 wt% of Si, or 0.03 to 0.10 wt% of C and Si.
Of 0.001 to less than 0.10 wt% and Mn of 0.5 to
A high-strength cold-rolled steel sheet containing 2.0 wt%, 0.01 to 0.10 wt% of P, and 0.50 wt% or less of Mo, respectively, but containing no oxides in the grain boundaries or grains of the surface layer. When hot-dip galvanizing is performed in a normal process, Si and Mn in steel are selectively oxidized by heating the steel sheet surface in the annealing process before plating, and oxides of Si and Mn are formed on the steel sheet surface. Since the oxides of Si and Mn are not reduced even by reduction annealing, the wettability between the steel sheet and molten zinc sharply decreases as the Si content in the steel increases, and so-called "non-plating" occurs.

However, in the present invention, since the oxide is formed in the surface layer of the steel sheet in advance, the surface concentration is suppressed and the oxide of Si or Mn is not formed on the surface of the steel sheet. The effect is that Si is 0.1 wt% or more, or Mn is 0.1% or less.
It is the largest when the content is 5 wt% or more.

The hot-dip galvanizing method applicable to the high-strength cold-rolled steel sheet according to the present invention is not limited to hot-dip galvanizing, and hot-dip aluminum plating or hot-dip aluminum-zinc plating 5% aluminum-zinc plating (commonly known as: Galvalume plating) etc. This is because oxides such as Si and Mn are suppressed from being concentrated on the surface, and thus wettability between a steel sheet and a molten metal such as aluminum is improved, not limited to zinc. Therefore, after all, by forming an oxide on the surface layer of the high-strength cold-rolled steel sheet in advance, Si
Since the concentration of oxides such as Mn and Mn on the surface is suppressed,
Even with a high-strength steel sheet containing a large amount of Si or Mn, the hot-dip galvanizability becomes good regardless of the metal species.

The same applies to alloying after plating, and it is not only the plating property that is correlated with the amount of surface concentration,
It has been confirmed that there is a correlation with the alloying rate, and that the smaller the amount of surface concentration, the better the plating property and the faster the alloying rate. Therefore, in order to dramatically improve the hot dip galvanizing property and the alloying processability of the high strength cold rolled steel sheet to which strengthening elements such as Si, Mn, Cr and P are added, after all, Si, Mn, etc. It is most effective and appropriate to remarkably suppress the surface thickening.

Next, the reasons for limiting the components and other conditions adopted in the present invention will be explained.

In the first invention, the lower limit of the Si amount is 0.1 wt%,
In the second invention, the lower limit of the Mn amount is set to 0.5 wt% because the range is less than this, even if the present invention is not applied, an ordinary radiant tube (RTH) type or non-oxidizing furnace (NOF) is used.
This is because good hot dip galvanizing can be performed using the mold CGL. Also, regarding the alloying reaction, there is no particular decrease in the alloying reaction rate, and the same alloying equipment and alloying temperature as before, alloying time, alloying time, heating rate during heating, cooling rate during cooling, etc. Since alloying is possible by the first invention, the Si content is 0.1 wt% or more in the first invention, and the Mn content is 0.5 wt% or more in the second invention.

In the first invention, the upper limit of the Si amount 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 with the plating bath is significantly reduced.

The second aspect of the present invention is to limit the Mn to 2.0 to 0.5 wt% as described above even when Si is less than 0.1 wt%, and only to limit the high strength cold rolling having the characteristics of the present application. A steel plate is obtained, and the lower limit of Si of 0.001 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 not sufficiently removed even by pickling, the wettability between the steel sheet and the molten zinc is significantly impaired, and so-called "non-plating" in which molten zinc does not adhere to the steel sheet often occurs.

However, in the present invention, when an oxide is generated at the crystal grain boundaries of the surface layer of the high-strength steel sheet, or within the grain boundaries, or within the crystal grain boundaries and the grain boundaries, Si, Mn, P, Cr
The surface thickening such as is suppressed, and these oxides are not formed on the surface of the steel sheet, so "non-plating" does not occur. Therefore, the present invention is also effective for a high-strength cold-rolled steel sheet containing Cr in an amount of 0.1 wt% or more and 2.0 wt% or less.

P is an essential element for high-strength steel sheets, since P has the ability to harden steel sheets with little deterioration in deep drawability 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 However, if P is contained in a large amount, it may cause alloying delay, so the content is set to 0.10 wt% or less, 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 thickness of the oxide layer is 0.1 or more and 10
What is limited to 0 μm or less is that it is 0.1 μm or less,
This is because the production amount of the oxide according to the present invention is so small that the surface concentration cannot be suppressed.
This is because if the thickness is 0 μm or more, the oxide is brittle, and the mechanical properties of the steel sheet itself may deteriorate.

[0034]

EXAMPLES The contents of the present invention will be supplemented below with reference to examples.

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 conditions are 860 to 860 after performing slab heating at 1200 to 1250 ° C. and then hot rolling.
Finish rolling was performed at 910 ° C., and coiling was performed 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
850 ° C., No. 2 is 880 ° C., No. 2 3 is 840
° C, No. 5 is 860 ° C., No. 5 6 is 870 ° C., No. 6
7 is 860 ° C., No. 7 8 is 850 ° C., No. 8 9 is 870
Performed at ° 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. Note that N
o. No. 8 has a chemical composition corresponding to the second aspect of the invention. In this case, the oxide effective in improving the plating property generated in the crystal grain boundaries and / or in the grains is MnO, since the Si content is small.
The main component is P ₂ O ₅ .

[0036]

[Table 2]

[0037]

[Table 3]

[0038]

[Table 4]

As a surface treatment method, the hot dip galvanizing bath was a bath containing 0.16% aluminum concentration and the plating temperature was 480 ° C. Regarding the appearance of the steel sheet that has been subjected to surface treatment methods such as hot dip coating and alloying, whether it is good by visual observation, whether "non-plating" has occurred, and whether there is "uneven amount of adhesion" Etc. The alloying treatment temperature was variously changed in the temperature range of 430 ° C to 600 ° C.
Regarding the alloying state, after alloying, uneven alloying visually,
It was evaluated after confirming that there was no delay in alloying. In addition, observation of the presence or absence of oxides just below the surface layer of the steel plate,
After polishing the cross section, etching was performed with a 1% nital solution.

The results of various surface treatments produced as described above are shown in Tables 3, 4, and 5 above. In the case of outside the scope of the claims of the present invention (comparative example), various problems such as “non-plating” and deterioration of mechanical properties of the steel sheet itself when the oxide layer was as thick as 150 μm occurred. However, within the scope of the claims of the present invention (Examples), no problems such as deterioration of mechanical properties of the steel plate were caused, let alone "non-plating".

[0041]

[Table 5]

[0042]

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

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Nobuo Totsuka             1, Kawashima-dori, Mizushima, Kurashiki City             Saki Steel Co., Ltd. Mizushima Steel Works

Claims (2)

[Claims] 1. A high-strength cold-rolled steel sheet used for hot dipping
Yes, contains Si of 0.1 wt% or more and 3.0 wt% or less
0.1-100μ from the steel plate surface layer of the high strength cold rolled steel plate
At the grain boundaries and / or within the crystal grains up to a depth of m, SiO
₂, MnO, FeSiO₃, Fe₂SiO_Four, MnSi
O₃, Mn₂SiO_FourAnd P₂O _FiveOne or more selected from
High-strength cold-rolled steel characterized by being formed with an oxide
Board. 2. A high-strength cold-rolled steel sheet used for hot dip coating, wherein C: 0.03 to 0.10 wt% Si: 0.001 to less than 0.10 wt% Mn: 0 as a composition element of the steel sheet. 0.5 to 2.0 wt% P: 0.01 to 0.10 wt% Mo: 0.5 to 1 wt% from the steel plate surface layer of the cold rolled steel sheet containing 0.50 wt% or less.
Within a grain boundary and / or grain to a depth of 00 μm,
SiO ₂ , MnO, FeSiO ₃ , Fe ₂ SiO ₄ , MnS
A high-strength cold-rolled steel sheet comprising one or more oxides selected from iO ₃ , Mn ₂ SiO _4, and P ₂ O ₅ .