JP2978096B2 - High strength hot-dip galvanized steel sheet with excellent plating properties - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-dip galvanized steel sheet (including a high-strength galvannealed steel sheet.) .).

[0002]

2. Description of the Related Art In recent years, it has been required to reduce the weight of an automobile body from the viewpoint of exhaust gas regulations. One method for reducing the weight of the vehicle body is to reduce the plate thickness. In this method, to ensure safety, it is necessary to increase the strength of the sheet by the thickness of the sheet, and therefore, alloy elements such as Si, Mn, Cr, and P are added to the steel to High strength must be achieved. Moreover, since excellent press workability is required for automotive steel sheets,
It is necessary to improve various material characteristics represented by r value as a material, but it has become clear that composite addition of alloying elements such as Si and Mn is necessary to obtain a high r value. . Against this background, conventional high-strength steel sheets for automobiles contain a large amount of alloying elements such as Si, Mn, Cr, and P.

[0003] When a high-strength steel sheet is annealed by hot-dip galvanizing equipment and plated, it is necessary to anneal at a high temperature of 800 ° C or higher in order to obtain excellent press workability. Usually, reduction annealing is performed in an N ₂ -H ₂ atmosphere.
A reducing atmosphere for Fe is an oxidizing atmosphere for Si, Mn, Cr and the like. Therefore, many of these elements are selectively oxidized to oxides, forming a so-called surface-concentrated film on the steel sheet surface. And these oxides significantly impair the wettability with the molten zinc and deteriorate the plating adhesion, so that the molten zinc hardly adheres to the steel sheet,
So-called non-plating often occurs. Therefore, there was a problem that hot-dip galvanizing of a high-strength steel sheet having excellent press workability was not possible.

As one of the methods for overcoming such problems, Japanese Patent Publication No. 9386/1986 proposes a method of applying a Ni base plating to the surface of a steel sheet prior to hot-dip plating. However, in this method, when a steel containing 0.2 to 2.0 wt% of Si is targeted, the adhesion amount is 10 g / m ² or more.
Ni plating must be performed, which has led to an increase in cost. Moreover, when such a large amount of Ni plating is applied, although the wettability of hot-dip galvanizing is improved,
There was a problem that defects caused by Si and Ni frequently occurred on the plating surface during the alloying process.

[0005] For example, Japanese Patent Application Laid-Open No. 57-70268 proposes a method of subjecting a steel sheet to a base plating of Fe prior to hot-dip plating. According to this method, it is possible to prevent the non-plating of the Si-added steel by the base plating, but for that purpose, it is necessary to apply Fe plating of 5 g / m ² or more, which is extremely uneconomical.

Another method is disclosed in Japanese Patent Application Laid-Open No. Sho 55-12
There are methods disclosed in JP-A-2865 and JP-A-4-254453. In these methods, a steel sheet is oxidized in advance to form an iron oxide film on its surface, and then subjected to reduction annealing to control the formation of an oxide film of an alloy element and to perform plating. However, since these methods control the thickness of the iron oxide film remaining before plating in reduction annealing to a certain value or more, it is excessively reduced during reduction annealing, and the alloy elements are concentrated on the surface to increase the plating property. However, there is a problem that the balance of the oxidation amount and the reduction amount is lost. Moreover, in order to prevent this excessive reduction, an enormous amount of iron oxide is required, so that the iron oxide film is peeled off by a roll or the like, and the selective oxidation of the alloy elements during the subsequent reduction annealing occurs, and However, there has been a problem that the properties are impaired, and the peeled-off iron oxide film is scattered in the furnace and adversely affects the operation.

[0007]

As described above,
In each of the above prior arts, the fact is that even high-strength steel sheets that are attractive as high-strength materials for automobiles lack practical means for hot-dip galvanizing them. The purpose of the present invention is
High-strength steel containing 0.1 to 2.0 wt% of Ni, has excellent plating properties comparable to ordinary steel, specifically, does not cause non-plating and has excellent plating adhesion even after pressing. It is to propose a high strength hot-dip galvanized steel sheet having characteristics.

[0008]

Means for Solving the Problems As a result of intensive research to realize the above-mentioned objects, when plating a high-strength steel sheet to which alloying elements such as Si and Mn are added in a complex manner, these components are not included during annealing. It has been found that non-plating occurs due to the formation of a film by surface concentration and inhibition of wettability with molten zinc. Therefore, when hot-dip galvanizing a high-strength steel sheet, it is necessary to suppress this surface concentration.

In this regard, the inventors' research has confirmed that there is a correlation between the above-mentioned surface concentration and the plating property and alloying rate. Has good plating properties and a high alloying speed. In addition, since there is a difference in the alloying speed depending on the amount of surface enrichment, if there is a place where the amount of enrichment is high and a place where the amount of enrichment is small in the same coil, a difference occurs due to the alloying speed even if alloying is performed under the same conditions. It has also been found that this causes poor alloying. In this case, a defective coil is formed and the operability deteriorates.

Therefore, in order to obtain a clue to the improvement of the plating property, using high-strength steel sheets manufactured by various methods, the structure of a portion having a different plating property and an alloying speed, that is, a portion having a different surface concentration amount is considered. Observed in detail. As a result, the inventors have found that oxides are generated at the grain boundaries of the base iron immediately below the plating layer in portions where the amount of surface enrichment is low (portions where the alloying speed is high), and conversely, the amount of surface enrichment is large. It was found that no oxide was formed in the portion (the portion where the alloying speed was low). From this new finding, if oxides are formed in advance at the grain boundaries on the steel sheet surface before plating, Si can be 0.1 to 2.
It has been concluded that good plating properties can be obtained even with a high-tensile steel containing 0 wt%, and the present invention has been completed. That is, the gist configuration of the present invention is as follows.

(1) A steel sheet containing 0.1 to 2.0 wt% of Si and having a plating layer on at least one side, characterized in that it has an oxide at a crystal grain boundary immediately below the plating layer, and has excellent plating properties. High-strength galvanized steel sheet.

(2) A high-strength hot-dip galvanized steel sheet obtained by alloying the plated layer according to claim 1.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. (1) Composition of steel When hot-dip galvanizing is performed in a normal process using a high-strength steel sheet containing 0.1 to 2.0 wt% of Si in the steel, the Si content in the steel during the annealing process before plating is increased. Is selectively oxidized by heating the surface of the steel sheet and diffuses into the surface layer of the steel sheet, so that the Si oxide forms a film on the steel sheet surface. Since this Si oxide is not reduced even by reduction annealing, it greatly inhibits wettability with molten zinc and lowers plating property. However, in the present invention, due to the presence of the above-mentioned grain boundary oxide, the surface concentration of Si is suppressed, and the Si oxide is not formed on the surface of the mother plate,
Plating can be performed without any problem. This effect is particularly apparent in a high-strength steel sheet containing 0.1 wt% or more of Si in steel. Therefore, the steel composition of the present invention is a steel containing 0.1 wt% or more of Si. This is because, when the content of Si is less than 0.1 wt%, hot-dip galvanizing can be performed with a normal radiant tube (RTH) type or non-oxidizing furnace (NOF) type CGL without particularly applying the present invention. On the other hand, 2.0 wt%
%, An oxide film is formed on the surface, and the adhesion to the plating bath is remarkably reduced. Therefore, the content of Si is set to 2.0 wt% or less.

In addition to the above-mentioned Si, Mn and Cr, which are often used as a strengthening element in high-strength steel and affect plating properties, will be described. This Mn and Cr also
During the annealing process, it is selectively oxidized by heating the surface of the steel sheet and diffused into the surface layer of the steel sheet, so that oxides of these elements form a film on the steel sheet surface. As a result, like Si,
It significantly impairs the wettability with molten zinc and reduces the plating property. However, in the present invention, these elements also suppress the surface concentration due to the presence of the above-mentioned grain boundary oxide, and Mn and Cr oxides are not formed on the surface of the mother plate, so that the plating property does not deteriorate. This effect is particularly effective in reducing Mn and Cr in steel by 0.5 wt% each.
%, 0.1 wt% or more. However, when Mn exceeds 2.0 wt%, the deep drawability is adversely affected, and when Cr exceeds 2.0 wt%, the strength improving effect is saturated. Therefore, the addition amount of Mn is 0.5 to 2.0 wt.
% And the amount of Cr added are preferably in the range of 0.1 to 2.0 wt%.

P is an element useful for strengthening steel with little deterioration in deep drawability, and B is an element useful for improving secondary work brittleness resistance of steel. These elements are selectively oxidized by heating the steel sheet surface during the annealing process and diffused into the surface layer of the steel sheet, but do not significantly impair the wettability with the molten zinc. In addition, in the present invention, since the surface concentration is suppressed by the presence of the grain boundary oxide, the influence is further reduced. Therefore, it is not necessary to limit the amounts of P and B added.

(2) Regarding Grain Boundary Oxide Immediately Below Plating Layer The greatest feature of the present invention is that the grain boundary oxide is present in the base plate immediately below the plating layer. FIG. 1 shows a cross-sectional structure of a hot-dip galvanized steel sheet (a) according to the present invention observed by an optical microscope (etching with a 1% nital solution for several seconds to several tens of seconds) in comparison with the conventional one (b). is there. FIG. 2 shows the result of analysis of this streak by EPMA. In FIG. 2, peaks of Si, Mn, P, and O are observed, and it is understood that the streaks shown in FIG. 1 are oxides composed of these components.
When the distribution of the streaks was observed by SEM, it was found that the streaks were located on the base plate side from the boundary between the plating layer and the base plate, and were present at the crystal grain boundaries. From these results, in the hot-dip galvanized steel sheet according to the present invention, a grain boundary oxide exists in the mother plate immediately below the plating layer, and this oxide has an effect of suppressing the surface concentration of components such as Si, Mn, and Cr. It can be said that it is bringing. On the other hand, in the conventional hot-dip galvanized steel sheet, as can be seen from the structure of FIG. 1, no grain boundary oxide as observed in the coated steel sheet according to the present invention is observed.

The mechanism by which the grain boundary oxide immediately below the plating layer improves the plating properties is not necessarily clear, but the inventors consider as follows. Usually, in reduction annealing in a hot-dip galvanizing line (CGL), Si, Mn, etc. are selectively oxidized and the surface is concentrated. However, as described in the present invention, when the above-mentioned grain boundary oxide is present in a plating base plate, The transfer of metal elements such as Si and Mn from the bulk to the surface is suppressed, and conversely, the transfer of oxygen to the inside is promoted, so that an internal oxide layer is formed and the surface concentration at the outermost surface is suppressed. You. In other words,
The surface oxide layer changes from outward diffusion of metal elements (surface concentration) to inward diffusion of oxygen (internal oxidation). Therefore, it is considered that there is no oxide film such as Si or Mn that deteriorates the plating adhesion on the steel sheet surface, and the plating adhesion is improved, and the non-plating is eliminated.

The reason why the grain boundary oxide immediately below the plating layer improves the adhesion during press working will be described. Usually, at the time of press working, plating peeling easily occurs mainly due to compressive stress at the time of working, and this phenomenon is particularly noticeable in alloyed hot-dip galvanized steel sheets. Here, when alloying a normal hot-dip galvanized steel sheet, Zn-Fe is diffused into the crystal grain boundary immediately below the plating layer of the base plate by thermal diffusion of Zn and Fe.
An Fe alloy is formed. On the other hand, when the grain boundary oxide is present in the mother plate immediately below the plating layer as in the present invention, Zn is more easily permeated through the intergranular gap than in the above-described normal plated steel sheet. Become. Therefore, the unevenness of the interface between the plating layer and the base plate increases, and the adhesion between the two becomes stronger. As a result, it is considered that the galvannealed steel sheet alloyed according to the present invention significantly improves the plating adhesion during press forming. Such improvement in plating adhesion at the time of press molding was confirmed by inspection with an optical microscope, and it was confirmed that even a small amount of grain boundary oxide was present at the crystal grain boundary immediately below the plating layer of the mother plate.

As a result of further investigation, the inventors have found that the grain boundary oxide which produces such an effect and is observed on the mother plate immediately below the plating layer is generated during hot rolling.
In particular, it was confirmed that the product was grown when the coil winding temperature was high and the cooling rate thereafter was low. FIG. 3 shows a cross-sectional observation result of a hot-rolled steel sheet. As shown in FIG. 3, the grain boundary oxide is observed immediately below the black scale of the hot-rolled steel sheet. The grain boundary oxide immediately below the black scale generated in the hot rolling stage as described above remains even after subsequent processes such as pickling, cold rolling, annealing, and plating. FIG. 4 shows the results of elemental analysis of a cold-rolled steel sheet after annealing and before plating by glow discharge (GDS) to a depth of about 10 μm from the surface (sputtering time: 750.0 seconds) . From Fig. 4, the sputtering time
The peaks of Si, Mn, and P, which appear at 37.5 to 225 seconds, that is, at a position of about 0.5 to 3 μm from the surface, correspond to grain boundary oxides.

Now, a method for producing a high-strength hot-dip galvanized steel sheet according to the present invention will be described. Hot rolling is
In order to generate a sufficient length of grain boundary oxide just below the black scale,
It is desirable to perform winding in the range of 690 to 750 ° C. Then, so as to remove the generated grain boundary oxides and remove only black scales, without performing any mechanical pretreatment such as grinding,
Pickling (pickling conditions are, for example, 3% to 10% HCl in 40 to 40%).
After treatment at 90 ° C. for 10 seconds to 30 seconds), cold rolling is performed as it is, and an oxide layer is present inside the surface layer so that the CGL line is passed. The plating layer is not particularly limited, but from the viewpoint of corrosion resistance and the like, as a steel sheet for automobiles, the adhesion amount of the Zn—Fe alloy is 25 to 90 g / m ² ,
The Zn content in the plating layer is suitably from 8 to 13% by weight. The conditions of the hot dip galvanizing bath are not particularly limited, but the Al concentration in the galvanizing bath is about 0.13 to 0.14 wt%,
The Fe concentration is preferably at least 0.01 wt% and the saturation concentration, and Pb, Mg, Mn, etc. may be further added to this bath. Further, if necessary, it is immediately heat-alloyed to produce an alloyed hot-dip galvanized steel sheet. The heat treatment at the time of alloying requires a long-time heating at a low temperature of less than 460 ° C. and lowers the productivity.

[0021]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below based on embodiments. High-strength steel material having the composition shown in Table 1 was hot-rolled (slab heating temperature 1100 to 1250 ° C, finish rolling temperature 850 to 95).
0 ° C.), 690-750 ° C. in the invention example, and 6 ° C. in the comparative example.
After winding at 00 to 650 ° C., only black scale was removed by pickling, followed by cold rolling, and then reduction annealing and plating with CGL to produce a high-strength galvanized steel sheet. In addition, a high-strength hot-dip galvanized steel sheet subjected to heat alloying at 500 ° C. after plating was also manufactured. The reduction annealing in CGL was performed at 850 ° C. for steel No. 1, 880 ° C. for steel No. 2,
Steel No. 3 was carried out at 840 ° C., steel No. 4 at 820 ° C., and steel No. 5 at 860 ° C. The plating bath was a bath to which 0.14% of Al was added, and the plating bath temperature was 480 ° C.

The resulting high-strength hot-dip galvanized steel sheet was polished and etched with a 1% nital solution to observe the presence or absence of grain boundary oxides. In addition, the presence or absence of non-plating is determined from the surface observation of the plated steel sheet, the alloyed plated steel sheet is bent and bent back by 90 °, the tape is peeled off the pressure-bonded side, and the amount of peeled zinc is measured by X-ray fluorescence. Then, the adhesiveness during press working was evaluated. The test results are shown in Tables 2 and 3 together with the hot-rolling winding temperature.

[0023]

[Table 1]

[0024]

[Table 2]

[0025]

[Table 3]

As shown in Table 2, the high-strength hot-dip galvanized steel sheets according to the present invention all had good surface appearance, and no non-plating occurred. On the other hand, in the comparative example, non-plating occurred. Also, as shown in Table 3, even when alloyed, the high-strength galvanized steel sheet according to the present invention had good surface appearance and good adhesion, but no plating occurred in the comparative example. The adhesiveness was also poor.

[0027]

As described above, according to the present invention,
The presence of grain boundary oxide directly under the plating layer allows Si
In addition to high-strength cold-rolled steel sheets containing Mn, Cr, etc.,
It is possible to improve non-plating resistance and plating adhesion, and to provide excellent plating properties comparable to ordinary steel.

[Brief description of the drawings]

FIG. 1 is a micrograph of a metal structure showing the presence or absence of a grain boundary oxide immediately below a plating layer.

FIG. 2 is a graph showing the results of analysis of grain boundary oxides by EPMA.

FIG. 3 is a metal micrograph showing grain boundary oxides in a hot-rolled steel sheet.

FIG. 4 shows the results of elemental analysis in the thickness direction of a cold-rolled steel sheet before plating.

Continuation of front page (58) Field surveyed (Int.Cl. ⁶ , DB name) C23C 2/00-2/40

Claims (2)

(57) [Claims] 1. A steel sheet containing 0.1 to 2.0 wt% of Si and having a plating layer on at least one surface, wherein an oxide is present at a crystal grain boundary immediately below the plating layer, and has high plating strength. Hot-dip galvanized steel sheet. 2. A high-strength galvanized steel sheet obtained by alloying the plating layer according to claim 1.