JP3097232B2 - Method for producing Si-containing high-strength galvannealed steel sheet with excellent coating uniformity and powdering resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a Si-containing high-strength alloyed hot-dip galvanized steel sheet mainly used as a material for automobiles and building materials. The present invention relates to a method for enabling production on a simple production line.

[0002]

2. Description of the Related Art In recent years, improvement in fuel efficiency of automobiles has been called for from the viewpoint of prevention of global warming and the like, and high strength and thin materials are strongly demanded from the viewpoint of weight reduction of a vehicle body and ensuring safety. On the other hand, alloyed hot-dip galvanized steel sheets have long been used as body materials from the viewpoint of extending the life of the vehicle body. Therefore, development of a high-strength alloyed hot-dip galvanized steel sheet has been performed in order to satisfy both of these characteristics.

[0003] Generally, to increase the strength of a steel sheet, addition of a solid solution strengthening element such as Si, P, addition of a precipitation element such as Nb, Ti, V, or a combination of both thereof is performed. I have. However, when strengthening is carried out only by the former, it is necessary to increase the addition amount of each strengthening element. In particular, Si is selectively oxidized on the steel sheet surface during annealing in a continuous galvanizing line (hereinafter referred to as CGL). However, there is a problem that the film properties are deteriorated (non-plating, poor alloying, poor powdering resistance, etc.). On the other hand, in the latter, since a large amount of precipitates are present in the steel, the recrystallization temperature rises, and further, there are difficulties in the material such as poor strength-ductility balance and poor hole expandability.

[0004] Apart from such a strengthening mechanism, it is also known that the steel is strengthened by a composite structure in which a hard second phase such as martensite is dispersed in soft ferrite. In this method, strength-ductility balance is excellent, and furthermore, the shape freezing property is improved because the yield ratio is reduced,
Although there is an advantage that press formability is improved, it is necessary to add a large amount of Si, Mn, Cr, etc. in order to stabilize the austenite phase in the CGL thermal cycle.
The same problem as that of the solid solution strengthening type described above occurs.

[0005]

In order to produce a high-strength alloyed hot-dip coated steel sheet having such an excellent strength-ductility balance, it is desirable to add Si or the like as a strengthening element to the steel. It was difficult to add a large amount of these elements from the viewpoint of deterioration of film properties due to selective oxidation.

[0006] In order to solve the above-mentioned problem of selective oxidation of Si, the following proposals have been made in the past. Japanese Patent Application Laid-Open No. Sho 60-262950: Si on steel sheet surface of steel containing Si by Ni pre-plating Japanese Patent Application Laid-Open No. 2-194156: Technology for Preventing Si Concentration on the Surface of Steel Sheet of Si-Containing Steel by Fe-B Preplating Japanese Patent Application Laid-Open No. 3-61352: CGL for Hot-rolled Steel Sheet Containing Si
Technology to improve appearance uniformity by surface grinding before

However, among these methods, there is a problem that an electroplating facility is required, and the Ni plating itself is expensive. Although the method is inexpensive as compared with the method, an electroplating facility is required in any case. Further, the method (1) has a problem that although the alloying unevenness is reduced, there is still a problem of surface oxidation at the time of CGL annealing, and in addition, there is a disadvantage that grinding flaws remain, which is not preferable for applications where the surface appearance is important.

The present invention has been made in view of the above-mentioned conventional problems,
The purpose of the present invention is to provide a method for manufacturing a high-strength galvannealed steel sheet having excellent strength-ductility balance and excellent surface appearance and powdering resistance with existing CGL equipment. is there.

[0009]

Means for Solving the Problems The inventors of the present invention assumed that a steel sheet containing Si was used as a plating base sheet.
After examining the problems on the plating film,
It has been found that the form and factor of the alloying abnormality of the Si-containing steel sheet can be arranged as follows.

(A) Non-uniform scale alloying: An alloying abnormality (unevenness) having a width of about 1 cm, which may leave traces even after painting, which is not preferable in appearance. This alloying anomaly occurs because Fe / Si-based low melting point composite oxides are partially formed on the slab surface during heating of the hot-rolled slab, and they easily remain after hot rolling and pickling. It was found that this was due to the occurrence of an alloying reaction. (B) Non-uniformity in selective oxidation alloying: Abnormality in alloying (unevenness) of about several hundred μm, locally increasing the amount of coating applied (abnormal alloying), and deteriorating powdering resistance. It has been found that this alloying abnormality is caused by the presence or absence of Si-based oxide formed by selective oxidation on the surface of the original sheet immediately before plating. (C) Non-plating: poor wettability with molten zinc, part where film is missing, scale- or selectively oxidizable original plate surface oxide remains particularly thick, and zinc and iron do not react at all (do not wet) ) It turned out to happen if. (D) Local Fe—
Generation of Zn reaction: It has been found that the reaction occurs when a Si-containing steel sheet is hot-rolled during hot rolling. This is presumed to be caused by ferrite grain boundary selective oxidation during hot rolling at high temperature.

The present inventors have studied the means for solving these problems based on the clarification of the various forms of the alloying abnormalities of the plating film and the causes thereof, and have obtained the following findings. (A) By optimizing the hot rolling conditions, it is possible to prevent the generation of Si-based scale (oxide) on the surface of the material steel sheet and the selective oxidation at the ferrite grain boundary, and to produce a plated original sheet having excellent surface uniformity. Can be. (B) A steel sheet containing 0.10 wt% or more of Si is CGL
Although it is industrially difficult to prevent the formation of Si-based oxides due to selective oxidation during annealing in the case of annealing in a furnace, a small amount of Si-based oxides are present on the surface of the original sheet immediately before plating by optimizing the plating and alloying treatment conditions. The influence of the Si-based oxides can be eliminated. (C) The powdering resistance of the coating can be improved by optimizing the alloying conditions in CGL and the subsequent cooling conditions.

The present invention solves the above-mentioned problems concerning the film properties of the Si-containing steel based on such knowledge, and provides a Si-containing high-strength alloyed molten zinc having both excellent film quality and strength-ductility balance. The successful development of a plated steel sheet is characterized by the fact that the content of Si is 0.1%.
A steel slab containing 10 to 0.60 wt%, Ts (° C.) ≦ 1190-67 [% Si] where [% Si]: hot rolling slab heating temperature Ts and hot rolling winding satisfying Si content (wt%) Temperature 60
After hot rolling under the condition of 0 ° C. or less, pickling the hot-rolled steel sheet and, if necessary, cold rolling, in a continuous galvanizing line, a zinc bath having an Al content of 0.15 wt% or less in the bath. After the plating, an alloying heat treatment is performed in an induction heating type alloying furnace so that the exit temperature of the furnace becomes 450 to 550 ° C., and after the surface molten zinc layer disappears, 10 ° C. to a temperature of 300 ° C. or less. / Sec or more.

[0013]

The production method of the present invention is based on the premise that a steel sheet containing 0.10 to 0.60 wt% of Si is used as a plating base sheet from the viewpoint of improving strength-ductility balance. Si
Is less than 0.10 wt%, the intended strengthening and ductility improvement effects cannot be obtained, and the above-mentioned problems regarding the film properties do not occur. On the other hand, if the Si content exceeds 0.60 wt%, the scale surface oxide or the selectively oxidizable original plate surface oxide remains particularly thick, so that even when the production conditions specified in the present invention are applied, non-plating or various alloying abnormalities occur. Cannot be prevented.

In the present invention, as described above, the Si content is 0.10 to 0.10.
Ts (° C.) ≦ 1190-67 [% Si] where steel slab containing 0.60 wt% is contained, [% Si]: hot rolling slab heating temperature Ts and hot rolling winding temperature satisfying Si content. : 6
Hot rolling is performed at a temperature not higher than 00 ° C.

FIG. 1 shows the result of examining the relationship between the heating temperature of the hot-rolled slab of the Si-containing steel and the tendency of the non-uniformity of scale alloying. In this test, slabs of steels a to d shown in Table 1 were hot-rolled by changing the heating temperature of hot-rolled slabs variously, wound at 550 to 595 ° C., and then cold-rolled. In this rolling step, steel a and steel b were hot-rolled to 3.2 mm, then cold-rolled to 0.7 mm, and steels c and d were hot-rolled to 3.6 mm, and then It was cold rolled to 0.8 mm. These steel sheets were subjected to CGL in a bath with an Al content of 0.115 wt%.
Hot-dip galvanizing in a galvanizing bath, followed by an alloying heat treatment in an induction heating type alloying furnace so that the exit sheet temperature becomes 480 to 522 ° C., and the cooling rate immediately after the molten zinc layer disappears on the surface of the plating film. : Cooled at 27 ° C / sec.

According to FIG. 1, it can be seen that the generation of non-uniformity in scale alloying is suppressed only when the hot-rolled slab heating temperature Ts satisfies Ts (° C.) ≦ 1190−67 [% Si]. For the above reasons, in the present invention, the hot rolling slab heating temperature Ts (° C.) is limited as described above.

FIG. 2 shows the result of examining the relationship between the hot-rolling winding temperature and the occurrence of a local Fe-Zn reaction at the grain boundaries of the base ferrite. In this test, the slabs of steels a to d shown in Table 1 were heated at a hot rolling slab heating temperature of 114.
The temperature was set to 0 to 1156 ° C., hot rolling was performed with various changes in the hot rolling winding temperature, and then cold rolling was performed. In this rolling process, steel a and steel b are hot-rolled to 3.2 mm, then cold-rolled to 0.7 mm, and steel c and d
After hot rolling to 3.6 mm for steel, 1.8 mm
Until cold-rolled. The steel sheet obtained in this way is CG
L, and after hot-dip galvanizing in a plating bath with an Al content of 0.13 wt% in the bath, an alloying heat treatment was performed in an induction heating type alloying furnace so that the furnace exit side sheet temperature was 500 to 525 ° C. The cooling was performed at a cooling rate of 25 ° C./sec immediately after the molten zinc layer on the surface layer of the plating film disappeared.

According to FIG. 2, it can be understood that abnormalities in alloying due to local Fe-Zn reaction are properly prevented by setting the hot-rolling winding temperature to 600 ° C. or less. For the above reasons, in the present invention, the hot rolling winding temperature is specified to be 600 ° C. or less.

The hot-rolled steel sheet is pickled, cold-rolled if necessary, and then passed through CGL. This CGL
May be 700 ° C. or more, which is a general annealing temperature necessary for obtaining a predetermined material, and an Ac ₃ transformation point or less.

After the above-described annealing, the steel sheet is plated in a hot-dip galvanizing bath.
The amount is specified as 0.15 wt% or less. The amount of Al in the bath was determined along with an alloying process using an induction heating method described below.
This is an important requirement for preventing non-plating and selective oxidation alloying abnormalities from occurring in a steel sheet containing 0.10 to 0.60% of Si.

FIG. 3 shows the result of examining the relationship between the amount of Al in the bath and the film properties. In this test, steels a and b shown in Table 1 were hot-rolled at a hot-rolled slab heating temperature of 1150 ° C. and a hot-rolling take-up temperature of 585 ° C., and then passed through CGL in which the amount of Al in the bath was varied. The plate temperature is 520 ° C on the exit side in the induction heating type alloying furnace.
After the alloying heat treatment, the cooling rate is 25 ° C.
/ Sec. According to FIG.
It is found that when the amount of l exceeds 0.15%, non-plating and alloying unevenness are apt to occur. For the reasons described above, in the present invention, the amount of Al contained in the hot-dip galvanizing bath is set to 0.15.
% Or less.

The hot-dip galvanized steel sheet is continuously alloyed. One of the features of the present invention is that the alloying treatment is performed in an induction heating (high frequency induction heating) type alloying furnace.
By performing the alloying heat treatment using such a heating method, abnormal alloying can be appropriately prevented.

As described above, Si: 0.10 to 0.60
%, By hot rolling under predetermined conditions, it is possible to improve the non-uniform alloying due to the non-uniformity of the surface of the original sheet caused in the hot rolling step including the slab heating step. However, during the annealing of CGL, selective oxidation occurs in accordance with the amount of the added element.
An i.Mn-based composite oxide or the like exists in an island shape. Such an oxide also causes alloying abnormality or, in a remarkable case, non-plating. When a heating furnace of the induction heating method is used for the alloying treatment, unlike the normally used gas heating method, the surface layer of the steel sheet is preferentially heated, so the surface layer is forcibly forced regardless of the unevenness of the steel sheet surface. It is considered that a reaction between iron and molten zinc can be caused, whereby abnormal alloying is suppressed.

The advantages of the alloying process using the induction heating method are specifically described as follows. First, by using an induction heating method in the alloying treatment, the surface layer of the steel sheet in contact with the plating film is directly heated, so that the Fe at the interface between the steel sheet and the plating film is compared with an atmosphere heating method such as gas heating. -The Zn reaction occurs uniformly in a short time and independently of the position on the steel sheet, so that there is no partial over-alloy or alloy phase remaining on the steel sheet,
Uniform film properties can be obtained.

Secondly, since the induction heating directly heats the surface layer of the steel sheet as described above, a microscopically uniform alloying reaction occurs. That is, in the conventional alloying treatment by gas heating, heat is applied from the outside of the coating, so that the heating tends to be nonuniform, and therefore, the alloying reaction tends to be microscopically nonuniform. In particular, since the crystal grain boundaries are highly reactive, a so-called outburst reaction is likely to occur. When such an outburst structure is generated, a Γ phase starts to grow from this portion, and the formation of the Γ phase lowers the powdering resistance. to degrade. In contrast, induction heating directly heats the surface layer of the steel sheet, so that there are few local variations in the alloying as described above, and there is also an oxide on the steel sheet surface and an alloying inhibitor (Fe ₂ Al) generated in the bath. ₅ ) is also easily diffused, so that a microscopically uniform alloyed film can be obtained.

Third, induction heating can shorten the growth time of the Γ phase because the plating can be alloyed in a short time, and thus the final amount of the Γ phase formed is small, which also improves the powdering resistance. It is considered to have contributed greatly.

Fourth, as an advantage of induction heating, uniform heating can be performed in the width and length directions of the steel sheet, so that strict control of the sheet temperature at the exit side of the heating furnace is possible. It is considered that this is due to the fact that there is no rise in the heated atmosphere gas (draft effect) unlike the atmosphere heating method such as above, so that over-alloy hardly occurs.

In the above alloying treatment, the temperature of the steel sheet on the furnace outlet side of the steel sheet is specified in the range of 450 to 550 ° C. If the temperature at the furnace outlet side is lower than 450 ° C., it takes time to alloy, while if it exceeds 550 ° C., the powdering resistance deteriorates.
In the present invention, the reason why the sheet temperature on the exit side of the induction heating furnace is controlled is that the temperature becomes the highest sheet temperature in the alloying heat cycle. In addition, since the growth rate of the alloy phase is maximized in this vicinity, it is possible to cause an alloying reaction at that temperature by controlling the outlet plate temperature.

After the molten zinc layer on the surface layer disappears in the above alloying treatment, cooling is performed at a cooling rate of 10 ° C./sec or more to a temperature of 300 ° C. or less. To improve powdering resistance,
In particular, it is important to prevent over-alloying after the alloying treatment. For this purpose, after the surface of the molten zinc layer has disappeared by the heating for the alloying, the temperature must be reduced to 10 ° C. until the temperature range (300 ° C. or less) where the alloying does not proceed. / Sec or more to prevent over-alloying.

[0030]

EXAMPLES Steel having the composition shown in Table 2 was melted in a 50-ton converter, and alloyed hot-dip galvanized steel sheets were manufactured under the respective manufacturing conditions shown in Tables 3 to 5. Tables 6 to 8 show the coating quality characteristics of the obtained plated steel sheets.

[0031]

[Table 1]

[0032]

[Table 2]

[0033]

[Table 3]

[0034]

[Table 4]

[0035]

[Table 5]

[0036]

[Table 6]

[0037]

[Table 7]

[0038]

[Table 8]

[Brief description of the drawings]

FIG. 1 is a graph showing the effect of the heating temperature of a hot-rolled slab of a steel containing Si on the occurrence of scale-related alloying unevenness of an alloyed plating film.

FIG. 2 shows that the hot-rolling winding temperature of the Si-containing steel is changed to the local Fe-Z at the ferrite grain boundary under the alloyed plating film.
Graph showing the effect on the occurrence of n reaction

FIG. 3 is a graph showing the effect of the amount of Al in a plating bath on occurrence of alloying abnormality in hot-dip galvanizing of a Si-containing steel sheet.

────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Masaru Sagiyama 1-2-1 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (56) References JP-A-4-168229 (JP, A) JP-A-2 −290955 (JP, A) JP-A-3-49561 (JP, A) JP-A-2-10685 (JP, A) JP-A-2-136056 (JP, U) Luboshin “Corrosion-proof plating and chemical plating” (February 25, 1961) Nikkan Kogyo Shimbun P69 (58) Field surveyed (Int. Cl. ⁷ , DB name) C23C 2/00-2/40 C21D 8/02, 9/46

Claims (1)

(57) [Claims] 1. A steel slab containing 0.10 to 0.60 wt% of Si, Ts (° C.) ≦ 1190−67 [% Si] where [% Si]: heat satisfying the Si content (wt%) Rolling slab heating temperature Ts, hot rolling winding temperature 60
After hot rolling under the condition of 0 ° C. or less, pickling the hot-rolled steel sheet and, if necessary, cold rolling, in a continuous galvanizing line, a zinc bath having an Al content of 0.15 wt% or less in the bath. After the plating, an alloying heat treatment is performed in an induction heating type alloying furnace so that the exit temperature of the furnace becomes 450 to 550 ° C., and after the surface molten zinc layer disappears, 10 ° C. to a temperature of 300 ° C. or less. A method for producing a Si-containing high-strength galvannealed steel sheet having excellent coating uniformity and powdering resistance, characterized by cooling at a cooling rate of not less than / sec.