JP3520741B2 - Galvannealed steel sheet with excellent plating adhesion - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an alloyed hot-dip galvanized steel sheet frequently used as a rust-preventive steel sheet for automobiles and a method for producing the same. The present invention relates to a galvannealed steel sheet. [0002] Currently, zinc-based hot-dip coated steel sheets and electroplated steel sheets have been developed and put into practical use as rust-preventive steel sheets for automobiles because of their excellent sacrificial corrosion protection ability. Among them, galvannealed steel sheets are widely used especially in actual vehicles among galvanized steel sheets because of their low production cost and high corrosion resistance. However, the galvannealed steel sheet is subjected to an alloying treatment usually at a temperature of around 500 ° C. after the hot-dip galvanization, and the mutual diffusion between the ground iron (base steel sheet) and the zinc as the plating layer is caused. Since it is produced by a method of generating an intermetallic compound of Zn-Fe, there is a problem that the adhesion between the plating layer and the base steel sheet is inferior to that of an electrical galvanized steel sheet. [0004] In the case of hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet, a method of improving plating adhesion by adding an appropriate amount of Al to a plating bath has been known as a measure for solving the above-mentioned problems. The hot-dip galvanized steel sheet currently produced in the process is plated in an Al-containing zinc bath. However, the above method alone is not always sufficient to ensure the adhesion of the plating layer required for a steel sheet for automobiles, and in the case of an alloyed hot-dip galvanized steel sheet, a good A technique for securing plating adhesion is disclosed. In the case of galvannealed steel sheets,
Previous studies have shown that there is a close relationship between the adhesion of the plating layer and the phase structure of the plating, and the presence of a high iron content intermetallic compound Γ phase formed at the plating layer / base iron interface is often present. It is shown that the adhesiveness deteriorates when it becomes. On the other hand, as described above, the plating layer of the galvannealed steel sheet is formed by mutual diffusion between the galvanized layer and the base steel sheet. Al phase is inevitably generated at the interface between the plating layer and the base steel sheet (base iron) even if Al exists in the steel. That is, by controlling the Al concentration in the bath and optimizing the alloying temperature as described above, the formation of the Γ phase can be suppressed to some extent, but the formation cannot be completely suppressed. It is hard to say that the plating adhesion, which is the performance, has reached a sufficient level even though it is relatively good. An object of the present invention is to solve the above-mentioned problems of the prior art and to provide an alloyed hot-dip galvanized steel sheet having excellent plating adhesion. [0008] The present invention SUMMARY OF] is at least one side has a galvannealed layer, the average ferrite grain size d ₁ and the base steel sheet of the base steel sheet surface just under the plating layer of the steel sheet the ratio between the average ferrite grain size d ₂ of the center of plate thickness [: d ₁ / d _2] satisfies the following formula (1), wherein the basis steel sheet
The average ferrite grain size d ₁ of the surface layer is excellent galvannealed steel sheet in a plating adhesion, characterized in der Rukoto than 13 .mu.m. D ₁ / d ₂ <0.8 (1) The present invention will be described in more detail below. The present inventors have studied the properties of the alloyed hot-dip galvanized steel sheet in view of the above-described points.As a result, the steel sheet surface is strained before annealing the material steel sheet in the alloyed hot-dip galvanized steel sheet manufacturing process. By making the crystal grain size of the surface layer of the base steel sheet immediately below the plating layer after alloying smaller than the crystal grain size at the center of the base steel sheet thickness, plating can be performed without impairing the mechanical properties of the base material (base steel sheet). It has been found that it is possible to dramatically improve the adhesion of the layers. As described above, the present invention provides an average ferrite grain size (hereinafter also referred to as a crystal grain size of a surface layer of a base steel sheet) d _{1 of a} surface layer of a base steel sheet immediately below a plating layer and an average ferrite grain diameter of a central portion of the sheet thickness of the base steel sheet. The ratio of the crystal grain size (hereinafter also referred to as the crystal grain size at the center of the thickness of the base steel sheet) d ₂ [: d ₁ / d ₂ ] satisfies the following formula (1), and the average ferrite crystal in the surface layer of the base steel sheet
In which the particle diameter d ₁ to provide a significantly better galvannealed steel sheet or der Rumekki adhesion 13 .mu.m. D ₁ / d ₂ <0.8 (1) The reason why the plating adhesion is improved by controlling the crystal grain size of the surface layer of the base steel sheet immediately below the plating layer is as follows. First, as described above, the adhesion of the alloyed hot-dip galvanized coating is greatly affected by the plating phase structure at the interface between the plating layer and the base steel sheet, and the plating adhesion is degraded when the Γ phase generated at the interface increases. . There is a close relationship between the formation of the Γ phase and the alloying conditions, and by controlling the alloying temperature to a certain temperature or less, good plating adhesion is ensured. Usually, for example, when the base steel sheet is an ultra-low carbon mild steel, an alloyed hot-dip galvanized steel sheet is often produced in an alloying temperature range of around 500 ° C, but generally, the alloying temperature depends on the line speed and the alloying temperature. It is determined by the furnace length of the furnace, and in order to increase the productivity, the alloying temperature must be increased, resulting in a problem that the plating adhesion is deteriorated. When the base steel sheet is a high-tensile steel sheet containing a large amount of Si, Mn, P, etc., the components in the steel have a function of delaying alloying. The alloying temperature must be further increased, and the plating adhesion is further deteriorated for the same reason as described above.
Therefore, if the alloying speed can be increased in any way, the above problem is solved. In the process of alloying the galvanized layer of a hot-dip galvanized steel sheet, generally, the galvannealed layer is generated by mutual thermal diffusion between the zinc of the galvanized layer and the base steel sheet, and the overall alloying speed is reduced by It is largely controlled by iron diffusion. Under normal production conditions for galvannealed steel sheets, the diffusion of iron from the crystal grain boundaries of the base steel sheet is dominant, and therefore the alloying speed increases as the number of grain boundaries in contact with zinc increases. Therefore, the smaller the crystal grain size of the surface of the base steel sheet immediately before plating is more advantageous from the viewpoint of promoting the alloying, and the alloying can be performed at a relatively low temperature without lowering the productivity. It is possible to obtain a plating layer having few phases and good adhesion. Furthermore, the plating adhesion is affected not only by the phase structure of the interface but also by the shape of the interface. The more irregularities at the interface, the more advantageous the adhesion. In the steel sheet disclosed in the present invention, in which the crystal grain size of the surface layer of the base steel sheet immediately below the plating layer is specified to be small, the unevenness of the interface is larger than that of a normal steel sheet, so that a so-called anchor effect is exhibited and the plating adhesion is improved. It can be significantly improved. As described above, the present invention reduces the crystal grain size on the surface of the base steel sheet, promotes alloying, and reduces the interface structure of the plating layer to have a small phase which is advantageous for adhesion, and further anchors the anchor. The effect is to significantly improve the plating adhesion. However, usually, the ferrite crystal grain size of a steel sheet is determined by heat treatment conditions, rolling reduction, components in steel, and the like, and these conditions are necessarily determined by the mechanical properties required of the material. . In other words, if the crystal grain size is reduced to improve only the plating adhesion of the galvannealed steel sheet, the required mechanical properties of the steel sheet cannot be satisfied. The inventors of the present invention have conducted intensive studies to solve the above-mentioned problems. As a result, it is found that it is the surface of the steel sheet that contributes to the promotion of alloying, more specifically, the crystal grains of the surface layer of the base steel sheet immediately below the galvannealed layer. By controlling the crystal grain size in this region, it has been found that plating adhesion can be improved without changing the mechanical properties of the material. Quantitatively, the ratio [: d ₁ / d ₂ ] of the crystal grain size d _{1 of the} surface layer of the base steel sheet immediately below the plating layer to the crystal grain size d ₂ at the center of the thickness of the base steel sheet is represented by d ₁ / d. _By setting < _2, it is possible to provide a galvannealed steel sheet having excellent plating adhesion without impairing the mechanical properties of the base steel sheet. The reason defined by the ratio of the two in the present invention is as follows. As described above, the crystal grain size at the center of the thickness of the base steel sheet is determined by the mechanical properties required for the material. Some materials have a crystal grain size. If the crystal grain size is simply defined to a certain value or less, it is meaningless from the viewpoint of achieving both mechanical properties of the material and promotion of alloying of the material. The ratio [: d ₁ / d ₂ ] of the crystal grain diameter d _{1 of the} surface layer of the base steel sheet immediately below the coating layer of the alloyed hot-dip galvanized steel sheet to the crystal grain diameter d ₂ of the central part of the base steel sheet thickness is represented by d ₁ / D ₂ <0.
The alloyed hot-dip galvanized steel sheet of the present invention set to 8 can be realized by locally imparting strain to the surface of the base steel sheet before recrystallization annealing performed in the continuous hot-dip galvanizing equipment (CGL). That is, due to the strain imparted only to the surface layer of the base steel sheet, during recrystallization annealing, the crystal grain size of the surface layer of the base steel sheet becomes smaller than the crystal grain size at the center of the thickness of the base steel sheet. The conversion promoting effect is exhibited. As a specific method of giving a strain to the surface layer of the base steel sheet, a method of grinding the steel sheet surface after cold rolling, before plating, that is, before annealing, if a galvannealed steel sheet using a cold rolled steel sheet as a material is exemplified. Is done. [0024] To a steel sheet properties required in the present invention, a steel sheet, the surface layer, by preferably 1 to 100 g / m ^2, more preferably 1 to 10 g / m ² Grinding, subsequent annealing process Thus, an alloyed hot-dip galvanized steel sheet having a surface layer having crystal grains finer than the central portion as described above can be manufactured. In this case, the grinding method is not particularly limited, and grinding using a scotch bright, a ceramic roll, or the like is possible. When the base steel sheet is a cold-rolled steel sheet, it is possible to locally introduce distortion to the surface of the base steel sheet by a method of increasing the surface roughness of the roll during cold rolling other than grinding. . The steel sheet having the strain introduced into the steel sheet surface by the above method is annealed before plating in a continuous hot-dip galvanizing line.In the present invention, the dew point of the atmosphere at the time of annealing is −10 ° C. or less, more preferably −20 ° C. It must be: The reason is that in a continuous hot-dip galvanizing line having a heating furnace using a radiant tube, annealing is usually performed in a reducing gas containing several% of hydrogen. Inevitably exist in
Alternatively, P intentionally added as an alloy element concentrates on the surface of the steel sheet, which significantly slows down the alloying speed. The method of galvannealing after the introduction of strain into the surface layer of the base steel sheet by the above-described method and the refinement of the crystal grains of the surface layer of the base steel sheet by annealing before plating is as follows. First, when the Al concentration in the plating bath during hot-dip plating is defined as a value obtained by subtracting the total iron concentration from the total Al concentration in the bath, the concentration needs to be 0.06 to 0.15 wt%. The Al concentration in the bath is defined as described above.
If the Al concentration is less than 0.06 wt%, a large amount of Γ phase is generated at the steel sheet / plating interface after alloying, no matter how the other conditions are limited, and sufficient adhesion cannot be ensured. Also,
The reason for specifying the upper limit of the Al concentration is that if the Al concentration exceeds 0.15 wt%, the alloying speed becomes extremely slow, and the adhesion is improved by controlling the crystal grain size of the steel sheet to accelerate the alloying speed. This is because it does not meet the original purpose of making it work. Regarding the alloying conditions after hot-dip plating, it is desirable that the alloying temperature be as low as possible from the viewpoint of the plating phase structure, and that the iron content in the plating layer be within a range in which the η phase does not remain. Lower is desirable. That is, the alloying temperature is 520 ° C or less, more preferably 490 ° C or less,
The iron content in the plating layer is 11% by weight or less, more preferably 10% by weight.
wt% or less and 7 wt% or more are given as target values. In the present invention, the base steel sheet is mainly a cold-rolled steel sheet, but the composition of the steel sheet is not particularly limited. Specifically, in recent years, Ti-based, Nb-based, Ti-Nb-based ultra-low carbon steel sheets, which have been widely used as materials for galvannealed steel sheets and have excellent mechanical properties, especially Examples thereof include low-carbon steel sheets, and high-strength steel sheets to which elements such as Mn, Si, and P are added as alloying elements. Further, in the galvannealed steel sheet of the present invention, as described above, the average ferrite grain size d _{1 of the} surface layer of the base steel sheet immediately below the plating layer of the galvannealed steel sheet and the thickness center of the base steel sheet The ratio [: d ₁ / d ₂ ] to the average ferrite crystal grain size d ₂ of the part may satisfy d ₁ / d ₂ <0.8, and furthermore, the ratio [: d ₁ / d ₂ ]
More preferably satisfies d ₁ / d ₂ ≧ 0.1. This is because when d ₁ / d ₂ <0.1, the alloying speed is abnormally accelerated, alloying occurs even during hot-dip galvanizing, and the alloy layer grows thicker during hot-dip galvanizing. This is because it is difficult to control the amount of plating applied by the gas wiping nozzle performed in the above. Further, the coating weight of the galvannealed steel sheet of the present invention is not particularly limited, but the coating weight = 10 to 100 g / m ²
Is more preferable. This is because when the coating weight is less than 10 g / m ² , the corrosion resistance is lowered, and when it exceeds 100 g / m ² , the effect of improving the corrosion resistance is practically saturated and is not economical. Hereinafter, the present invention will be described in detail with reference to examples. The surface of a cold-rolled steel sheet having a composition shown in Table 1 and having a thickness of 0.7 mm was ground in a laboratory by Scotch Bright (Examples 1 to 4 and Comparative Examples 1 to 3). Table 2 shows the grinding amount obtained by the weight method during grinding. In a laboratory, a ground steel sheet (Examples 1 to 4 and Comparative Examples 1 to 3) and a cold-rolled steel sheet (Comparative Examples 4 to 6) which do not impart distortion to the surface of the base steel sheet by final grinding are used as materials. Using a rigid hot-dip galvanizing apparatus, annealing and hot-dip galvanizing were performed under the following conditions following alkaline electrolytic degreasing and hydrochloric acid pickling. (Annealing conditions :) Atmosphere gas; composition: 5 vol% H ₂ -N ₂ , dew point: -40 ° C Heating rate: 10 ° C / sec Annealing temperature: 800 to 850 ° C Annealing time: 20 sec Cooling rate: 10 ° C / sec ( Hot-dip galvanizing conditions :) Plating bath composition; Al: 0.14 wt%, Fe: 0.04 wt%, Pb: 0.008
wt%, balance: Zn bath temperature; 475 ° C Penetration plate temperature; 475 ° C Immersion time: 1 sec Coating weight: 50g / m ² or more of hot-dip galvanized steel sheet produced by direct heating furnace After the temperature was raised in the furnace and the alloying treatment was performed at the alloying temperature shown in Table 2, the alloy was cooled by blowing nitrogen gas to produce a galvannealed steel sheet. The plating layer of the galvannealed steel sheet (: GA) obtained as described above was dissolved in hydrochloric acid containing an inhibitor, and the solution was analyzed by ICP emission spectroscopy. The iron content was determined. Table 2 shows the obtained coating weight and the iron content of the plating layer. The crystal grain size of the galvannealed steel sheet was measured according to the following method. First, a galvannealed steel sheet (G
The plating layer of A) was dissolved and removed with hydrochloric acid containing an inhibitor. Thereafter, the surface of the steel sheet was polished to a polishing thickness of about 5 μm so that the structure of the base steel sheet could be observed. After the nital corrosion, a structure photograph was taken. From the obtained structure photograph, an arbitrary line segment LL in the rolling direction of the base steel sheet and a line segment L orthogonal thereto are shown.
The number of crystal grains cut by C is defined as nL and nC, respectively, and the ferrite crystal grain diameters dL and dC are defined as dL.
= LL / nL, determined by dC = LC / nC, and d ₁₁ obtains an average of both the average of each of the values determined by the same methods by changing the field of view 10 times, the average ferrite crystal grain size of the base steel sheet surface layer It was determined d _1. The length of the line segment was selected so that the number of crystals in the line segment was at least 20 or more. In addition, regarding the crystal grain size at the center in the thickness direction of the base steel sheet, one side polishing until the thickness of the base steel sheet is reduced to half,
With respect to the polished surface, the average ferrite grain size d ₂ at the center of the thickness of the base steel sheet was determined in exactly the same manner as described above. Table ₁ shows d ₁ and d ₂ thus determined and the ratio d ₁ / d ₂ of both. Further, as a performance test of the plating layer, a powdering test was performed on the test piece of the alloyed hot-dip galvanized steel sheet, and the Zn count number (CPS) measured by X-ray fluorescence analysis was measured. And the powdering index. Table 2 shows the obtained powdering index, which is the count number (CPS) of Zn. <Powderability test:> Specimen size: width 40 mm × length 100 mm 90 degree bending back → tape peeling → tape peeling surface As shown in Table 2 for fluorescent X-ray analysis, according to the present invention, annealing was performed. the strain was applied before the base steel sheet surface, the ratio between the average ferrite grain size d ₁ of the base steel sheet surface just under the galvannealed layer, the average ferrite grain size d ₂ of the base steel sheet thickness center [ : D
₁ / d ₂ ] is set to d ₁ / d ₂ <0.8 , and
By setting the average ferrite crystal grain size d _{1 of the} layer to 13 μm or more, it became possible to dramatically improve the plating adhesion of the galvannealed steel sheet. [Table 1] [Table 2] According to the present invention, it has become possible to significantly improve the plating adhesion of an alloyed hot-dip galvanized steel sheet without impairing the mechanical properties of the base material.

Continuation of front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C22C 38/00 301 C23C 2/02

Claims (1)

(57) [Claims] [Claim 1] A steel sheet has an alloyed hot-dip galvanized layer on at least one side thereof, and has an average ferrite grain size d _{1 of the} surface layer of the base steel sheet immediately below the coating layer and a thickness of the base steel sheet The ratio [: d ₁ / d ₂ ] to the average ferrite grain size d ₂ at the center is expressed by the following formula:
Satisfies (1) and the average ferrite crystal of the surface layer of the base steel sheet
Excellent galvannealed steel sheet coating adhesion particle diameter d ₁ is characterized by der Rukoto than 13 .mu.m. Note d ₁ / d ₂ <0.8 ............ (1)