JPH06158254A - Manufacture of hot-dip galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To establish a means for stably mass producing hot-dip galvannealed steel sheet having high quality regardless of kind of steel to the base material without needing a special new equipment. CONSTITUTION:At the time of pickling and descaling to a hot-rolled steel sheet for galvanizing, shot blasting treatment is executed and successively, if necessary, the surface is brushed or after executing cold-rolling having >=30% rolling ratio, desirably, heating treatment in strong oxidizing atmosphere to this hot- rolled steel sheet in a continuous hot-dip galvanizing line, and then, the hot-dip galvanizing is executed. This steel sheet is heated to execute the alloying treatment. Even in the case of being the high strength steel sheet having chemical composition of 0.002-0.2% C, <=2.0% Si, 0.5-2.5% Mn, <=0.1% P and the balance Fe with inevitable impurities in the base material, the stable treatment can be executed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for stably producing a galvannealed steel sheet.

[0002]

[Prior art and its problems] "Alloyed molten zinc" is formed by heat-treating a galvanized steel sheet after hot dip galvanizing to form a Zn-Fe alloy plating layer by thermally diffusing iron of a plating base material into the plating layer. Since "plated steel sheet" generally exhibits excellent corrosion resistance and paintability, it has been widely used as a material for automobiles, home appliances, building materials, etc.

On the other hand, in recent years, weight reduction of products (car bodies) has become an important issue from the viewpoint of resource saving and energy saving including automobiles, and in order to cope with this, high tensile strength steel plates are used as materials for various members. Its use has become widely considered.
Therefore, in consideration of the "corrosion resistance" that is essential for these members, there is an increasing demand for alloyed hot-dip galvanized steel sheets to which high-strength steel is applied. A high-strength alloyed hot-dip galvanized material used as a material is strongly desired.

However, as described above, the "alloyed hot-dip galvanized steel sheet" is obtained by heat-treating the hot-dip galvanized steel sheet to thermally diffuse the iron of the base metal into the plated layer.
Since it forms an alloy plating layer, certain elements present in the base material may affect the plating processability and alloying processability significantly during the manufacturing process, and depending on the steel type, stable properties can be obtained. The product was said to be very difficult to manufacture.

For example, in the case of high-strength steel, Si, Mn and the like are commonly used as additive elements, but these elements are concentrated on the surface of the steel sheet during the heat treatment before plating to improve plating wettability. It is known that it inhibits and causes "non-plating".

As a countermeasure against this, for example, JP-A-57-7
As seen in Japanese Patent No. 0268, a method has been proposed in which a base material is electrolytically plated with iron in advance, and then hot-dip galvanizing is performed. However, in this method, it is necessary to newly provide a pre-plating treatment facility, and the process becomes complicated, and there are many problems in practical use.

In addition to Si and Mn, P may be considered as a general additive element used in high-strength steel, but in addition to the fact that these elements easily cause the above-mentioned "non-plating", It is also known that when alloying treatment is carried out, the alloying speed is remarkably delayed and "unalloyed treatment" is likely to occur. And, when such "unalloyed treatment" occurs, not only the visual appearance is inferior, but also the chemical conversion processability, which is important as a coating base treatment, is deteriorated, causing problems such as corrosion resistance after coating. is there.

[0007] Of course, Japanese Patent Laid-Open No. 3-2455 proposes a countermeasure therefor, but the way to do it is to "preliminarily perform Ni-based plating on the base material" and It was not possible to avoid problems such as "excessive plating equipment addition" and "complexity of process". In addition to this, a method of "performing a low speed operation to secure a sufficient processing time" may be considered in order to cope with a delay in alloying, but in this case, there was a problem in productivity. .

On the other hand, as a countermeasure from the aspect of continuous galvanizing operating conditions for steel types that tend to have a slow alloying rate, a method of hot dip galvanizing by controlling the galvanizing bath temperature according to the Al concentration in the galvanizing bath (JP-B-64). No. 11111), and a method of defining heating rate conditions, cooling rate conditions and the like during alloying treatment (Japanese Patent Application Laid-Open No. 62-130268).

However, it is very difficult to significantly change the operating conditions of only a special steel type in a continuously operated hot-dip galvanizing line, which is preferable for stably producing an alloyed hot-dip galvanized steel sheet having constant properties. It was not a means.

In view of the above, the object of the present invention is to provide a high-quality alloy without requiring any special equipment expansion, even if the base material is a steel type that is difficult to process. The aim is to establish means for stable mass production of galvanized steel sheets.

[0011]

[Means for Solving the Problems] The inventors of the present invention conducted various studies to achieve the above-mentioned object, and said, "In consideration of the actual operation in a galvannealed steel sheet production line, the change of the operation condition is performed. It is more advantageous to avoid the method that involves the above as much as possible, and to adopt the "method of performing some kind of pretreatment so as not to be affected by the base material steel type", apart from the pre-plating method, which has a cost problem. " Upon further conclusion and further research, we were able to obtain the following findings.

A) When a hot-rolled steel sheet is used as the base material of the hot-dip galvanized steel sheet, the steel sheet may be descaled by shot blasting. This shot blasting treatment has a peculiar effect in ordinary hot-dip galvanizing. Although not confirmed to be associated with the above, it acts to remarkably increase the alloying rate, particularly when producing a galvannealed steel sheet. That is, when the hot-rolled steel sheet before the galvannealing is subjected to the shot blasting treatment, the diffusion rate of iron from the base metal is remarkably increased during the alloying treatment.

B) This shot blasting treatment can basically be expected to have an effect of promoting alloying regardless of which step is performed before plating, but the shot blasting treatment is performed especially during pickling descaling. When it is carried out, the effect of pickling and descaling is remarkably improved, and the production efficiency of the galvannealed steel sheet is further improved.

C) Further, it has been reported so far that "removing the impurity layer existing on the surface layer of the plating base material with a grinding brush or the like leads to uniform alloying". (See, for example, Japanese Patent Laid-Open No. 3-207845),
Actually, the effect was not always sufficient. However, if the pretreatment that combines the above-mentioned shot blasting treatment and brush grinding is performed, even if the high-strength steel of high Si level, which was difficult to uniformly alloy with the conventional brush surface grinding alone, was used as the base material. Therefore, uniform alloying is possible without requiring special control of continuous hot dipping / alloying treatment conditions.

D) Moreover, when cold rolling is performed after shot blasting instead of the above brush grinding, the same effect as in the case of combined brush grinding can be obtained. e) On the other hand, in a normal continuous hot-dip galvanizing / alloying line, a hot-dip galvanizing tank consisting of a heating zone, a reduction zone, a cooling zone, etc. is placed next to a hot-dip galvanizing tank. It is designed to be guided to the hot-dip galvanized layer after being heated and annealed to obtain it. However, in the hot-rolled steel sheet subjected to shot blasting during pickling and descaling, if the heating and annealing atmosphere is set to a strong oxidizing atmosphere, " Plating wettability "
And “alloying growth” are further improved, and the manufacturability of high-grade alloyed hot-dip galvanized steel sheet becomes more stable.

The present invention has been completed on the basis of the above-mentioned findings and the like. "A shot blasting treatment is also performed at the time of pickling and descaling of hot-rolled steel sheet to be hot-dipped, and then, if necessary, After the surface of the hot-rolled steel sheet is brush-ground or cold-rolled at a rolling ratio of 30% or more, the hot-rolled steel sheet is heat-treated in a continuous hot-dip galvanizing line, preferably in a strong oxidizing atmosphere, and then hot-dip galvanized. By heating and alloying this, a uniform and high-quality alloyed hot-dip galvanized steel sheet can be stably manufactured ”.

1 to 4 are schematic explanatory views of an example of the manufacturing process of the galvannealed steel sheet according to the present invention.

There is no particular limitation on the kind of the hot rolled steel sheet as the base material in the present invention, but the effect is extremely high when the high tensile hot rolled steel sheet for which galvannealing is difficult is applied. The more remarkable effect is C: 0.002.
~ 0.2% (Hereafter,% representing the component ratio shall be% by weight),
It can be sufficiently confirmed when a high-strength hot-rolled steel sheet containing Si: 2.0% or less, Mn: 0.5 to 2.5%, P: 0.1% or less, and the balance being Fe and inevitable impurities is used as a base material.

By the way, the high-strength hot-rolled steel sheet having the above-mentioned specific chemical composition is prepared by adjusting the components of C, Si, Mn, and P for the following reasons. If the C ... content is less than 0.002%, it is necessary to add an alloying element to supplement the strength, which is a cost problem. On the other hand, if the content exceeds 0.2%, the weldability becomes a problem. If the Si content exceeds 2.0%, the alloying treatment may be insufficient even if the measures specified in the present invention are taken, which is not preferable. If the Mn content is less than 0.5%, the strength is insufficient, while 2.
If it is contained in excess of 5%, there will be a cost problem. P ... Added to secure the strength of the steel sheet, but if its content exceeds 0.1%, deterioration of secondary work embrittlement becomes a problem.

The present invention will be described in more detail below along with the action of each processing step.

In the present invention, it is not clear why the hot-rolled steel sheet as the base material is shot-blasted to promote alloying during the alloying treatment. However, when investigating the growth state of the alloy phase, it is considered that alloying is promoted by some change in the surface of the base material because the alloying is growing from the shot hit portion.

The reason why the alloying is promoted by the shot blasting treatment is that "the same effect as that obtained by the grinding treatment which will be described later" by removing the impurity layer on the surface "can also be obtained by the shot blasting treatment". Not unexpectedly, it shows almost the same alloying growth rate both when blasting the hot-rolled scale plate before pickling and when blasting after the scale removal by pickling. Therefore, it is considered that this reason is not correct.

As described above, the shot blasting treatment can be expected to have a certain degree of alloying promoting effect in any step before plating, but especially in the case of pickling descaling. Shot blasting is very preferable because the pickling and descaling effect is also improved and the productivity of the galvannealed steel sheet is further improved.

Further, the reason why alloying is further promoted when the hot-rolled steel sheet is subjected to pickling descaling treatment accompanied by shot blasting and further brush grinding is carried out is the descaling by pickling.
It is considered that even in the rolled steel sheet, "a layer of impurities that adversely affects alloying" remains on the surface layer and is removed by brush grinding.

However, the "high-strength steel" containing a large amount of impurity elements other than Fe as compared with the ordinary steel has a rolling temperature of 800 ° C or higher in the hot rolling process, and therefore the scale layer formed on the surface. Not only becomes thicker, but also becomes partly very uneven. This tendency is remarkable in high-strength steel containing a large amount of Si. Of course, when this is used as a plating base material, the scale layer is removed by pickling, but the scale layer is non-uniform, so the surface roughness after the scale is very large. turn into. Therefore, it is difficult to completely remove the impurity layer left by brush grinding even after the entire surface is pickled uniformly, and when the alloying treatment is performed after hot dipping, the alloying growth rate is partially increased. Differences may occur and processing unevenness may occur. However, even in this case, if the shot blast is strengthened, the above-mentioned adverse effects can be reduced.

However, if the "amount of the surface of the base material to be completely scraped" is secured as the brush grinding amount, there is no concern as described above, and the shot blasting process may be unnecessary. It is very difficult to secure the quantity, and there is a problem in productivity.

Note that FIG. 5 shows the results of the investigation of "the relationship between the amount of grinding of the surface of a hot rolled steel sheet (steel type C in the examples described later) by a grinding brush and the degree of alloying growth" during galvannealing treatment. It is a graph which shows. From this Figure 5,
When the grinding amount is 6 g / m ² or more, the improvement of the grinding effect is hardly recognized, and it can be seen that the effect is saturated.

As a method of brush grinding, a method of performing strong grinding using a grinding brush instead of "a brush which is usually used for removing sludge" after shot blast pickling, and "alkali cleaning before hot dipping" It is possible to use a grinding brush instead of a "brush" for strong grinding. Of course, there is almost no effect of promoting alloying even if it is simply washed with an alkaline cleaning brush after pickling without using a grinding brush, but the surface of the steel sheet is cleaned before it is used as a plating base material, especially when it is transported to the hot dip coating line. Roll (Her
In order to suppress damage to the scroll, such an alkali cleaning brush may be appropriately additionally used.

The alloying is further promoted when the hot-rolled steel sheet is subjected to pickling descaling treatment accompanied by shot blasting and then cold rolling, because the reason is that after descaling. It is considered that this is because the impurity layer remaining on the surface layer of the steel sheet is "broken" or "thinned" by cold rolling. Then, as a result of investigating the relationship between the cold rolling rate and alloying growth, the cold rolling rate must be 30% or more because the alloying promoting effect is insufficient if the cold rolling rate is less than 30%. found.

However, even in this case, if the surface roughness after removing the scale by pickling is large, rolling may not be carried out or insufficient rolling may occur. It is necessary to be careful because uneven alloying treatment tends to occur in the direction of stripes, but strengthening the shot blast reduces such uneven alloying treatment, and can form a uniform alloyed plating film. It is possible. Although the effect of cold rolling cannot be expected, it is preferable to use a skin pass from the viewpoint of the characteristic value of the steel sheet or the surface shape, and it is preferable to use it in combination.

By the way, as mentioned earlier, the "phenomenon in which the surface roughness partially increases due to the non-uniformity of the scale layer" is greatly affected by the Si content in steel, and the Si content is 0.2 It becomes remarkable when it becomes more than%. Further, C, Mn, P, etc. also slow down the alloying rate, so they cannot be said to be preferable components for alloying hot-dip galvanizing, but if the treatment accompanied by the shot blasting treatment according to the present invention is carried out, C: 0.002 to 0.2% , Si:
Even when the base material is "high-strength hot-rolled steel sheet, which is difficult to be hot-dipped or alloyed", which contains 0.2 to 2.0%, Mn: 0.5 to 2.5%, P: 0.1% or less and the balance is Fe and inevitable impurities. It is possible to obtain an alloyed hot-dip galvanized steel sheet of good quality.

The base material thus pretreated is passed through a normal continuous galvannealing line. In a normal continuous galvanizing hot-dip galvanizing line, a hot dip galvanizing tank is arranged continuously in a heating annealing furnace consisting of a heating zone, a reduction zone, a cooling zone, etc., and the plated steel sheet is immediately subjected to heat treatment (alloying). Processed). Therefore, when manufacturing an alloyed hot-dip plated steel sheet, first, the steel sheet is subjected to heat annealing to secure a predetermined characteristic value, and subsequently a predetermined basis weight (adhesion amount: generally 25 to 80 g / m ² ).
Hot-dip galvanizing is performed. Then, immediately after that, the plate temperature is heated to 400 to 600 ° C. for 3 to 20 seconds and held to allow thermal diffusion of Fe in the base material to the plated layer, and the Fe content in the plated film is 7 to 15%, and the balance remains. Fe-Zn consisting of iron
The alloy plating layer is formed.

Therefore, in a high-strength steel sheet containing elements in steel such as Si, Mn, and P, many elements in the steel concentrate on the surface during heating and annealing, which impedes wettability with hot-dip galvanizing. The thickening element also hinders alloying growth. As a countermeasure, there is a method of performing annealing in a non-oxidizing atmosphere, but there are many problems in terms of equipment and cost.

On the other hand, on the contrary, a large amount of oxide scale is once formed in an extremely strong oxidizing atmosphere, and a large amount of reduced iron is formed on the surface in the subsequent reduction zone. Also, a preferable annealing method can be considered. However, the high-strength steel sheet is unlikely to form an oxide scale even in such a strong oxidizing atmosphere, and it is usually difficult to apply the above method. However, when the above-mentioned base material pretreatment according to the present invention is applied, the reason why the steel sheet surface is oxidized is unknown.
Layer is formed reasonably enough to ensure plating wettability and alloying growth, making it possible to perform more stable alloying hot dip plating even under high-strength steel sheets under the same operating conditions as ordinary steel. . Therefore, when a high-strength steel sheet is used as the base material, it is recommended that the heat treatment is preferably performed in a strong oxidizing atmosphere prior to the hot dip galvanizing.

In order to form the above strongly oxidizing atmosphere, it is effective to control the air-fuel ratio of the heating furnace, and the air-fuel ratio is preferably 1.0 to 1.2. In this case, the air-fuel ratio
If it is less than 1.0, the oxide scale may not be sufficiently formed even if the pretreatment of the base material according to the present invention is performed, and untreated alloying may occur, which is not preferable. On the other hand, if the air-fuel ratio is larger than 1.2, too many oxidation scales are generated, which may damage the conveying roll in the annealing furnace and cause a flaw on the treated steel sheet, which is not preferable.

Next, the present invention will be described more specifically by way of examples.

[Examples] First, a hot-rolled steel sheet black material having the chemical composition shown in Table 1 was prepared and passed through an ordinary continuous galvannealed galvanizing line to carry out a production test of the galvannealed steel sheet.

[0036]

[Table 1]

However, in the above continuous alloy galvanizing line, a shot blasting device is arranged in the pickling tank, and instead of the "brush used for removing sludge" installed near the outlet side. A grinding brush was set, and some modifications were made so that the atmosphere in the heating zone of the annealing furnace could be converted into a strong oxidizing atmosphere.

The conditions of shot blasting, pickling, brush grinding, heat annealing, hot dip galvanizing and alloying treatment were as follows. a) Shot blast: Shot ... Steel ball (diameter: 0.3 mm), Density… 80 kg / m ² · min.

B) Pickling: Using a sulfuric acid pickling solution (concentration: 15%, temperature: 80 ° C.). c) Brush grinding: Grinding brush: core thread diameter 8 mmφ, abrasive material: SiC, abrasive grain size: # 240.

D) Heat annealing: Heating zone: Oxidation state control is possible by controlling the O ₂ concentration in the atmosphere Non-oxidizing atmosphere (O ₂ concentration: 5 ppm or less), weakly oxidizing atmosphere (O ₂ concentration: 100 to 1000 ppm), strong Oxidizing atmosphere (O ₂ concentration: over 1000 ppm). Reduction zone ... Atmosphere: 25% H ₂ -75% N ₂ , 30 at 800 ° C
Hold for seconds.

[0041] e) hot-dip galvanizing: bath temperature ... 460 ° C., bath Al concentration ... 0.15%, weight per unit area ... 50 ± 5g / m ^2. f) Alloying: Heating temperature: 550 ° C. at plate temperature, processing time: 40 seconds.

Next, the "galvanized finish appearance", "alloying degree" and "alloying degree" of the galvannealed steel sheet obtained by the above test were investigated. In addition, apart from this, in some tests, the amount of oxide scale formed due to the difference in atmosphere during heating was measured (the work was stopped after heating, the sample was taken out, and the amount of oxide scale was measured by film dissolution. (Measurement) was also performed. The results of these investigations are shown in Table 2 and FIGS.

The above survey results are shown in Table 2 as follows. <Appearance of plated finish (macroscopic plating defect)> 3: No plating (good finish), 2: Non-plating in pits, 1: Non-plating on the entire surface. <Degree of alloying treatment> 3: Alloying to the plating surface, 2: Partial alloying, 1: Whole surface untreated. <Degree of alloying> Displays the Fe content obtained by melting the plating film and performing chemical analysis.

[0044]

[Table 2]

From the results shown in Table 2 and FIGS.
The following is clear. That is, for example, by comparing test numbers 1, 2, 3, and 4 in Table 2, it can be seen that the alloying is promoted when the shot blasting process is incorporated. Further, by comparing Test Nos. 7, 8, and 9, for example, even in the case of steel grades where the alloying promotion effect is not sufficient only by shot blasting, alloying is promoted by combining brush grinding or cold rolling. I understand.
Then, by comparing test numbers 10, 11, 12, and 14, even if alloying is insufficient by brush grinding and cold rolling, alloying treatment can be performed by combining shot blasting treatment. I understand. By comparing test numbers 15 and 16, it is understood that when cold rolling is performed, a sufficient alloying promotion effect cannot be secured unless the cold rolling rate is 30% or more.

On the other hand, from FIG. 5 described above, the alloying degree by brush grinding becomes saturated at a grinding amount of 6 g / m ² or more unless shot blasting is performed. It can be seen that the alloying degree can be remarkably improved.

From FIGS. 6 and 7, the atmosphere of the heating zone in the heat annealing is a strong oxidizing atmosphere (O ₂ concentration ≧ 100).
It can be seen that by setting the content to 0 ppm), even if the desired alloying treatment could not be expected, sufficient alloying treatment can be performed.
It can also be seen that there is a correlation between the scale amount and the degree of alloying during heating and oxidation. In addition, the shot blast treatment increases the amount of scale produced during thermal oxidation,
It is also clear that alloying is promoted.

[0047]

[Summary of Effects] As described above, according to the present invention,
Even when using a high-strength hot-rolled steel sheet containing high Si and Mn, which is said to be extremely difficult to alloy hot-dip galvanizing by the conventional method, high-quality alloyed hot-dip galvanizing is performed without changing the operating conditions. It enables stable production of steel sheets.

As described above, the hot-rolled high-strength steel sheet can be galvannealed at the same timing as that of the ordinary steel without making a large change in equipment, so that there is no restriction on manufacturing opportunities. The process and cost advantages of producing galvannealed steel sheets are very large. Also, in the heat annealing process prior to hot dip galvanizing, it is possible to further improve the effect of alloying promotion simply by adding a means of heating in a stronger oxidizing atmosphere than in the past, so it is regulated because of the processability of hot dip galvannealing. It can be expected that the restrictions on the base material that had been adopted will become even looser.

By applying the present invention, it becomes possible to provide a high-strength surface-treated steel sheet having a high rust preventive performance at a low cost. For example, as a measure for reducing the vehicle body weight, which is currently being studied by automobile manufacturers and the like. It can be said that the industrial effect is very large, such as significant progress can be expected.

[Brief description of drawings]

FIG. 1 is a schematic explanatory diagram of an example of a process for producing a galvannealed steel sheet according to the present invention.

FIG. 2 is a schematic explanatory diagram regarding another example of the alloyed hot-dip galvanized steel sheet manufacturing process according to the present invention.

FIG. 3 is a schematic explanatory diagram regarding still another example of the process for producing a galvannealed steel sheet according to the present invention.

FIG. 4 is a schematic explanatory diagram regarding an example other than the above of the alloyed hot-dip galvanized steel sheet manufacturing process according to the present invention.

[Fig. 5] "Relationship between degree of alloying and degree of grinding of hot-rolled steel sheet surface by grinding brush" during galvannealing treatment
It is a graph which shows.

FIG. 6 is a graph showing the “relationship between the atmosphere in the heating zone of the heating and annealing furnace, the amount of oxide scale, and the degree of alloying” during the galvannealing treatment when shot blasting was not performed.

FIG. 7 is a graph showing the “relationship between the atmosphere in the heating zone of the heating and annealing furnace, the amount of oxide scale, and the degree of alloying” during hot dip galvanizing in the case of shot blasting.

Continuation of front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display area C22C 38/04

Claims (5)

[Claims] 1. A hot-rolled steel sheet to be hot-dipped is subjected to shot blasting during pickling and descaling. The hot-rolled steel sheet is hot dip galvanized in a continuous hot-dip galvanizing line and then heated to form an alloy. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises treating. 2. A hot-rolled steel sheet to be hot-dipped is subjected to shot blasting during pickling and descaling, the surface is brush-grinded, and then hot-rolled steel sheet is hot-dip galvanized in a continuous hot-dip galvanizing line. A method for manufacturing an alloyed hot-dip galvanized steel sheet, which comprises heating and alloying the same. 3. A hot-rolled steel sheet to be hot-dipped is subjected to shot blasting during pickling and descaling, followed by cold rolling at a rolling rate of 30% or more, and then the steel sheet is melted in a continuous hot-dip galvanizing line. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises performing galvanizing and heating the alloy for alloying treatment. 4. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 1, wherein hot-dip galvanizing is performed after heat treatment in a strong oxidizing atmosphere. 5. The hot-rolled steel sheet subjected to hot dip coating has a weight ratio of C: 0.002 to 0.2%, Si: 2.0% or less,
Mn: 0.5 to 2.5%, P: 0.1% or less, Fe and unavoidable impurities: the remaining chemical composition.
5. A method for manufacturing an alloyed hot-dip galvanized steel sheet according to any one of 4 to 4.