JP2007291498A - Manufacturing method of high-strength hot-dip-galvanized steel sheet excellent in appearance and plating adhesion - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method of manufacturing a hot-dip-galvanized steel sheet which has no non-deposited area, has a beautiful surface appearance and is excellent in plating adhesion when using a high-Si-content steel sheet as a parent material. <P>SOLUTION: The steel sheet comprising, by mass, ≤0.25% C, 0.1-3.0% Si, 0.5-3.0% Mn and 0.01-3% Al, wherein the Mn/Si ratio is ≤2, is subjected to heating (A-zone heating) at 400-750°C in an atmosphere containing ≥0.1% O<SB>2</SB>and ≥1% H<SB>2</SB>O, subsequently to heating (B-zone heating) at 600-850°C in an atmosphere containing <0.1% O<SB>2</SB>and ≥1% H<SB>2</SB>O, subsequently to heating (C-zone heating) in an atmosphere which contains 1-50% H<SB>2</SB>and has a dew point of 0°C and subsequently to a hot-dip galvanizing treatment. Since oxides at the surface, which conventionally react with a roll at C-zone heating and are picked up, are reduced and cut down by B-zone heating, the occurrence of pick-up at C-zone heating can be prevented. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for producing a hot-dip galvanized steel sheet using a Si-containing high-strength steel sheet as a base material, and more particularly, to a method for producing a hot-dip galvanized steel sheet having a beautiful surface appearance without unplating and excellent plating adhesion. .

In recent years, in the fields of automobiles, home appliances, building materials, etc., surface-treated steel sheets that give rust prevention to raw steel sheets, especially hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that can be manufactured inexpensively and have excellent rust prevention properties in use.

Generally, a hot dip galvanized steel sheet is manufactured by the following method. First, using a thin steel plate obtained by hot-rolling, cold-rolling or heat-treating the slab, the base steel plate surface is degreased and / or pickled and cleaned in the pretreatment step, or the pretreatment step is omitted. After the oil on the surface of the base steel plate is burned and removed, recrystallization annealing is performed by heating in a non-oxidizing atmosphere or a reducing atmosphere. Then, the steel sheet is cooled to a temperature suitable for plating in a non-oxidizing atmosphere or a reducing atmosphere, and in a molten zinc bath to which a small amount of Al (about 0.1 to 0.2%) is added without being exposed to the air Immerse in.
An alloyed hot-dip galvanized steel sheet is produced by subsequently heat-treating the steel sheet in an alloying furnace after hot-dip galvanizing.

By the way, in recent years, weight reduction has been promoted with higher performance of raw steel sheets, and higher strength of raw steel sheets has been demanded, and the amount of high-strength hot-dip galvanized steel sheets that have rust prevention properties is increasing. .
Addition of solid solution strengthening elements such as Si, Mn, P, and Al is performed to increase the strength of the steel sheet. Among these, Si and Al have an advantage that the strength can be increased without impairing the ductility of the steel, and the Si-containing steel plate is promising as a high-strength steel plate. In particular, a relatively low Mn / Si ratio in steel tends to ensure relatively good mechanical properties. However, it tries to manufacture hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets based on high-strength steel sheets that contain a small amount of Mn in steel and contain a large amount of Si and Al (that is, low Mn / Si ratio in steel). If you have the following problems.

  As described above, the hot dip galvanized steel sheet is subjected to hot dip galvanizing treatment after heat annealing at a temperature of about 600 to 900 ° C. in a reducing atmosphere. However, Si and Al in steel are easily oxidizable elements and are selectively oxidized in a reducing atmosphere that is generally used to concentrate on the surface to form oxides. Since these oxides reduce the wettability with molten zinc during the plating process and cause non-plating, the wettability rapidly decreases with the increase of the Si and Al concentrations in the steel, and non-plating occurs frequently. In addition, even when non-plating does not occur, there is a problem that the plating adhesion is poor.

  Further, when Si in steel is selectively surface oxidized and concentrated on the surface, a significant alloying delay occurs in the alloying process after hot dip galvanizing. As a result, productivity is significantly inhibited. If an alloying treatment is attempted at an excessively high temperature in order to ensure productivity, there is a problem that the powdering resistance is deteriorated, and it is difficult to achieve both high productivity and good powdering resistance. In addition, the addition of Si and Al facilitates the formation of residual γ phase and has the advantage of good mechanical properties, but the residual γ phase becomes unstable when alloyed at high temperature to avoid alloying delay. The mechanical properties may deteriorate and the benefits of adding Si and Al cannot be enjoyed.

Furthermore, even with the same amount of Si in the steel, if the Mn / Si ratio in the steel is low, a large amount of SiO ₂ with poor wettability and poor alloying reactivity after plating is formed. It is difficult to produce Mn ₂ SiO ₄ having good reactivity. Therefore, even if the Mn / Si ratio is low and the mechanical characteristics are good, it is difficult to achieve compatibility with the plating characteristics. Thus, a high-strength hot-dip galvanized steel sheet having good mechanical properties and plating properties could not be produced.

  Furthermore, in recent years, such high-strength steel sheets may be used in places that are easily visible, such as the outer panels and some undercarriage parts, and in addition to the above, the appearance of the surface is strictly required. It came to be.

Several techniques have been disclosed for such problems.
Patent Document 1 discloses a technique in which wettability with molten zinc is improved by heating a steel sheet in an oxidizing atmosphere in advance to form iron oxide on the surface, followed by heating and reduction annealing.

Prior to hot dipping, sulfur or a sulfur compound is deposited on the steel sheet surface as an S amount of 0.1 to 1000 mg / m ^2, and then a preheating step is performed in a weakly oxidizing atmosphere, and then in a non-oxidizing atmosphere containing hydrogen. A method of annealing is disclosed in Patent Document 2.

Moreover, the technique which ensures favorable plating quality by controlling atmospheres, such as oxygen concentration during preheating, is disclosed by patent document 3. However, in the case of a steel plate with a high Si amount in steel and a low Mn / Si ratio When the oxidation conditions are strengthened to secure the oxidation amount, the plating adhesion is improved and the non-plating is improved. However, since the oxidation amount is large, the oxide scale adheres to the in-furnace roll, and the steel sheet is pressed, so-called A pickup phenomenon occurs. When such a phenomenon occurs, the plating appearance deteriorates and is not suitable as an automobile part.
Patent registration No. 2587724 Japanese Patent Laid-Open No. 11-50223 Patent Registration No. 3415191

  The present invention has been made in view of such circumstances, and produces a hot dip galvanized steel sheet having a beautiful surface appearance free of non-plating and excellent plating adhesion when a high Si content steel sheet is used as a base material. It aims to provide a method. In particular, it improves the plating characteristics of low Mn / Si steel, which is difficult to improve even though the mechanical characteristics are good.

Plating adhesion improves when high-Si steel is strengthened by oxidation in the heating zone, but the oxide scale peels off in the reduction zone, resulting in creases. In order to solve this, the inventors conducted research focusing on the oxidation conditions of the heating zone.
As a result, by controlling the conditions on the heating zone exit side, it becomes possible to reduce the steel sheet surface once oxidized in the heating zone front stage to the middle stage on the heating zone exit side, and as a result, installed after the heating zone. It was found that the pickup in the reduction zone furnace can be suppressed.
That is, (1) oxidation heating (hereinafter referred to as A-band heating) for strengthening oxidation of high-Si steel was performed in the preceding stage to the middle stage of the heating zone, and then (2) the heating zone exit side was once oxidized. It is possible to reduce the surface of the steel sheet and perform heating under conditions where pickup does not occur, that is, under conditions of low-temperature reduction in a low O ₂ concentration atmosphere (hereinafter referred to as B-band heating). The steel plate surface once oxidized in the middle stage is subjected to a reduction treatment, and (3) high-temperature reduction heating (hereinafter referred to as C-band heating) is performed in the reduction zone. At this time, the surface oxide that has conventionally reacted with the roll in the C-band heating and caused the pickup is already reduced and reduced by the B-band heating in the present invention. Will not occur and will be prevented.

In addition, by controlling the ratio of FeO that has a low melting point and is likely to cause pick-up on the outermost surface of the steel plate, which is mainly composed of Fe-based oxide scale, after B-band heating, more pick-up occurs during C-band heating. It can be further prevented, and by making the steel sheet surface layer part after C-band heating have no unreduced Fe-based oxide remaining, pickup can be further prevented, and alloying delays and non-plating defects can be prevented. It was found that generation can be suppressed.
As described above, improvement in plating adhesion due to oxidation strengthening and non-plating are eliminated, and at the same time, pressing is suppressed, and an extremely beautiful plating appearance can be secured.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] Hot dip galvanizing on steel sheet with mass%, C: 0.25% or less, Si: 0.1-3.0%, Mn: 0.5-3.0%, Al: 0.01-3%, and Mn / Si ratio of 2 or less When the steel sheet is subjected to heating, the steel sheet is heated at a temperature of 400 to 750 ° C. in an atmosphere containing O ₂ ≧ 0.1% and H ₂ O ≧ 1% (A-band heating), and then O ₂ <0.1%, H ₂ Heat in an atmosphere containing O ≧ 1% at a temperature of 600 to 850 ° C. (B-band heating), then heat in an atmosphere containing H ₂ = 1 to 50% and having a dew point of 0 ° C. or less (C A method for producing a high-strength hot-dip galvanized steel sheet excellent in appearance and plating adhesion, characterized by subjecting to hot-dip galvanizing treatment after band heating.
[2] In the above [1], the A-zone heating is performed by a direct fire burner (DFF) or a non-oxidizing furnace (NOF) under the condition of an air ratio ≧ 1, and the B-zone heating is performed by a direct fire burner (DFF) or A method for producing a high-strength hot-dip galvanized steel sheet with excellent appearance and plating adhesion, characterized in that it is performed in a non-oxidizing furnace (NOF) under conditions of an air ratio <1.
[3] In the above [2], the A-band heating is performed under the condition that the air ratio is α or more and 1.5 or less represented by the following formula (1), and the B-band heating is performed using the following formula (2). 3. The method for producing a high-strength hot-dip galvanized steel sheet excellent in appearance and plating adhesion according to claim 2, characterized in that it is carried out at a value of not less than β and less than 1.0.
α = 0.3 [Si] + [P] +0.65 (where α ≧ 1.0) --- (1)
β = 0.05 [Si] + 0.25 [P] + 0.675 (2)
However, [Si] and [P] represent mass% of Si and P in the steel sheet, respectively.
[4] In any one of the above [1] to [3], before performing the A-band heating, a solution containing S is applied to the steel sheet, and S is adhered to the steel sheet surface by 10 to 1000 mg / m ^2. A method for producing a high-strength hot-dip galvanized steel sheet having excellent appearance and plating adhesion.
[5] The method for producing a high-strength hot-dip galvanized steel sheet having excellent appearance and plating adhesion, wherein the alloying treatment is performed after the hot-dip galvanizing treatment in any one of the above [1] to [4].
[6] In any one of [1] to [5], the steel鈑最surface after the B band heating, in mol%, FeO is less _{30%, Fe, Fe 2 O} 3, Fe 3 O _A method for producing a high-strength hot-dip galvanized steel sheet excellent in appearance and plating adhesion, characterized in that the sum of ₄ is 70% or more.
[7] In any one of the above [1] to [6], in the steel plate after the C-band heating, FeO, Fe ₂ O _3, Fe ₃ may be formed in a region from the steel plate outermost surface to a depth direction of 5 μm. A method for producing a high-strength hot-dip galvanized steel sheet excellent in appearance and plating adhesion, characterized by not containing O ₄ .

  In addition, in this specification,% which shows the component of steel is all mass%.

  According to the present invention, there can be obtained a hot dip galvanized steel sheet having a beautiful surface appearance without plating and excellent plating adhesion. The present invention is also effective when a high Si content steel sheet is used as a base material, and improves the plating characteristics of low Mn / Si steel, which is difficult to improve the plating characteristics despite good mechanical characteristics. It can be said that the invention is useful as a method.

Hereinafter, the present invention will be specifically described.
First, the steel plate used in the present invention will be described. The composition of the steel sheet of the present invention is as follows.

C: 0.25% or less,
In order to easily form a residual γ phase, 0.05% or more is preferable. However, in the present invention, the lower limit is not particularly specified. On the other hand, if the upper limit exceeds 0.25%, the weldability deteriorates. Therefore, the C content is 0.25% or less.

Si: 0.1-3.0%
A steel plate as a plating base plate is a high-strength steel plate containing Si, Mn, Al and the like in the steel. In particular, Si and Al are residual γ-forming elements and are most important because they are effective in improving the mechanical properties of steel sheets. For that purpose, the Si content needs to be 0.1% or more. However, if it exceeds 3.0%, it is difficult to suppress the formation of an oxide film and it is difficult to improve adhesion. Therefore, the Si content is 0.1% or more and 3.0% or less.

Mn: 0.5-3.0%
To increase the strength, it is more effective to add Mn ≧ 0.5%. On the other hand, if Mn exceeds 3.0%, it becomes difficult to ensure weldability, plating adhesion, and strength ductility balance. Therefore, the Mn content is 0.5% or more and 3.0% or less.

Al: 0.01 to 3%
Al is an element added complementary to Si. If Si is added sufficiently without containing Al, the mechanical properties can be secured, but it is inevitably mixed in the steelmaking process, so Al needs to be contained in an amount of 0.01% or more. On the other hand, if the amount of Al added exceeds 3%, it is difficult to suppress the formation of an oxide film and it is difficult to improve the adhesion. Therefore, the Al content is 0.01% or more and 3% or less.

Mn / Si ratio is 2 or less
The lower the Mn / Si ratio, the better the mechanical properties.

  The steel of the present invention can achieve the desired properties with the above essential additive elements, but can contain the following elements according to the desired properties.

  P is inevitably contained, and is preferably 0.001% or more in order to delay the precipitation of cementite and delay the progress of transformation. On the other hand, if it exceeds 0.10%, not only the weldability deteriorates, but also the surface quality deteriorates, so the plating adhesion deteriorates during non-alloying, the alloying temperature rises during alloying, and the ductility deteriorates at the same time. The adhesion of the alloyed plating film may deteriorate. Therefore, when contained, the P content is 0.001% or more and 0.10% or less.

In order to control the high strength ductility balance, if necessary, at least one of 0.01 ≦ Cr ≦ 1.0%, 0.01 ≦ Mo ≦ 1.0%, 0.005 ≦ Nb ≦ 0.2%, 0.005 ≦ Ti ≦ 0.2%, and residual γ In order to promote phase formation, at least one of 0.01 ≦ Cu ≦ 0.5%, 0.01 ≦ Ni ≦ 1.0%, 0.0005 ≦ B ≦ 0.01% may be added as necessary. These elements not only improve mechanical properties, but Cr, Mo, Nb, and Cu alone or in combination of two or more promote the internal oxidation of Si and Al and suppress the surface concentration. Has an effect. Therefore, it can also be added to obtain good plating adhesion. If Cr is less than 0.01%, strength adjustment and internal oxidation promotion effects are difficult to obtain, and if it exceeds 1.0%, the surface concentration of Cr is rather concentrated, which may deteriorate the plating adhesion and weldability. If the Mo content is less than 0.01%, it is difficult to obtain the effect of adjusting the strength or improving the adhesion of the plating when combined with Nb, Ni or Cu, and if it exceeds 1.0%, the cost may increase. If Nb is less than 0.005%, it is difficult to obtain the effect of adjusting the strength and the effect of improving the plating adhesion when combined with Mo, and if it exceeds 0.2%, the cost may increase. If Ti is less than 0.005%, the effect of adjusting the strength is difficult to obtain, and if it exceeds 0.2%, plating adhesion may be deteriorated. If Cu is less than 0.01%, it is difficult to obtain the effect of promoting the formation of the residual γ phase and the effect of improving the plating adhesion when combined with Ni or Mo, and if it exceeds 0.5%, the cost may increase. If Ni is less than 0.01%, it is difficult to obtain the effect of promoting the formation of the residual γ phase and the effect of improving the plating adhesion when combined with Cu or Mo, and if it exceeds 1.0%, the cost may increase. If B is less than 0.0005%, the effect of promoting the formation of the residual γ phase is difficult to obtain, and if it is 0.01% or more, the plating adhesion may deteriorate.
S is an element that is inevitably contained as in the case of P. However, since a weldability deteriorates when contained in a large amount, S is preferably 0.2% or less.

Next, a method for producing a high-strength hot-dip galvanized steel sheet will be described.
Hot dip galvanizing is performed on the steel sheet having the above composition. In the present invention, the steel sheet is subjected to plating treatment after being heated in the following three steps. This heating is an important requirement in the present invention, and in particular, B-band heating is the most important requirement. By performing B-band heating between A-band heating and C-band heating under the following conditions, the amount of surface oxide in C-band heating is reduced to an amount that does not cause pickup, and the surface Si It becomes possible to adjust to an amount sufficient to prevent the formation of the system oxide.
A-band heating: heating at a temperature of 400 to 750 ° C. in an atmosphere containing O ₂ ≧ 0.1% and H ₂ O ≧ 1% B-band heating: containing O ₂ <0.1% and H ₂ O ≧ 1% In the atmosphere, heating at a temperature of 600 to 850 ° C. C-band heating: In an atmosphere including H ₂ = 1 to 50% and having a dew point of 0 ° C. or less, the heating consisting of these three steps will be described below.

A-Band Heating Heating in the A-band is performed to positively oxidize the steel sheet. Therefore, O ₂ must have a sufficient amount for oxidation and is set to 0.1% or more. An upper limit is not set, but it is preferably 20% or less of the atmospheric level for economic reasons. H ₂ O is made 1% or more in order to promote oxidation. An upper limit is not set, but 50% or less is preferable in consideration of humidification costs. Further, the heating temperature is less than 400 ° C., and it is difficult to oxidize.

B-Band Heating Heating in the B-band is most important in the present invention in order to suppress the pressing and ensure a beautiful appearance. Therefore, in the B-band heating, it is possible to reduce the surface of the steel plate once oxidized, and heating is performed under conditions where pick-up does not occur, that is, low-temperature reduction heating in a low O ₂ concentration atmosphere. The steel plate surface once oxidized in the middle stage is reduced to the extent that it reacts with the roll and does not pick up in the next C-band heating. If O ₂ is 0.1% or more, it cannot be reduced, so O ₂ should be less than 0.1%. H ₂ O is made 1% or more so as not to reduce too much. Heating temperatures below 600 ° C are difficult to reduce, and heating temperatures above 850 ° C require heating costs.

  As described above, the heating zone needs to be divided into at least two zones of the A zone and the B zone in order to satisfy the above conditions. In addition, it is also possible to perform A zone | band heating and B zone | band heating in a separate furnace. However, in consideration of industrial productivity and improvement of the current production line, it is preferable to divide into two zones or more by changing the condition setting in the same furnace.

  The A-zone heating is performed by a direct fire burner (DFF) or non-oxidizing furnace (NOF) under the condition of an air ratio ≧ 1, and the B-zone heating is performed by a direct fire burner (DFF) or non-oxidizing furnace (NOF). The air ratio is preferably less than 1. Direct fire burners (DFF) or non-oxidizing furnaces (NOF) are widely used as hot dip galvanizing furnaces (CGL furnaces), and the air ratio can be easily controlled. It is preferable to use a burner (DFF) or a non-oxidizing furnace (NOF). In addition, if the air ratio is outside the above conditions, the oxygen concentration may be outside the above range, and the effects of the present invention may not be obtained. The air ratio is less than 1.

Further, it is preferable that the air ratio of the A band is performed under the condition of α or more and 1.5 or less represented by the following formula, and the B band heating is performed with the air ratio of β or more and represented by the following formula or less than 1.0.
α = 0.3 [Si] + [P] +0.65 (where α ≧ 1.0) --- (1)
β = 0.05 [Si] + 0.25 [P] + 0.675 (where β <1.0) --- (2)
However, [Si] and [P] represent mass% of Si and P in the steel sheet, respectively.

  In the A band, the air ratio needs to be 1 or more in order to oxidize Fe of steel added with alloy elements such as Si and Al by increasing the air ratio. Furthermore, if the mass% of Si and P in steel increases, the oxidation of Fe is further promoted and the ratio of Fe oxide in the oxide is not lowered. It turned out not to be good. Therefore, a technology to control the air ratio by mass% of Si and P in steel was devised. Figures 1 and 2 show the relationship between the mass% of Si and P in steel, the appearance of the plated steel sheet, and the plating adhesion. Note that the measurement method and evaluation (◯ and Δ in the figure) are the same as in the examples described later, and the plating appearance and adhesion evaluation level (○, Δ, ×) of each evaluation material were the same. These are shown together with one evaluation symbol. When the lower limit of the air ratio at which these characteristics are particularly good is α, the above equation (1) is obtained in relation to α and the mass% of Si and P in the steel.

  In the B zone, the air ratio is lowered to reduce the Fe oxide on the steel surface. However, the reduction is to suppress the roll pickup by using the Fe oxide on the surface in contact with the roll as the metal Fe in the C zone. When the amount of reduction increases, as a result, oxides such as Si are concentrated on the surface and the plating appearance and plating adhesion deteriorate, and the oxidation effect in the A band cannot be obtained. Figures 3 and 4 show the relationship between the mass% of Si and P in steel, the appearance of the plated steel sheet, and the plating adhesion. The measurement method and evaluation (◯ and Δ in the figure) are the same as those in FIGS. When the lower limit of the air ratio at which these characteristics are particularly favorable is β, the above equation (2) is obtained in relation to β and mass% of Si and P in steel.

In addition, the outermost surface of the steel plate after B-band heating is mainly composed of an Fe-based oxide scale. By controlling the FeO generation ratio to be low on the outermost surface, pickup in C-band heating can be more reliably prevented. FeO (wustite: melting point 1370 ° C) has a lower melting point than Fe ₂ O ₃ (hematite: melting point 1550 ° C), Fe ₃ O ₄ (magnetite: melting point 1538 ° C), Fe (iron: melting point 1535 ° C), It is easy to sinter with rolls at high temperatures, and pick-up is likely to occur compared to other Fe-based oxide scales. Therefore, the pickup can be further suppressed by lowering the FeO generation ratio. For the reasons described above, in the present invention, the FeO production ratio is preferably 30% or less. In addition, the analysis method of an oxide scale composition includes a method in which a C-band heating and a plating bath are evacuated, a steel plate that has been left with B-band heating is collected, and quantified by an X-ray diffraction method. The compositional composition ratio of each oxide can be obtained based on the measurement result of the standard sample for quantification. It is also possible to determine the composition of the oxide scale by installing an X-ray diffractometer on the B-band heating outlet side and measuring the X-ray intensity of a specific plane orientation of each scale online.

It is installed immediately after the C zone heating and heating zone and performs a reduction treatment. For this reason, the atmosphere in the reduction zone has H _{2 in the} range of 1% to 50%. The dew point is 0 ° C or less. If this condition is not met, the oxide scale generated in the heating zone is difficult to reduce, so even if sufficient oxide scale, internal oxidation, or nitride is generated to ensure plating adhesion, the plating characteristics tend to deteriorate. It can be seen. Also, if H ₂ exceeds 50%, the cost will increase. Moreover, since it is difficult to implement industrially when the dew point is less than -60 ° C, the dew point is preferably -60 ° C or higher.

In order to completely reduce the oxide film formed in the B band in the C band, FeO, Fe ₂ O ₃ , Fe _{3 in} the region from the outermost surface to the depth direction of 5 μm in the steel sheet after the C band is heated. It is important not to contain O ₄ . By controlling in this way, generation | occurrence | production of a pickup is suppressed further and the alloying reaction after plating is accelerated | stimulated. If oxides such as FeO, Fe ₂ O ₃ , and Fe ₃ O ₄ remain on the surface of the steel sheet, naturally pick-up is likely to occur, and it also becomes a barrier to alloying that is a Fe-Zn diffusion reaction. There is a problem when alloying is performed. Even when the alloying treatment is not performed, the adhesion of the plating during bending is deteriorated starting from the oxide. The remaining oxide scale can be confirmed by measuring the region from the outermost surface of the steel sheet taken through the plating bath to 5 μm by XRD. If the iron oxide peak is not detected by this method, there is no problem. You may use EPMA together as needed. Furthermore, the presence or absence of iron oxide can also be confirmed by analyzing the cross section of the steel sheet after plating.

Furthermore, in order to promote oxidation under the above conditions, particularly uniform and non-uniform oxidation, in the present invention, before performing A-band heating, a solution containing S is applied to the steel sheet, and 10 to 1000 mg of S is applied to the steel sheet surface. It is preferable to attach / m ² . By applying S to the steel plate surface in advance, there is an effect of promoting oxidation of the steel plate surface. At this time, if it is less than 10 mg / m ² , uneven oxidation tends to occur. On the other hand, when it exceeds 1000 mg / m ² , the effect is hardly obtained.

For the production of hot-dip galvanized steel sheets that are hot-dip galvanized after heating in the above three steps (A-band heating to C-band heating), a galvanizing bath with a bath temperature of 440 to 550 ° C and an Al concentration in the bath of 0.14 to 0.24% In the production of the alloyed hot-dip galvanized steel sheet, a zinc plating bath having a bath temperature of 440 to 550 ° C. and an Al concentration in the bath of 0.10 to 0.20% is used. If the bath temperature is less than 440 ° C, there is a possibility that the solidification of Zn may occur in places where the temperature variation in the bath is large. There are operational problems due to adhesion. Furthermore, since alloying proceeds during plating, it tends to be overalloyed.

  When the alloying treatment is not performed, the Al concentration in the bath is preferably about 0.14 to 0.24%. If it is less than 0.14%, the Fe—Zn alloying reaction proceeds during plating, resulting in uneven appearance. If it exceeds 0.24%, the Fe-Al alloy layer formed at the plating / base metal interface in the plating layer during plating is formed thick, so that the weldability deteriorates. Also, since there is a lot of Al in the bath, a large amount of Al oxide film is formed on the surface of the plated steel sheet, and not only the weldability but also the appearance is greatly deteriorated.

  The Al concentration in the bath during the alloying treatment is preferably about 0.10 to 0.20%. If it is less than 0.10%, a hard and brittle Fe-Zn alloy layer is formed at the plating / base metal interface at the time of plating, so that the plating adhesion deteriorates. If it exceeds 0.20%, the Fe—Al alloy layer formed at the plating / base metal interface grows thick immediately after plating, so the weldability deteriorates.

  In addition, Mg may be added to the plating bath to improve the corrosion resistance. This technique can be applied not only to hot dip galvanizing but also to hot dip Al, hot dip Zn-5% Al, hot dip Zn-55% Al plating, and the like.

The alloying process is optimally performed at a temperature higher than 460 ° C. and lower than 570 ° C. Below 460 ° C, alloying progresses slowly, and at 570 ° C and above, not only does the hard and brittle Zn-Fe alloy layer formed at the iron-iron interface due to overalloying deteriorate the plating adhesion, but also the residual austenite phase Since it decomposes, the strength ductility balance also deteriorates. The plating adhesion amount is not particularly defined, but is preferably 10 g / m ² or more from the viewpoint of corrosion resistance and plating adhesion amount, and preferably 120 g / m ² or less from the viewpoint of workability.

  Hereinafter, the present invention will be specifically described based on examples.

A slab having a steel composition shown in Table 1 was heated in a heating furnace at 1260 ° C. for 60 minutes, subsequently hot-rolled to 2.8 mm and wound at 540 ° C. Next, the black scale was removed by pickling and cold rolled to 1.6 mm. Then, using DFF type or NOF type CGL having a heating zone divided into 4 zones, changing the oxidation conditions to the range shown in Table 2 and performing heat treatment (A zone heating & B zone heating), Table 2 Annealing (C-band heating) was performed under the conditions shown in FIG. Subsequently, hot dip galvanizing was performed with Al-containing Zn at 460 ° C., followed by alloying treatment to obtain a hot dip galvanized steel sheet. In addition, 0.08 to 0.20% Al-containing Zn bath was used as the Al concentration in the bath. The amount of adhesion was adjusted to 40 g / m ² per side by gas wiping. The alloying treatment was performed at 500 to 580 ° C. Moreover, the steel sheet after B-band heating was obtained by performing C-band heating and plating bath without performing C-band heating and plating treatment, and collecting the steel sheet. Moreover, the steel plate after heating C zone | band was obtained by emptying only a plating bath and extracting a steel plate, without performing a plating process. These steel sheets were quantified by an X-ray diffraction method, and the presence and amount of Fe, FeO, Fe ₂ O ₃ and Fe ₃ O ₄ were confirmed.

With respect to the hot dip galvanized steel sheet obtained as described above, the plating appearance and the tight adhesion were investigated by the method described below. The obtained results are shown in Table 2 together with the conditions.
<Plating appearance>
Judging by visual inspection for appearance defects such as non-plating and push rods, it is particularly good (○) when there is no appearance defect, good (△) when there is a slight appearance defect but generally good, and appearance When there was a defect, it was determined as a defect (x).
<Plating adhesion>
Measure the amount of peel per unit length when cellotape (registered trademark) is applied to an alloyed hot-dip galvanized steel sheet and the tape surface is bent and bent back at 90 ° C as the Zn count by fluorescent X-ray. In light of this, those with ranks 1 and 2 were evaluated as particularly good (◯), good (Δ), and three or more as bad (x).
X-ray fluorescence count rank
0-500 1 (good)
500-1000 2
1000-2000 3
2000-3000 4
3000 or more 5 (poor)
About the hot-dip galvanized steel sheet which is not alloyed, the ball impact test was performed, the processed part was peeled off with cello tape (registered trademark), and the plating adhesion was evaluated by visually judging the presence or absence of peeling of the plating layer.

○: Plating layer is not peeled ×: Plating layer is peeled

  From Table 2, it can be seen that the (alloyed) hot dip galvanized steel sheet of the present invention has a beautiful surface appearance without unplating and excellent plating adhesion despite containing Al and Si. In particular, even in the low Mn / Si steel, which is considered difficult to improve the plating characteristics, the plating properties are improved, and it has excellent plating appearance and adhesion.

As in Example 1, a slab having a steel composition shown in Table 3 was heated in a heating furnace at 1260 ° C. for 60 minutes, subsequently hot-rolled to 2.8 mm and wound at 540 ° C. Next, the black scale was removed by pickling and cold rolled to 1.6 mm. After that, using a DFF type or NOF type CGL having a heating zone divided into 4 zones, the oxidation conditions were changed to the ranges shown in Table 3 and heat treatment (A zone heating & B zone heating) was performed. Annealing (C-band heating) was performed under the conditions shown in FIG. Subsequently, hot dip galvanizing was performed with Al-containing Zn at 460 ° C., followed by alloying treatment to obtain a hot dip galvanized steel sheet. In addition, 0.08 to 0.20% Al-containing Zn bath was used as the Al concentration in the bath. The amount of adhesion was adjusted to 40 g / m ² per side by gas wiping. The alloying treatment was performed at 500 to 580 ° C. With respect to the hot-dip galvanized steel sheet obtained as described above, the plating appearance and the plating adhesion were investigated in the same manner as in Example 1. The obtained results are shown in Table 3 together with the conditions.

  From Table 3, by adjusting the air ratios of the A and B bands optimally, the alloyed hot-dip galvanized steel sheet of the present invention contains Al and Si, but has a particularly beautiful surface appearance and plating adhesion. It can be seen that

  Since it has good mechanical properties and is excellent in plating appearance and plating adhesion, it is expected to be used in a wide range of applications, especially in the fields of automobiles, home appliances, building materials, and the like.

It is a figure which shows the relationship between Si and P content rate in steel, and the appropriate air ratio minimum (alpha) in A zone | band heating. It is a figure which shows the relationship between Si and P content rate in steel, and the appropriate air ratio minimum (alpha) in A zone | band heating. It is a figure which shows the relationship between Si, P content rate in steel, and the appropriate air ratio lower limit (beta) in B zone heating. It is a figure which shows the relationship between Si, P content rate in steel, and the appropriate air ratio lower limit (beta) in B zone heating.

Claims (7)

When hot-dip galvanizing is applied to a steel sheet containing mass%, C: 0.25% or less, Si: 0.1-3.0%, Mn: 0.5-3.0%, Al: 0.01-3%, and an Mn / Si ratio of 2 or less. ,
Steel plate
In an atmosphere containing O ₂ ≧ 0.1% and H ₂ O ≧ 1%, heating at a temperature of 400 to 750 ° C. (A-band heating)
Next, in an atmosphere containing O ₂ <0.1% and H ₂ O ≧ 1%, heating at a temperature of 600 to 850 ° C. (B-band heating)
Next, after heating (C-band heating) in an atmosphere containing H ₂ = 1 to 50% and having a dew point of 0 ° C. or less,
A method for producing a high-strength hot-dip galvanized steel sheet excellent in appearance and plating adhesion, characterized by performing hot-dip galvanizing treatment.   The A-zone heating is performed by a direct fire burner (DFF) or a non-oxidizing furnace (NOF) under the condition of an air ratio ≧ 1, and the B-zone heating is performed by a direct fire burner (DFF) or a non-oxidizing furnace (NOF). The method for producing a high-strength hot-dip galvanized steel sheet having excellent appearance and plating adhesion according to claim 1, which is performed under a condition of ratio <1. The A-band heating is performed under the condition that the air ratio is α or more and 1.5 or less represented by the following formula (1), and the B-band heating is performed with the air ratio of β or more and less than 1.0 represented by the following formula (2). 3. The method for producing a high-strength hot-dip galvanized steel sheet having excellent appearance and plating adhesion according to claim 2.
α = 0.3 [Si] + [P] +0.65 (where α ≧ 1.0) --- (1)
β = 0.05 [Si] + 0.25 [P] + 0.675 (2)
However, [Si] and [P] represent mass% of Si and P in the steel sheet, respectively. Before performing A-band heating, a solution containing S is applied to a steel sheet, and S is adhered to the steel sheet surface in an amount of 10 to 1000 mg / m ^2. A method for producing a high-strength hot-dip galvanized steel sheet with excellent plating adhesion.   The method for producing a high-strength hot-dip galvanized steel sheet excellent in appearance and plating adhesion according to any one of claims 1 to 4, wherein the alloying treatment is performed after the hot-dip galvanizing treatment. The outermost surface of the steel sheet after the B-band heating is mol%, FeO is 30% or less, and the total of Fe0, Fe ₂ O ₃ and Fe ₃ O ₄ is 70% or more. The manufacturing method of the high intensity | strength hot-dip galvanized steel plate which is excellent in the external appearance property and plating adhesiveness in any one of 1-5. In the steel sheet after the C-band heating, the region from the outermost surface of the steel sheet to the depth direction of 5 μm does not contain FeO, Fe ₂ O ₃ , or Fe ₃ O ₄ . The manufacturing method of the high intensity | strength hot-dip galvanized steel plate which is excellent in the external appearance property and plating adhesiveness in any one.

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