JP6137002B2 - Method for producing hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet, hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a method for producing a hot dip galvanized steel sheet and an alloyed hot dip galvanized steel sheet, and in particular, hot dip galvanizing that efficiently determines the maximum heating temperature in oxidation treatment and combines good mechanical properties and plating properties. The present invention relates to a method for producing a steel sheet and an galvannealed steel sheet, and a galvanized steel sheet and an galvannealed steel sheet produced by the method of the present invention.

  In recent years, in the fields of automobiles, home appliances, building materials, etc., surface-treated steel sheets with rust-preventing properties, especially hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that can be manufactured at low cost and have excellent anti-rust properties. Is used.

  Generally, a hot dip galvanized steel sheet is manufactured by the following method. That is, first, the surface of a thin steel plate obtained by hot-rolling, cold-rolling or heat-treating a slab is cleaned by degreasing and / or pickling in the pretreatment step, or the pretreatment step is omitted and the steel plate is removed in the preheating furnace. After the oil on the surface is removed by combustion, 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 hot dip galvanizing bath to which a small amount of Al (about 0.1 to 0.2% by mass) is added without being exposed to the air. Immerse and galvanize. Thus, a hot-dip galvanized steel sheet can be manufactured. Furthermore, an alloying hot dip galvanized steel plate can be manufactured by heat-processing a steel plate in an alloying furnace following a hot dip galvanization process.

  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. . Here, in order to increase the strength of the steel sheet, it is common to add a solid solution strengthening element such as Si, Mn, P, or Al to the steel. Among these, Si and Al have an advantage that the strength can be increased without impairing the ductility of the steel. In particular, a steel plate containing Si is promising as a high strength steel plate.

  For this Si-containing steel sheet, the lower the ratio of the Mn content to the Si content in the steel sheet (hereinafter referred to as “Mn / Si ratio”), the more easily the residual γ phase is formed, ensuring good mechanical properties. Cheap. However, when manufacturing hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets using a high-strength steel sheet containing a small amount of Mn in the steel sheet and containing a large amount of Si or Al as a raw steel sheet, the following problems exist.

  That is, as described above, when a hot dip galvanized steel sheet is manufactured, hot dip galvanizing treatment is performed after recrystallization annealing is performed at a temperature of, for example, about 600 to 900 ° C. in a reducing atmosphere. However, since Si and Al in steel are easily oxidizable elements, even in a generally used reducing atmosphere, selective surface oxidation is performed and the surface is concentrated to form Si and Al oxides. These oxides have a problem that the wettability with molten zinc at the time of the plating treatment is lowered, and the plating adhesion is deteriorated even when non-plating occurs or non-plating does not occur.

  Further, when Si in steel is selectively surface oxidized and concentrated on the surface, a remarkable alloying delay occurs during the plating process, and as a result, the productivity is remarkably lowered. Therefore, if alloying is performed at an excessively high temperature in order to increase productivity, a new problem that deteriorates the powdering resistance occurs, and it is difficult to achieve both high productivity and good powdering resistance. is there. Further, the alloying treatment at a high temperature may cause the residual γ phase to become unstable and the mechanical characteristics to deteriorate, so that the benefits of adding Si or Al may not be enjoyed.

Furthermore, even if the amount of Si in the steel is the same, if the Mn / Si ratio in the steel is low, the wettability is poor, and a large amount of SiO ₂ with low alloying reactivity after plating is formed. It is difficult to form Mn ₂ SiO ₄ having relatively good reactivity after plating. Therefore, even if the Mn / Si ratio is low and the mechanical properties are good, it is difficult to achieve both good plating properties.

  Thus, it is difficult to produce a high-strength hot-dip galvanized steel sheet that has good mechanical characteristics and plating characteristics. In addition, in recent years, such high-strength steel sheets may be used in places where they are easily visible, such as outer panels and some undercarriage parts, and the appearance of the surface is also strictly required. It became so.

  Several techniques have been disclosed for such problems. For example, Patent Document 1 discloses that a steel sheet is preheated in an oxidizing atmosphere having an air ratio of 0.95 to 1.10, rapidly oxidized at an average oxidation rate of 30 angstroms / second to form iron oxide on the steel sheet surface, and then the hydrogen concentration. It describes a technique for improving adhesion and appearance uniformity after hot-dip galvanization by heating a steel sheet in a reducing atmosphere of 10% or less and performing reduction annealing.

Further, in Patent Document 2, prior to hot dip galvanizing treatment, 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. Hot-dip galvanized steel sheet with good surface appearance, no linear marks, high strength, excellent plating film uniformity, and excellent adhesion by annealing in a non-oxidizing atmosphere containing hydrogen Is described.

  Furthermore, Patent Document 3 describes a technique for ensuring good plating characteristics by controlling the oxygen concentration and the amount of water vapor in the atmosphere in the oxidation treatment.

Japanese Patent No. 2587724 Japanese Patent Laid-Open No. 11-50223 Japanese Patent No. 3415191

  By the way, as for the technique described in the said patent documents 1-3, after heating the steel plate used as a raw material in an oxidizing atmosphere and performing the oxidation process which forms iron oxide in a steel plate surface layer, in a reducing atmosphere By applying a reduction treatment that reduces the iron oxide formed by heating and oxidation treatment, so-called oxidation / reduction treatment is performed to suppress selective surface oxidation of Si-based oxides and improve wettability with molten zinc. It is based on technology. In this technique, in order to suppress the selective surface oxidation of the Si-based oxide in the reduction treatment, it is important to secure a necessary and sufficient amount of oxidation in the oxidation treatment.

  The oxidation amount of the steel sheet can be controlled mainly by the maximum heating temperature or the atmosphere in the heating furnace in the oxidation treatment. Among these, since it is difficult to change the atmosphere in the heating furnace quickly in an actual process, it is preferable to control the oxidation amount by changing the maximum heating temperature. Development of high-strength steel sheets has been actively carried out, and many various new steel types have been developed. In order to develop these into actual processes, it is necessary to determine an appropriate maximum heating temperature for each steel type. Hereinafter, “appropriate maximum heating temperature” means that sufficient iron oxide is formed on the steel sheet surface layer in the oxidation process in actual production (actual operation), and plating defects such as non-plating and poor plating adhesion do not occur. It refers to the maximum heating temperature during oxidation treatment that can form a plating having both good mechanical properties and plating properties.

  However, in order to find this appropriate maximum heating temperature, it is necessary to produce steel sheets annealed at various maximum heating temperatures in a laboratory and find a condition that can secure a necessary and sufficient amount of oxidation. This is a very complicated operation, and even if a new steel type is developed, it takes a long time to develop it into an actual process, which has been a major development issue.

  Accordingly, an object of the present invention is to provide a method for producing a hot-dip galvanized steel sheet having both good mechanical characteristics and plating characteristics by efficiently determining an appropriate maximum heating temperature in the oxidation treatment.

  The inventors diligently studied how to solve the above problems. For that purpose, an in-situ observation experiment was performed on the surface of the steel sheet using an environmental scanning electron microscope (E-SEM) to simulate the oxidation process during actual production (actual operation). The formation status of iron oxide on the surface layer of steel plate in various oxidation treatments was grasped. As a result, as will be described in detail below, it has been found that there is a temperature at which the iron oxide particle size in the steel sheet surface layer rapidly increases and the form of the iron oxide transitions.

  FIG. 1 is an SEM image of the surface of a steel plate containing 1.8% by mass of Si when the temperature is raised in an oxidizing atmosphere while observing with E-SEM. Here, (a) to (d) correspond to the case where the temperature of the steel sheet surface is (a) 750 ° C., (b) 860 ° C., (c) 890 ° C. and (d) 950 ° C., respectively.

  Comparing FIGS. 1 (a) and 1 (b), the iron oxide particle size is about 0.1 μm to 0.2 μm at any temperature, and even if the temperature is increased from 750 ° C. to 860 ° C., the oxidation The iron form remained almost unchanged. However, when the temperature was raised from 860 ° C. to 890 ° C., the form of iron oxide changed greatly. That is, as shown in FIG. 1C, the iron oxide particle size rapidly increased to about 0.2 μm to 1 μm in a narrow temperature increase from 860 ° C. to 890 ° C. This rapid increase in particle size does not cause the individual iron oxide particles to gradually become coarser during the temperature increase from 860 ° C. to 890 ° C., but changes in temperature within 1 ° C. in a part of the observation region. This phenomenon occurred in various places, and this phenomenon occurred in various places. When the temperature was finally raised to 890 ° C., the particle diameter proceeded to increase almost over the entire observation region. In the present invention, the phenomenon in which the iron oxide particle size of the steel sheet surface layer increases rapidly is referred to as “iron oxide form transition”, and the temperature at which the iron oxide form transition occurs is referred to as “iron oxide form transition temperature”. .

  The inventors have examined the observation results in more detail, and as the iron oxide form transitions, the amount of iron oxide on the steel sheet surface layer increases rapidly, and iron oxide sufficient to obtain good plating properties is formed. It also turned out to be. For this reason, the inventors determined that the iron oxide form transition temperature at which this iron oxide form transition occurs was determined by trial and error after performing oxidative measurement and plating experiments in the laboratory. It was speculated that this would correspond to an appropriate maximum heating temperature in the oxidation treatment during (actual operation).

Therefore, the inventors investigated the iron oxide form transition temperature of steel plates of various existing steel types for which an appropriate maximum heating temperature in the oxidation treatment has been established. As a result, the appropriate maximum heating temperature Xa for the steel plate X is a steel plate in which the iron oxide form transition temperature Xt of the steel plate X and the appropriate maximum heating temperature Wa in the oxidation treatment are established (hereinafter referred to as “contrast steel plate”). It has been found that the following relational expression (A) holds between the iron oxide form transition temperature Wt of W.
Xa = Wa + Xt−Wt (A)

  Therefore, if the iron oxide form transition temperatures Xt and Wt are obtained in advance for the steel plate X whose proper maximum heating temperature Xa in the oxidation treatment is unknown and the comparative steel plate W whose proper maximum heating temperature Wa is established, From the above formula (A), it was found that an appropriate maximum heating temperature Xa of the steel plate X can be determined efficiently, and the present invention has been completed.

That is, the gist of the present invention is as follows.
(1) With respect to a steel plate containing 0.1 to 3% by mass of Si, an oxidation treatment in which the steel plate is heated in an oxidizing atmosphere to form iron oxide on the surface of the steel plate, and the steel plate after the oxidation treatment In manufacturing a hot-dip galvanized steel sheet through a reduction treatment in which the iron oxide is reduced by heating in a non-oxidizing atmosphere and a plating treatment in which hot-dip galvanization is performed on the steel plate after the reduction treatment, the steel plate and the oxidation For the comparative steel sheet for which an appropriate maximum heating temperature Wa has been established in the treatment, the iron oxide formation state on the steel sheet surface layer in the heating process in an oxidizing atmosphere is grasped, and the iron oxide in the steel sheet is determined from the iron oxide formation state. The temperature Xt at which the form transitions and the temperature Wt at which the form of iron oxide in the contrast steel plate transitions are determined, and Xa is determined from these Xt, Wt and Wa according to the following formula (A), and Xa is the maximum heating temperature. As said oxidation treatment Gastric line method for producing a hot-dip galvanized steel sheet wherein the heating process of the oxidizing atmosphere is characterized by rows Ukoto at a heating rate of 5 ° C. / min or higher 50 ° C. / min or less.
(Record)
Xa = Wa + Xt−Wt (A)

(2) The method for producing a hot-dip galvanized steel sheet according to (1) above, wherein the iron oxide formation status is grasped by observation using an environmental control scanning electron microscope or X-ray diffraction.

(3) Alloying characterized by heating the hot dip galvanized steel sheet produced by the method described in (1) or (2) above and subjecting the hot dip galvanized steel to alloying Manufacturing method of hot dip galvanized steel sheet.

(4) A hot-dip galvanized steel sheet produced by the method described in (1) or (2) above.

(5) An alloyed hot-dip galvanized steel sheet produced by the method described in (3) above.

  According to the present invention, it is possible to efficiently determine an appropriate maximum heating temperature in the oxidation treatment, and to manufacture a hot-dip galvanized steel sheet having both good mechanical characteristics and plating characteristics.

It is a SEM image of the surface of the Si containing steel plate at the time of a simulation oxidation treatment.

Hereinafter, the present invention will be specifically described.
As the steel sheet according to the present invention, a high-strength steel sheet containing Si is used. In addition, unless otherwise indicated, the "%" display regarding the steel plate component described below shall mean the mass%.

Si: 0.1% or more and 3% or less As described above, Si is an element that contributes to strength improvement without reducing the ductility, workability, and the like of steel. However, when the content is less than 0.1%, the effect of addition is poor. On the other hand, if the content exceeds 3%, it is concentrated as an oxide on the surface of the steel sheet, which not only causes non-plating, but also deteriorates the appearance properties, so that this method is difficult to apply. Therefore, Si is intended for steel sheets containing 0.1% to 3%.

  The steel sheet according to the present invention contains Si having the above composition as a basic element. About the composition other than this, although it can add suitably according to the characteristic of the hot-dip galvanized steel plate requested | required, the following is mention | raise | lifted as an example of a basic component composition.

Mn: 0.5% to 3.5% It is more effective to add 0.5% or more of Mn to increase the strength. On the other hand, if Mn exceeds 3.5%, it becomes difficult to ensure weldability, plating adhesion, and strength ductility balance. Therefore, it is preferable to use, as a material, a steel sheet that contains Mn in a range of 0.5% to 3.5%.

C: 0.05% or more and 0.25% or less C is an element that contributes to improving the strength of steel. However, if the content exceeds 0.25%, the weldability deteriorates. Therefore, it is preferable to use a steel plate containing C in a range of 0.05% to 0.25% as a raw material.

Al: 0.005% or more and 3.0% or less Al is an element added complementarily to Si. Although it is possible to ensure mechanical properties if Si is sufficiently added without containing Al, it is inevitably mixed in the steel making process, so Al is usually preferably contained in an amount of 0.005% or more. On the other hand, when the Al addition amount exceeds 3.0%, it is difficult to suppress the formation of an oxide film and it is difficult to improve the adhesion. Therefore, it is preferable to use, as a material, a steel plate containing Al in a range of 0.005% to 3.0%.

P: 0.001% or more and 0.10% or less P is unavoidably 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, it is preferable to use as a material a steel sheet containing P in a range of 0.001% to 0.10%.

S: 0.2% or less S is an element which is unavoidably contained like P, but if contained in a large amount, weldability deteriorates. When the amount of S in steel increases, it causes hot red hot brittleness and may cause problems such as breakage of a hot-rolled sheet during the manufacturing process. In addition, inclusion MnS is formed on the steel sheet, and is present as a plate-like inclusion after cold rolling, so that the ultimate deformability of the material is particularly lowered, and the formability such as stretch flangeability is reduced. It is preferable to use a steel plate with an upper limit of% as the material. Further, it is preferably 0.0050% or less. More preferably, it is 0.0030% or less.

Nb: 0.010 or more and 0.080% or less Nb is an element that contributes to improving the strength of the steel sheet by solid solution strengthening or precipitation strengthening. However, if the content is less than 0.010%, the addition effect is poor. On the other hand, if the content exceeds 0.080%, the hot-rolled sheet becomes hard, and the rolling load during hot rolling or cold rolling is increased. Therefore, a steel plate contained in the range of 0.010% to 0.080% may be used as a material.

  When the steel sheet having such a component composition is subjected to oxidation treatment and reduction treatment, and then subjected to hot dip galvanizing treatment, and further to alloying treatment thereafter, it is preferable to apply the present invention. A hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet having both mechanical properties and plating properties can be produced efficiently. Hereinafter, although each process is demonstrated, it is not limited to this, The conditions currently used when manufacturing a normal plated steel plate and an galvannealed steel plate can be used.

(Oxidation treatment)
The difference in the oxidation means does not hinder the effect of the present invention, and any means may be used as long as the steel sheet can be oxidized. Accordingly, the heating means actually used in the production may be any conventional heating method such as burner heating, induction heating, radiant heating, and current heating, and is not particularly limited. For example, as the burner heating method, a conventionally used heating furnace such as an oxidation furnace or a non-oxidation furnace can be used. In the case of a non-oxidizing furnace, for example, the steel sheet can be easily oxidized by setting the air-fuel ratio of the direct fire burner to exceed 1.0.

In addition, in the case of the induction heating method, the radiant heating method, and the energization heating method, the steel plate can be easily oxidized by setting the atmosphere in the vicinity of the steel plate to be heated to an oxidizing atmosphere. The oxidizing atmosphere is generally an atmosphere containing one or more oxidizing gases such as oxygen, water vapor and carbon dioxide, but these may be mixed with nitrogen and used to oxidize the steel sheet. If it can do, it will not specifically limit.
In addition, the above showed a typical example, and in any case, it is sufficient that the steel sheet can be oxidized, and the means is not particularly limited.

(Reduction treatment)
Next, the reduction method in the reduction process may be a conventionally used method, and is not particularly limited. For example, it is common to perform a reduction treatment at a temperature of about 600 to 900 ° C. in a reducing atmosphere containing hydrogen in a radiant heating type annealing furnace, but if the oxide film on the steel sheet surface can be reduced Any means can be used.

(Plating treatment)
After the above reduction treatment, after cooling to a temperature suitable for plating in a non-oxidizing or reducing atmosphere, it is immersed in a plating bath and hot dip galvanized. This hot dip galvanizing treatment may be performed in accordance with a conventional method. For example, the plating bath temperature is about 440 to 520 ° C., the temperature when the steel sheet is immersed in the plating bath is approximately equal to the plating bath temperature, and the Al concentration in the galvanizing bath is generally about 0.1 to 0.2 mass%. However, it is not particularly limited.

  Depending on the application of the product, the plating conditions such as the plating temperature and the plating bath composition may be changed, but the difference in the plating conditions does not affect the effect of the present invention and is not particularly limited. Absent. For example, even if elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn are mixed in the plating bath other than Al, the effect of the present invention is not changed. .

  Furthermore, the method for adjusting the thickness of the plated layer after plating is not particularly limited, but generally, gas wiping is used, and the gas pressure of gas wiping, the distance between the wiping nozzle and the steel plate, etc. are adjusted. By adjusting, the thickness of the plating layer is adjusted. At this time, the thickness of the plating layer is not particularly limited, but is preferably about 3 to 15 μm. This is because if the thickness is less than 3 μm, sufficient rust resistance cannot be obtained, while if it exceeds 15 μm, not only the rust resistance is saturated but also the workability and economy are impaired. However, the difference in the thickness of the plating layer does not hinder the effect of the present invention and is not particularly limited.

(Alloying treatment)
Moreover, in this invention, it is also possible to give an alloying process after the above hot dip galvanizing.
As described above, according to the present invention, since Si surface concentration during annealing can be suppressed, the problem in the prior art of significant alloying delay in the Si-containing steel sheet can be solved. As a result, an alloyed hot-dip galvanized steel sheet having excellent powdering resistance can be produced without impairing productivity. As the alloying treatment method, any conventionally used heating method such as gas heating, induction heating, and current heating may be used, and it is not particularly limited. For example, the alloying plate temperature is generally about 460 to 600 ° C., and the alloying holding time is generally about 5 to 60 seconds.

In the case of performing oxidation treatment as described above, reduction treatment, hot dip galvanization treatment, and further alloying treatment thereafter, in the oxidation treatment, the steel sheet having the above component composition, By using the maximum heating temperature during oxidation treatment obtained by applying the present invention, efficiently producing hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets having good mechanical properties and plating properties Can do.
Hereinafter, a method for obtaining an appropriate maximum heating temperature during the oxidation treatment, which is the gist of the present invention, will be described.

(Determine iron oxide form transition temperature)
Prepare a steel plate X having a composition of Si: 0.1% or more and 3% or less, which is intended to produce a hot dip galvanized steel plate or an alloyed hot dip galvanized steel plate. While observing the above, the temperature is raised and heated, and a simulated oxidation treatment for forming iron oxide on the surface of the steel plate X is performed. At that time, the iron oxide formation state on the steel sheet surface layer in the heating process is grasped, and the iron oxide form transition temperature Xt is obtained. In addition, for the comparative steel sheet W for which an appropriate maximum heating temperature Wa has been established, the iron oxide formation state on the steel sheet surface layer in the heating process in an oxidizing atmosphere is also grasped, and the iron oxide form transition temperature in the comparative steel sheet W Wt is obtained.

  The grasp of the iron oxide formation state is to observe the iron oxide formed on the steel sheet surface layer during the simulated oxidation treatment and grasp the change in the particle size of the iron oxide. Generally, as the temperature rises, the particle size of iron oxide increases rapidly with the iron oxide form transition, so this temperature is set as the iron oxide form transition temperature.

There is no particular limitation on the method for observing the steel sheet surface layer during the above-described simulated oxidation treatment, and it can be performed by E-SEM, X-ray diffraction (XRD), or the like. For example, when using E-SEM, set the observation field of about 900μm ^{2 on} the steel sheet surface layer, pay attention to 50 iron oxides in the observation field, and plot the average particle size against the heating temperature, The inflection point in the temperature region where the average particle diameter increases rapidly may be the iron oxide form transition temperature. The above observation is preferably performed under conditions close to the oxidation treatment atmosphere during actual production (actual operation), but since experiments under atmospheric pressure are not possible, oxygen close to the oxygen partial pressure of the actual furnace What is necessary is just to perform in the oxidizing atmosphere of partial pressure. For example, when the oxygen partial pressure of the actual furnace is 100 Pa, the experiment may be performed in a 100 Pa pure oxygen atmosphere. In addition, the oxidizing atmosphere at this time may be a pure oxygen atmosphere, or may be an atmosphere containing water vapor, a rare gas, and nitrogen in addition to oxygen.

  In addition, observation methods other than E-SEM are mainly methods that monitor the amount of iron oxide on the steel sheet surface layer, starting with XRD. When XRD is used, the increase in signal intensity due to iron oxide is plotted against the heating temperature, and the inflection point in the temperature region where the iron oxide signal increases rapidly is defined as the iron oxide form transition temperature. Since XRD allows experiments under atmospheric pressure, it can be performed in an atmosphere close to the partial pressure of oxygen in the actual furnace. For example, when the oxygen partial pressure of the actual furnace is 100 Pa, the experiment can be performed under a 0.1% by volume oxygen-nitrogen atmosphere (the oxygen partial pressure corresponds to about 100 Pa).

Regardless of which E-SEM or XRD is used, the temperature rise rate is very slow compared to the actual machine because the temperature rises while measuring in the laboratory apparatus. However, as a result of intensive studies by the inventors, the above formula (A) is established even at a heating rate of about 5 ° C./min to 50 ° C./min, which is possible with a laboratory apparatus, and is appropriate for the oxidation treatment. It has been found that the maximum heating temperature can be determined.
Here, when the temperature is less than 5 ° C./min, the experiment takes a long time and the efficiency is deteriorated. On the other hand, if it exceeds 50 ° C / min, data cannot be acquired with sufficient time resolution by current ESEM and in situ X-ray diffraction. Therefore, it is set to 5 ° C./min or more and 50 ° C./min or less. If data can be acquired with sufficient time resolution due to technological progress, the temperature rise rate may be equivalent to that of an actual furnace. The atmosphere in the heating furnace is adjusted as described above, and the steel plate X introduced into the furnace is heated until it exceeds the iron oxide form transition temperature.
The above conditions describe the case where the heating is performed in one stage. However, the heating can be performed in two or more stages and changing the temperature increase rate in each stage.

(Determining an appropriate maximum heating temperature during oxidation treatment)
From the iron oxide form transition temperature Xt of the steel sheet X thus obtained, the iron oxide form transition temperature Wt of the contrast steel sheet W, and the appropriate maximum heating temperature Wa of the contrast steel sheet W, Xa is determined according to the following formula (A).
Xa = Wa + Xt−Wt (A)
Here, in actual production (actual operation), the appropriate maximum heating temperature of the contrast steel sheet W has a certain range, but any temperature within the range may be set to Wa.
Further, according to the study by the inventors, if the maximum heating temperature during the oxidation treatment of the steel sheet X is lower than Xa obtained by setting the lower limit of the appropriate maximum heating temperature range established in the contrast steel sheet W as Wa, The amount produced is insufficient and non-plating occurs. In addition, if the maximum heating temperature during oxidation of the steel sheet X is higher than Xa obtained by setting the upper limit of the appropriate maximum heating temperature range of the contrast steel sheet W as Wa, the iron oxide roll pick-up or non-plating due to peroxidation, plating adhesion Defects occur.

  Conventionally, in order to develop various new steel types that have been developed into actual processes, it is complicated to find the conditions for producing necessary and sufficient oxidation amounts by producing steel plates annealed at various maximum heating temperatures for each steel type in the laboratory. However, according to the present invention, an appropriate maximum heating temperature in the oxidation treatment can be efficiently determined without such a complicated operation. And the maximum heating temperature determined in this way is applicable to the normal oxidation process performed about the steel plate containing 0.1-3.0 mass% Si.

(Example)
Examples of the present invention will be described below.
First, various cold-rolled steel sheets having a composition shown in Table 1 and having a thickness of 0.7 mm were prepared. Steel plate No. in Table 1 P of 1-4 was 0.001 mass% or more and 0.10 mass% or less, and S was 0.2 mass% or less. Among these, steel plate No. 4 is a comparative steel sheet in which an appropriate maximum heating temperature in the oxidation treatment is established in actual production (actual operation). Steel plate No. The appropriate maximum heating temperature (Wa) of 4 is 720 to 750 ° C. The iron oxide formation process on the surface of these steel plates was observed, and the iron oxide form transition temperature of each obtained steel plate was determined. The iron oxide form transition temperature was determined by in-situ observation using E-SEM. E-SEM conditions were as follows: heating from room temperature to 950 ° C. at a rate of 30 ° C./min in an oxygen atmosphere of 100 Pa, and in-situ observation of the iron oxide formation process on the surface of each steel plate. Asked. Table 2 shows the iron oxide form transition temperatures of the obtained steel sheets. Steel plate No. No. 4 iron oxide form transition temperature difference (° C.) [Xt−Wt] and steel plate No. Table 2 shows the appropriate maximum heating temperature (Wa) in the actual production (actual operation) of No. 4 and the sum [Wa + Xt−Wt] thereof.
Subsequently, in order to investigate the plating property using each steel sheet having a thickness of 0.7 mm and 70 mm × 180 mm, a practical experiment (actual operation) was simulated and a plating experiment was performed using a CGL simulator. Steel plate No. 1 to 4 are set in the simulator, heated at 10 ° C / second in an atmosphere of 0.1% O ₂ -N ₂ (dew point 20 ° C) and gas flow rate 40 l / min. Oxidation was performed by heating to “heating temperature”. After reaching the maximum heating temperature, cooling was performed with N ₂ gas. The N ₂ gas flow rate during cooling is 200 l / min. Thereafter, the steel plate after the oxidation treatment was reduced in a 10% by volume hydrogen + nitrogen atmosphere (dew point: −35 ° C.) under conditions of a plate temperature of 830 ° C. and a holding time of 45 seconds. The plating conditions were 0.14% by mass of Al (Fe saturated), 460 ° C zinc plating bath, intrusion plate temperature: 460 ° C and immersion time: 1 ^second. A hot dip galvanized steel sheet was prepared.

(Evaluation)
<Plating appearance (with or without plating)>
The obtained hot-dip galvanized steel sheet is visually observed with a magnifying glass and visually. When there is no plating, no plating is indicated (O). In the case where only non-plating occurred, the case where there was no significant plating (Δ), and the case where non-plating could be observed with the naked eye was judged as non-plating (x).

<Plating adhesion>
A ball impact test was performed on the obtained hot-dip galvanized steel sheet. In the test, a 2.8 kg weight was dropped from a height of 1 m using a punch with a diameter of 1/2 inch in accordance with ISO 6272. Then, the plating adhesion was evaluated by peeling the processed part with cello tape (registered trademark) against plating on the convex part side of the steel sheet on which the convex part was formed by impact, and visually judging the presence or absence of peeling of the plating layer. The plating peeling state after the test was visually observed, and the case where there was no plating peeling was regarded as adhesion pass, and the case where plating peeling was present was regarded as adhesion failure.
The evaluation results are also shown in Table 2.

  From Table 2, steel plate No. 1 to 3 were determined from the iron oxide form transition temperature difference (Xt-Wt) determined by a simple method in the laboratory and the appropriate maximum heating temperature (Wa) in actual production (actual operation) (Wa + Xt-Wt). It can be seen that those plated with a steel plate subjected to oxidation treatment at a maximum heating temperature have no unplating, good appearance, good plating adhesion, and excellent mechanical properties. Further, when the oxidation treatment was performed with the temperature outside this range as the maximum heating temperature, the results were not good.

  Thus, according to the present invention, steel plates annealed at various maximum heating temperatures in a laboratory are prepared, and the appropriate maximum heating temperature in the oxidation treatment is searched without searching for conditions that can ensure the necessary and sufficient amount of oxidation. It can be seen that a hot-dip galvanized steel sheet with good plating characteristics can be manufactured.

Claims (3)

With respect to a steel sheet containing 0.1 to 3% by mass of Si, the steel sheet is heated in an oxidizing atmosphere to form iron oxide on the surface of the steel sheet, and the oxidized steel sheet is non-oxidizing In producing a hot-dip galvanized steel sheet through a reduction process of reducing the iron oxide by heating in an atmosphere, and a plating process in which hot-dip galvanizing is performed on the steel sheet after the reduction process,
For the steel sheet and the comparative steel sheet in which an appropriate maximum heating temperature Wa in the oxidation treatment is established, grasp the iron oxide formation status on the steel sheet surface layer in the heating process in an oxidizing atmosphere, and from the iron oxide formation status, The temperature Xt at which the form of iron oxide in the steel sheet transitions and the temperature Wt at which the form of iron oxide in the contrast steel sheet transitions were determined, and these Xt, Wt, and Wa were determined according to the following formula (A). A method for producing a hot-dip galvanized steel sheet, wherein the oxidation treatment is performed with Xa as the maximum heating temperature, and the heating process in the oxidizing atmosphere is performed at a temperature increase rate of 5 ° C / min to 50 ° C / min.
(Record)
Xa = Wa + Xt−Wt (A)   The method for producing a hot-dip galvanized steel sheet according to claim 1, wherein the iron oxide formation status is grasped by observation using an environmental control scanning electron microscope or X-ray diffraction. A hot-dip galvanized steel sheet produced by the method according to claim 1 or 2 is heated to perform an alloying treatment for alloying the plated hot-dip galvanized steel sheet. Method.