KR100685034B1 - Method for manufacturing high strength galvannealed steel sheet - Google Patents

Abstract

A method for manufacturing a high strength galvannealed steel sheet, which can prevents transformation of residual austenite and can improve powdering resistance when performing alloying treatment on a steel sheet containing components that are hardly plated, is provided. In a method for manufacturing a high strength galvannealed steel sheet comprising one or more of 0.01 to 3.0 wt.% of Si, 0.1 to 3.0 wt.% of Mn and 0.1 to 3.0 wt.% of Al, the method comprises leading a steel sheet comprising a wustite-based iron oxide layer with a thickness of 0.1 to 1 mum and a porosity of 5 to 30% formed in a surface layer thereof into a hot-dip galvanizing bath at a temperature of 500 to 550 deg.C to galvanize the steel sheet; and alloying the galvanized steel sheet at a temperature of 460 to 500 deg.C. The hot-dip galvanizing bath has a composition comprising 0.10 to 0.135 wt.% of Al and the balance of Zn and other inevitable impurities.

Description

Manufacturing method of high strength alloyed hot-dip galvanized steel sheet {METHOD FOR MANUFACTURING HIGH STRENGTH GALVANNEALED STEEL SHEET}

Japanese Laid-Open Patent Publication 1980-122865

Japanese Laid-Open Patent Publication 1992-027057

Japanese Laid-Open Patent Publication 1994-172953

Japanese Laid-Open Patent Publication 2001-226742

Korean Patent Application No. 2004-0113084

Korean Patent Application No. 2004-0113085Y

The present invention relates to a method for manufacturing a high strength alloyed hot-dip galvanized steel sheet containing a non-plating component, and more particularly, to a method for manufacturing an alloyed hot-dip galvanized steel sheet which improves powdering resistance while securing material properties by lowering alloying temperature. It is about.

Recently, as fuel economy regulations are tightened as a task for global environmental preservation, automobile body weight has been actively reduced. As one of the countermeasures, it is effective to reduce the weight of the automobile material by increasing the strength of the steel sheet. High-strength automotive materials include precipitation hardening steel, shobu hardening steel, employment hardening steel, and transformation hardening steel. Even in high strength steels, solid solution hardened steels, especially transformation hardened steels, have poor plating properties.

Solid solution hardened steel is a steel which achieves high strength by a reinforcing mechanism in which a solid solution such as Si, Mn, P, Cr, or the like penetrates or is substituted to form a strain field.

Transformed reinforced steels include dual phase steel (hereinafter simply referred to as DP) and transformation induced plasticity (hereinafter also referred to as TRIP). These metamorphic reinforced steels are also called Advance High Strength Steel (AHSS). DP steel is a steel grade in which hard martensite is finely and uniformly dispersed in soft ferrite to secure high strength. TRIP steel is a type of steel that secures high strength and ductility by causing martensite transformation when micro homogeneously dispersed residual austenite is processed at room temperature. DP and TRIP steels usually have a high content of Mn and Si, which are hard plating components.

The high-strength steels include Si, Mn, and Al, which are non-plating components, which are concentrated on the surface and have poor wettability with molten zinc. Particularly, in the case of Si, when 0.1 wt% or more is contained in the steel, Si is diffused to the surface of the steel sheet during the hot rolling process and the continuous annealing heat treatment process, so that the concentration is about 10 to 100 times higher than that of the bulk material. As such, silicon concentrated in the grain boundary or in the mouth forms a SiO ₂ oxide film in response to the trace amount of moisture or impurities in the furnace atmosphere, thereby greatly reducing wettability with molten zinc in the molten zinc plating process. As a result, it is difficult to secure the adhesion, so the unplated phenomenon is frequently caused, or even when the plating is hot, the plating adhesion is greatly deteriorated and plating peeling occurs during processing, and alloying is greatly delayed during the alloying heat treatment.

As a method for improving the hot-dip galvanizing adhesion by the non-plating components, (1) plating bath component management, (2) lead gold technology, (3) redox method, and the like are known.

(1) Plating bath ingredient management

This technique reduces SiO ₂ oxides and the like remaining in the steel sheet in the plating bath to prevent deterioration of the hot dip of wet plating due to the oxide film. That is, the amount of Al added in the plating bath is managed to a high level of 0.21 to 0.25% by weight at the level of 0.10 to 0.20% by weight. This increases the amount of Zn-Fe-Al-Si-based and Fe-Al-Si-based alloy layers formed at the interface between the base iron and the plating layer, thereby reducing the wettability of the molten plating due to the oxide film. To prevent it. However, the increase in aluminum in the plating bath is likely to cause plating peeling over time since grain boundary corrosion occurs easily when co-existing with Pb, which is inevitably added to the plating bath during the manufacture of the mini-spangle steel sheet. In addition, the increase in the aluminum concentration in the plating bath increases the amount of upper dross generated in the plating bath, and there is a problem of significantly delaying the alloying reaction.

(2) leading technology

It is a method to secure wettability with the hot dip zinc layer by pre-coating electroless plating or electroplating of various alloy elements such as Fe, Ni, and iron oxide films before hot dip plating. Even if the alloying element is concentrated on the iron interface, it is concentrated under the preplating layer to block the reaction with moisture in the atmosphere during annealing or heating. For this reason, since the oxidation of silicon is prevented, plating adhesion and alloying treatment property are improved significantly. However, since the method of lead plating on the surface of the steel sheet is usually performed by electroplating, when the hot-calculated fine steel sheet having large unevenness of base iron is used as the plating material, a short plating process causes variations in the amount of plating deposition on the uneven portion. That is, since the uneven portion has a gap difference from the anode than the smooth plating layer surface, the convex portion has more plating adhesion than the smooth surface, but the concave portion may have less plating adhesion or no plating at all. . In order to prevent this, there is a method of lengthening the electroplating process or performing a deceleration operation, but it is not preferable because overplating occurs in the convex portion. In particular, the lead metal in this way has a problem that plating peeling occurs during processing when the plating adhesion amount is large as an element having a high hardness and insufficient ductility. In addition, when the leading gold is electroplated in the pretreatment process, it is not economically preferable because of the complicated equipment and high manufacturing cost.

(3) redox method

Generally, about 0.05 to 0.1 탆 thin oxide film remains on the surface of the steel sheet after cold rolling. Since the thin oxide film is completely reduced during the annealing process, the Si and Mn in the steel cannot be prevented from being concentrated on the surface, thereby significantly reducing the plating adhesion. Therefore, the annealing furnace is divided into the oxidizing zone and the reducing zone, and the oxidizing zone adopts the direct furnace method to intentionally form an oxide film by injecting excess air from the direct furnace, and in the reduction zone, the oxide film is reduced with the reducing atmosphere, such as Si. Research on the redox method that suppresses surface thickening is actively progressing. Prior arts related to the redox method include Japanese Patent Application Laid-Open Nos. 1980-122865, 1992-27057, 1994-172953, 2001-226742, and the like.

Japanese Laid-Open Patent Publication No. 1980-122865 is a technique of oxidizing a thickness of an oxide film to 0.04 to 1 탆 on a steel plate of a non-plating material, followed by annealing in an atmosphere containing hydrogen and hot dip galvanizing. This technique ensures plating adhesion by suppressing the surface concentration of Si by the iron oxide film.

Japanese Unexamined Patent Publication Nos. 1992-276057 and 1994-172953 insist that Japanese Unexamined Patent Application Publication No. 1980-122865 claims that it is practically impossible to control the reduction time of an oxide film of iron. Therefore, it has been pointed out that a long reduction time can cause the surface concentration of Si, and when the reduction time is shortened, an iron oxide film remains on the surface of the steel sheet, resulting in poor plating adhesion.

In order to solve this problem, Japanese Laid-Open Patent Publication No. 1992-276057 discloses that a steel sheet containing 0.2% or more of Si is reduced to 0.05 μm or less in a reduction furnace after oxidation in a direct combustion furnace having a combustion air ratio of 0.9 to 1.2. To reduce the residual oxide film by Mn and Al in the bath by performing a melt plating process on a zinc plating bath of 0.01 to 2%, Al to 0.01 to 2%, and remaining Zn.

In addition, Japanese Patent Application Laid-Open No. 1994-172953 discloses a high-strength steel sheet having a Si content of 0.2 to 2.0% so that the thickness of the iron oxide film remains 0.02-0.2 μm in a reduction zone after oxidation in a direct combustion furnace having a combustion air ratio of 0.9 to 1.2. After reduction, Al 0-10% and Mn 1-10% are contained, and a technique of applying a vibration when performing a hot plating process in the zinc plating bath of the remaining Zn to peel off and remove the remaining oxide film is proposed.

The conventional redox method is a technique of suppressing the surface concentration of Si during the redox process and the hot dip galvanizing process and finally removing iron oxide between the steel plate surface and the plating layer.

On the other hand, Japanese Laid-Open Patent Publication No. 2001-226742 is a technique in which the redox method is actually applied to TRIP steel, a high strength steel.

Japanese Laid-Open Patent Publication No. 2001-226742 proposes a technique of applying a lead or / and redox method to TRIP steel containing Si: 0.3 to 2.5% and Mn: 0.5 to 3.0%. As the leading gold, one, two or more of Ni, Fe, and Cu are used. In the redox method, a technique of oxidizing a combustion air ratio of 0.9 to 1.2 to give a Fe oxide of 0.02 to 1 µm on the surface of a steel sheet before reduction annealing has been proposed. This technique simply applies the redox method of Japanese Patent Application Laid-Open No. 1980-122865 to TRIP steel. As disclosed in Japanese Laid-Open Publication Nos. 1992-276057 and 1994-172953, this technique is intended to control the reduction time of the oxide film of iron. There is no suggestion. Therefore, if the reduction time is prolonged, the surface of Si may be thickened. If the reduction time is short, iron oxide film remains on the surface of the steel sheet, and as a result, it is judged that the problem of poor plating adhesion is not overcome.

As described above, various methods for applying the redox method to high strength steel sheets have been proposed. However, the application of molten zinc plating to steel grades, especially TRIP steel, having a high content of non-plating components such as Si, does not produce actual products.

On the other hand, the present inventors in the Republic of Korea Patent Application No. 2004-0113084 and 2004-0113085 so that the iron oxide layer with the fine pores are distributed between the steel plate and the plating layer so that the iron oxide layer blocks the surface thickening of the non-plating components It has been proposed a technique for improving the plating adhesion by the locking effect of the hot dip galvanized layer due to the fine pores of the iron oxide layer. This technique has little consideration in the case of alloying.

When the iron oxide layer with fine pores is galvanized and alloyed on the high-strength steel sheet formed on the surface layer, the concentration of Si, Mn, and Al is significantly reduced, so the Fe diffusion is easy and the alloying temperature can be lowered. have. However, in the case of TRIP steel, even at a lowered alloying temperature, residual austenite is transformed into tempered martensite, making it difficult to secure material properties, and a thick gamma phase is formed in the plating layer, which may cause powdering.

The present inventors provide a method for producing an alloyed hot-dip galvanized steel sheet which can prevent transformation of residual austenite and improve powdering resistance when alloying a steel plate including a hard plated component.

In the manufacturing method of the alloyed hot-dip galvanized steel sheet of the present invention for achieving the above object, in the production method of high-strength alloyed hot-dip galvanized steel sheet containing one or two or more of Si, Mn, Al,

And galvanizing the steel sheet in a hot dip galvanizing bath at a steel sheet drawing temperature of 500 to 550 ° C., followed by alloying at 460 to 500 ° C.

In the steel sheet to be plated of the present invention, an iron oxide layer having a thickness of 0.1 to 1 μm is present on the surface layer, and the iron oxide layer preferably has a porosity of 5 to 30% observed at 10000 magnification. . According to the present invention, the pore size observed at 10000 magnification is preferably 0.2 to 1 μm. In addition, the iron oxide layer is preferably an excellent titite (FeO). The steel sheet that can be applied to the present invention may be applied as long as it contains a non-plating component, and examples thereof include steel including Si, Mn, Al, P, Cr, and the like. Specifically, for example, the content of Si is 0.01 to 3.0% by weight, the content of Mn is 0.01 to 3.0% by weight, Al: 0.1 to 3.0% by weight. The present invention is preferably applied to a cold rolled steel sheet subjected to annealing heat treatment. In the present invention, the zinc plating bath preferably contains 0.10 to 0.135% of Al.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention confirmed that the plating adhesion is improved when the iron oxide layer remains between the zinc plated layer and the steel sheet during the research and experiment for applying the redox method to the production of the hard plated material. Proposed in 0113084 and 2004-0113085.

In the case where the iron oxide layer in which the fine pores are distributed in the high strength steel sheet is formed on the surface layer, the concentration of Si, Mn and Al is significantly reduced at the interface between the galvanized layer and the steel sheet or at the grain boundary of the steel sheet, so it is easy to diffuse Fe during alloying. . In addition, the diffusion of Fe is also promoted through the pores of the iron oxide layer.

If Fe is easily diffused in the alloying treatment, the alloying treatment temperature can be lowered, but it is difficult to lower the alloying treatment temperature below 500 ° C. Zinc plating bath is formed on the Al ₅ Fe ₂ in the plating procedure, since a small amount of Al contained, it is in the alloying process of Fe ₂ Al ₅ interfere with the diffusion of Fe. Therefore, it is difficult to lower the alloying treatment temperature below 500 ° C. in order to destroy Fe ₂ Al ₅ . In addition, in the case of TRIP steel, when the alloying temperature exceeds 500 ° C., the retained austenite is transformed into tempered martensite during the alloying process, and a thick gamma phase is formed on the alloy plated layer.

In the present invention was derived in the course of studying the method for further lowering the alloying treatment temperature, if the steel sheet inlet temperature is increased to 500 ~ 550 ℃ Fe ₂ Al ₅ is formed at the same time during the galvanizing process does not interfere with the alloying treatment It is based on the fact that

The steel sheet which is the object of the present invention is a steel sheet containing a non-plated component which is concentrated on the surface and does not have good wettability with molten zinc. As the non-plating component, Si, Mn, Cr, P, Al and the like are known. Specifically, for example, the content of Si is 0.01 to 3.0% by weight, the content of Mn is 0.01 to 3.0% by weight, Al: 0.1 to 3.0% by weight. In particular, the present invention applies to TRIP steel. As for the steel plate which is the object of this invention, a cold rolled steel plate is preferable.

It is preferable that an iron oxide layer exists in the surface layer of such a steel plate.

As for the thickness of the said iron oxide layer, 0.1-1 micrometer is preferable. The micropores (area fraction) of the iron oxide layer is preferably 5 to 30%. The porosity and pore size of these micropores is observed at a high magnification of 10,000 times. In the present invention, the phase of the iron oxide layer is preferred to excellent urethane (FeO) for the steel sheet adhesion.

Next, a method for producing an alloyed hot dip galvanized steel sheet will be described.

In the present invention, the steel sheet is subjected to oxidation zone and reduction zone of the annealing furnace, followed by hot dip galvanizing and alloying treatment.

In the present invention, it is preferable to appropriately control the heat treatment conditions of the oxidation zone and the reduction zone to form the iron oxide layer having fine pores.

First, it is preferable to perform an oxidation zone at the temperature of 550-750 degreeC on condition that oxygen concentration is 1.0-21 vol%.

As for the thickness of the iron oxide obtained by the heat processing of an oxidation zone, 0.2-2 mm is preferable.

The heat treatment in the oxidation zone as described above and then the heat treatment in the reduction zone, which is preferably carried out in a reducing atmosphere of 5 to 20% hydrogen concentration. As for the heat processing temperature of a reducing zone, 750-850 degreeC is preferable. By heat treatment of such a reducing zone, it is preferable to obtain a non-tight iron oxide layer having a thickness of 0.1 to 1 μm, porosity of 5 to 30%, and pore size of 0.2 to 1 μm.

In accordance with the present invention, the steel sheet subjected to redox heat treatment in an annealing furnace is hot dip galvanized after cooling.

The molten zinc bath is more preferably contained 0.1 to 0.135% by weight of Al content. If the Al content is less than 0.1% in the molten zinc bath, the initial diffusion inhibitory layer of iron-aluminum does not cover the surface of the base iron, thus promoting the formation of a weak gamma phase in the absence of diffusion inhibitory layer or thin powder, thereby greatly increasing the amount of powdering. Increases. On the other hand, if the Al content exceeds 0.135%, the initial diffusion suppression layer is formed thick and the alloying reaction between iron and zinc is very suppressed. Therefore, in order to form an alloy layer made of delta phase, the alloying temperature must be heated higher than 550 ° C. This deteriorates and the powdering is severely generated.

As for the steel plate introduction temperature which enters such a molten zinc bath, 500-550 degreeC is preferable. When the temperature of the steel sheet is less than 500 ° C, the alloying temperature is set to 500 ° C or higher in order to destroy the initial diffusion inhibiting layer Fe ₂ Al _{5. In} this case, the residual austenite is transformed into tempered martensite during the alloying process. In addition, a thick gamma phase is formed in the alloy layer, so that powdering occurs severely. On the other hand, when the temperature of the steel sheet is higher than 550 ° C, the occurrence of dross defects increases due to a large temperature difference with the plating bath, and a weak gamma image is formed thick, thereby causing severe powdering.

It is preferable to lead the galvanizing bath and then to galvanize the alloy at 460 to 500 ° C. Zinc plating bath has a small amount, since Al is included is formed Fe ₂ Al _{5 5} In the plating process in the alloying process of Fe ₂ Al ₅ is to prevent the diffusion of Fe. Therefore, in order to destroy Fe ₂ Al ₅ which is a thermodynamically stable alloy phase, it is preferable that the alloying treatment temperature be 460 ° C or higher. In addition, in the case of TRIP steel, if the alloying treatment temperature exceeds 500 ° C, the residual austenite is transformed into tempered martensite during the alloying treatment, and a thick gamma phase is formed on the alloy plating layer, thereby causing severe powdering.

Hereinafter, the present invention will be described in detail by examples.

Example 1

Oxidation heat treatment for 780Mpa grade TRIP cold rolled steel sheet containing 0.20%, Si: 1.0%, Mn: 1.6%, Al: 0.5%, consisting of the remaining Fe and other unavoidable impurities and 1.2mm thick 21% oxygen) using an induction furnace for 1 second. Heat treatment conditions of the reducing zone in the annealing furnace were annealed at 800 ° C. for 60 seconds in a reducing furnace containing 10% hydrogen.

After the reduction heat treatment, the steel plate was immersed at 460 ℃ galvanizing bath composed of Al: 0.13% by weight and Fe: 0.02% by weight at different steel sheet inlet temperatures for 3 seconds to obtain 50g / m ² of plating deposition on a cross-sectional basis. Hot dip galvanizing and alloying treatment were performed by changing the alloying temperature. The mechanical properties, residual austenite fraction and powdering properties of the alloyed hot-dip galvanized steel sheet thus obtained were observed. Mechanical properties were measured by tensile test and tensile strength test using a JIS No. 5 specimen, the residual austenite fraction of the iron was measured by X-ray diffraction method. In addition, powdering properties were quantitatively evaluated by measuring the amount of dropping of the plating layer using a cup forming tester.

division Inlet temperature of steel sheet (℃) Alloying temperature (℃) Tensile Strength (MPa) Elongation (%) Residual austenite fraction (%) Powdering amount (mg) Inventive Example 1 500 490 835 25 6.1 8 Inventive Example 2 540 480 823 26 5.3 5 Comparative Example 1 460 580 650 18 0.5 35 Comparative Example 2 480 550 683 21 1.1 27

As shown in Table 1, Inventive Example (1-2), which satisfies the conditions of the present invention, is formed at the same time as Fe ₂ Al ₅ is formed in the galvanizing process, thereby lowering the alloying temperature below 500 ° C. The amount of transformation to de-martensite is very small, so the mechanical properties are excellent, and the fragile gamma image is suppressed, and the powdering property is very good. However, when the steel sheet inlet temperature is lower than 500 ° C (Comparative Example 1-2), the alloying temperature is usually higher than 550 ° C, and most of the residual austenite is transformed into tempered martensite, resulting in deterioration of the material and thickening of the gamma phase. Powdering properties deteriorated greatly.

As described above, in the present invention, the alloying temperature is lowered to improve the material properties and the powder-resistant properties of the steel sheet.

Claims (4)

delete In the manufacturing method of high strength alloyed hot-dip galvanized steel sheet containing one or two or more of Si, Mn and Al, The steel sheet has a wustite-based iron oxide layer having a thickness of 0.1 to 1 μm and a porosity of 5 to 30%. The steel sheet is introduced into a hot dip galvanizing bath at 500 to 550 ° C. and galvanized. A method of producing a high strength alloyed hot-dip galvanized steel sheet comprising after the alloying treatment at 460 ~ 500 ℃. The method of claim 2, wherein the zinc plating bath contains 0.10 to 0.135% by weight of Al, and is composed of the remaining Zn and other unavoidable impurities. According to claim 2, wherein the content of Si is 0.01 to 3.0% by weight, the content of Mn is 0.1 to 3.0% by weight, the content of Al is 0.1 to 3.0% by weight of the high strength alloyed hot-dip galvanized steel sheet Manufacturing method.