JP2011058013A - High-strength hot-dip-galvanized steel sheet excellent in peeling resistance of plating layer - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high-strength hot-dip-galvanized steel sheet that comprises a high-Si-content steel sheet as a base metal and is excellent in resistance to peeling of a plating layer when subjected to high degree of processing. <P>SOLUTION: The steel sheet comprises, by mass, 0.01-0.15% C, 0.8-2.0% Si, 1.0-3.0% Mn, ≤0.025% P, ≤0.01% S and the balance being Fe and unavoidable impurities. Among Si and/or Mn oxides present at a boundary between the plating layer and the steel sheet and within a plating layer, the number of film-like oxides having an area of ≥4 μm<SP>2</SP>is 10 pieces or less in total within an area corresponding to 10,000 μm<SP>2</SP>of the galvanized steel sheet. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a high-strength hot-dip galvanized steel sheet using a Si-containing high-strength steel sheet as a base material.

  2. Description of the Related Art In recent years, surface-treated steel sheets imparted with rust resistance to raw steel sheets, particularly hot dip galvanized steel sheets with excellent rust resistance, have been used in fields such as automobiles, home appliances, and building materials. In addition, from the viewpoint of improving automobile fuel efficiency and improving automobile collision safety, the application of high-strength steel sheets to automobiles is aimed at reducing the thickness by increasing the strength of the body material and reducing the weight and strength of the body itself. Has been promoted. There are hot-dip galvanized steel sheets that have been subjected to post-plating alloying treatment and those that have not been subjected to alloying treatment. When it is particularly necessary to distinguish, the alloyed galvanized steel sheet is described as alloyed, and the non-alloyed galvanized steel sheet is described as not.

  In general, hot dip galvanized steel sheet is a thin steel sheet obtained by hot rolling or cold rolling a slab as a base material. The base steel plate is recrystallized and annealed in a CGL annealing furnace, and then hot dip galvanized. A non-alloyed hot-dip galvanized steel sheet, which is a hot-dip galvanized steel sheet that is not subjected to alloying treatment, is manufactured, and after hot-dip galvanizing, further alloying treatment is performed to produce an alloyed hot-dip galvanized steel sheet.

  As a hot dip galvanized steel sheet having excellent plating peel resistance using a high-strength steel sheet containing a large amount of Si and Mn as a base material, in Patent Document 1, for galvanized steel sheet, in Patent Document 2, for galvannealed steel sheet, It defines the shape of oxides when observed from a cross section orthogonal to the plating / steel interface and the element distribution of the base steel sheet. Moreover, in patent document 3, about the hot dip galvanized steel plate, the coverage of the oxide formed in a plating / steel plate interface is prescribed | regulated by surface observation after dissolving a plating layer.

  However, Patent Document 1 only defines the form of oxide in the plating / steel plate interface and the base steel plate, and does not define the oxide present in the plating layer. Moreover, in patent document 2, although presence about the oxide in a plating film is prescribed | regulated, the shape and form are not prescribed | regulated. Moreover, in patent document 3, it has shown about the oxide and plating adhesiveness of the plating / steel plate interface in a non-alloyed hot-dip galvanized steel plate.

  Furthermore, in recent years, the application of high-strength hot-dip galvanized steel sheets to more severely-processed parts has progressed, and the anti-plating anti-peeling property at the time of high processing has become important. Specifically, when the plated steel sheet is bent over 90 ° and bent to a more acute angle, or when a severer impact is applied, the steel sheet is required to suppress plating peeling at the processed part. . In order to satisfy these characteristics, not only a large amount of Si is added to the steel to ensure the desired steel sheet structure, but also to suppress crack propagation that causes plating peeling, only at the plating / steel sheet interface. In addition, advanced control is also required for the form of oxide present in the plating layer. However, such control is difficult in the prior art, and it has not been possible to provide a hot-dip galvanized steel sheet excellent in anti-plating resistance at the time of high processing using a Si-containing high-strength steel sheet as a base material.

JP 2001-288550 A Japanese Patent No. 399860 JP-T-2006-517257

  This invention is made | formed in view of this situation, Comprising: It makes it a subject to provide the high intensity | strength hot-dip galvanized steel plate which is excellent in the plating-peeling-proof property at the time of the high processing which used the high Si content steel plate as a base material.

  In the steel sheet to which Si and Mn are added, in the annealing process before hot dip galvanization, oxides of Si and Mn are deposited on the steel plate surface, and after hot dip galvanization, this exists at the plating / steel plate interface. In addition, when alloying treatment is performed, since the alloying reaction of Fe-Zn proceeds from the plating / steel sheet interface, some of the oxides of Si and Mn existing at the plating / steel sheet interface are plated films. Disperse in.

The present inventors pay attention to the oxides present in the plating / steel interface of the hot dip galvanized steel sheet and the plating film, the oxide contained in the plating film of the hot dip galvanized steel sheet, and the resistance to plating during high processing. As a result of investigating the relation of peelability, among the oxides present in the plating / steel interface and plating film, a film-like oxide with an area of 4 μm ² or more deteriorates the plating peel resistance during high processing. I understood.

The present invention is based on this finding, and the means of the present invention for solving the above-mentioned problems is mass%, C: 0.01 to 0.15%, Si: 0.8 to 2.0%, Mn: 1 0.0 to 3.0%, P: 0.025% or less, S: 0.01% or less, the balance being Fe and unavoidable impurities, a steel plate, in the plating / steel interface and plating layer Of the existing oxides of Si and / or Mn, the number of the film-like oxides having an area of 4 μm ² or more is 10 or less in total in the area corresponding to 10000 μm ² of the galvanized steel sheet. It is a high-strength hot-dip galvanized steel sheet that has excellent anti-plating resistance.

  According to the present invention, it is possible to obtain a high-strength hot-dip galvanized steel sheet having excellent plating peel resistance at the time of high processing using a high Si-containing steel sheet as a base material.

An example of the scanning electron micrograph of the film-form Si and Mn oxide which exists in the plating / steel-plate interface of a hot-dip galvanized steel plate and a plating film is shown, (a) is an alloyed hot-dip galvanized steel plate, (b) Is the case of a non-galvanized steel sheet. It is a scanning electron micrograph of a film-like Si, Mn oxide observed in a network.

  Hereinafter, the present invention will be specifically described. In the following description, the content of each element of the steel component composition and the unit of the content of each element of the plating layer component composition are all “mass%”, but the following is simply “%” unless otherwise specified. Show.

  First, the steel component composition will be described.

C: 0.01 to 0.15%
C makes it easy to improve workability by forming martensite or the like in the steel structure. For that purpose, 0.01% or more is desirable. If it exceeds 0.15%, the weldability deteriorates. Therefore, the amount of C is limited to 0.01 to 0.15%.

Si: 0.8 to 2.0%
Si is an element effective for strengthening steel and obtaining a good material. If Si is less than 0.8%, there is no problem in the plating peeling resistance at the time of high processing even if the present invention is not applied, and if it exceeds 2.0%, it is difficult to improve the plating peeling resistance at the time of high processing. . Therefore, the Si amount is set to 0.8 to 2.0%.

Mn: 1.0-3.0%
Mn is an element effective for increasing the strength of steel. In order to ensure mechanical properties and strength, it is necessary to contain 1.0% or more. If it exceeds 3.0%, it becomes difficult to ensure weldability, plating adhesion, and strength ductility balance. Therefore, the amount of Mn is set to 1.0 to 3.0%.

P: 0.025% or less Inevitable. If it exceeds 0.025%, not only the weldability is deteriorated but also the surface quality is deteriorated, and the alloying treatment temperature cannot be increased unless the alloying treatment temperature is raised during the alloying treatment. Increasing the alloying treatment temperature to reduce the degree of ductility degrades the ductility and at the same time the adhesion of the alloyed plating film, so that the desired degree of alloying, good ductility, and the alloyed plating film must be compatible. I can't. Therefore, the P content is preferably 0.025% or less.

S: 0.01% or less An element inevitably contained. Although a lower limit is not specified, 0.01% or less is preferable because weldability deteriorates when contained in a large amount.

In order to control the balance of strength and ductility, Al: 0.01 to 0.1%, B: 0.001 to 0.005%, Nb: 0.005 to 0.05%, Ti: 0.005 to 0 0.05%, Cr: 0.05-1.0%, Mo: 0.05-1.0%, Cu: 0.05-1.0%, Ni: 0.05-1.0% One or more selected elements may be added as necessary. Among these elements, Cr, Mo, Nb, Cu and Ni are used alone or in combination of two or more, and when the annealing atmosphere is a humid atmosphere containing a relatively large amount of H ₂ O, the internal oxidation of Si May be added to obtain good plating adhesion rather than to improve mechanical properties.

  The reason for limiting the appropriate addition amount in the case of adding these elements is as follows.

  Since Al is most easily thermodynamically oxidized, it is oxidized prior to Si and Mn, and has the effect of promoting the oxidation of Si and Mn. This effect is obtained at 0.01% or more. If it exceeds 0.1%, the cost increases. Therefore, the Al content is 0.01 to 0.1%.

When Cr is less than 0.05%, it is difficult to obtain the effect of promoting internal oxidation when the hardenability or the annealing atmosphere is a humid atmosphere containing a relatively large amount of H ₂ O. Due to concentration, plating adhesion and weldability deteriorate.

  If Mo is less than 0.05%, it is difficult to obtain the effect of adjusting the strength and the effect of improving the adhesion of plating when combined with Nb or Ni or Cu, and if it exceeds 1.0%, the cost increases.

  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 at the time of composite addition with Mo, and if it exceeds 0.05%, the cost increases.

  If Ti is less than 0.005%, the effect of adjusting the strength is difficult to obtain, and if it exceeds 0.05%, the plating adhesion deteriorates.

  If Cu is less than 0.05%, it is difficult to obtain the effect of promoting the formation of the residual γ phase and the effect of improving the plating adhesion at the time of composite addition with Ni or Mo.

  When Ni is less than 0.05%, it is difficult to obtain the effect of promoting the formation of the residual γ phase and the effect of improving the plating adhesion upon the combined addition of Cu and Mo.

  If B is less than 0.001%, it is difficult to obtain a quenching effect, and if 0.005% or more, the plating adhesion deteriorates.

  The balance other than the above is Fe and inevitable impurities.

  Next, the most important plating / steel interface in the present invention and Si and Mn oxides present in the plating layer will be described.

  Usually, a hot dip galvanized steel sheet is manufactured as follows. After annealing the steel sheet in a reducing atmosphere with a continuous annealing facility, dip it in a galvanizing bath, apply galvanizing, and lift it from the galvanizing bath and adjust the amount of coating with a gas wiping nozzle to unalloyed hot dip galvanizing Manufacture steel sheets. Alternatively, an alloying hot-dip galvanized steel sheet is produced by subjecting the plating film to alloying treatment in an alloying heating furnace.

  In order to increase the strength of a hot-dip galvanized steel sheet, it is effective to add Si, Mn, etc. to the steel as described above. However, in the steel sheet to which these elements are added, the added Si and Mn oxides are deposited on the surface of the steel sheet in the annealing process before hot dip galvanizing, and the steel sheet is immersed in the galvanizing bath and galvanized. This will be present at the plating / steel interface after application. Moreover, in the alloyed hot-dip galvanized steel sheet that has been subjected to alloying treatment, since the alloying reaction of Fe-Zn proceeds from the plating / steel sheet interface, the oxides of Si and Mn present at the plating / steel sheet interface A part is dispersed in the plating film. The form of these oxides greatly affects the plating peel resistance during high processing.

  The present inventors pay attention to the oxides present in the plating / steel interface of the hot dip galvanized steel sheet and the plating film, the oxide contained in the plating film of the hot dip galvanized steel sheet, and the resistance to plating during high processing. The relationship of peelability was investigated.

  FIG. 1 shows an example in which oxides present in a plating / steel interface of a hot dip galvanized steel sheet and in a plating film are observed with a scanning electron microscope. In FIG. 1, (a) is an example of an galvannealed steel sheet, and (b) is an example of a non-alloyed galvanized steel sheet. The oxides shown in FIGS. 1 (a) and 1 (b) are film-like, and it was found from the results of EDS elemental analysis that the oxides are mainly composed of Si and Mn.

  It was confirmed from the analysis of the cross section that the film-like oxide was present at the plating / steel plate interface after hot dip galvanizing and was also present in the plating layer after the alloying treatment. Since measurement of the form and size of the film-like oxide is difficult by cross-sectional observation, the plating layer was peeled off and obtained from analysis of the peeling residue as described later.

The inventors of the present invention have improved the plating peeling resistance at the time of high processing when a large amount of oxide formed in such a relatively large film is present in the plating / steel interface of the hot dip galvanized steel sheet and in the plating film. I found it inferior. As a result of examining in more detail, evaluating the area and the number of film-like oxides per one piece, and investigating the relationship with the plating peeling resistance, it exists in the area corresponding to 10000 μm ² of the hot dip galvanized steel sheet. It was found that when the number of film-like oxides having an area of 4 μm ² or more is 10 or less in total, the plating peeling resistance during high processing is excellent.

  As a method for extracting and observing the oxide in the plating / steel plate interface and the plating layer, the zinc plating layer is dissolved electrochemically in a non-aqueous solution, the solution is filtered through a filter, and the resulting residue is filtered. There is a method of observing with a scanning electron microscope or a transmission electron microscope.

In order to observe the area corresponding to 10000 μm ² of the hot dip galvanized steel sheet, for example, a galvanized layer having an area of A (mm ² ) is electrochemically dissolved in a non-aqueous solution, and the solution is dissolved in an area of B ( When filtering with a filter of mm ² ), it is possible to observe any 5 fields of 2000 × B / A (μm ² ) and evaluate the total.

  Further, in order to measure the area of the oxide, it is possible to measure the observed image after performing contrast adjustment and binarization by image processing. The area of the oxide at that time is, for example, the area excluding the hole portion of the oxide observed in a mesh shape as shown in FIG.

Oxidation the number of Si and / or Mn is observed as 4 [mu] m ² or more area in the observation area by converting the number of area corresponding to 10000 ^2, obtaining the number of the area corresponding to 10000 ^2.

  When the plated layer of the hot dip galvanized steel sheet is dissolved electrochemically in a non-aqueous solution, the oxide present in the plating film and at the plating / steel plate interface is not dissolved but dispersed in the non-aqueous solution. By filtering the non-aqueous solution in which the oxide is dispersed, the oxide can be captured in the filtration filter while maintaining the shape of the oxide. Moreover, in order to dissolve only the plating layer of the hot dip galvanized steel sheet electrochemically, the plating layer can be dissolved without causing dissolution of the underlying steel sheet by performing constant potential electrolysis by controlling the electric potential at which only the plating layer is dissolved. Can be dissolved.

  In the hot dip galvanized steel sheet, the improvement of the plating peel resistance at the time of high processing can be realized by suppressing the propagation of cracks generated in the plating layer at the time of high processing. However, if there is an oxide of Si and / or Mn formed in a film form in the plating film, cracks are easily propagated from there. It is estimated that this leads to peeling. Similarly, when a film-like Si and / or Mn oxide is present at the plating / steel sheet interface, the crack generated in the plating film easily propagates when it reaches the plating / steel sheet interface. Therefore, it is considered that peeling of the plating / steel plate interface is likely to occur.

  The existence of the Si and / or Mn oxide in the form of a film in the plating film and / or at the plating / steel sheet interface seems to be greatly influenced by the form of the oxide present on the surface of the steel sheet before plating. That is, in the case where an oxide of Si and / or Mn is formed in the form of a film on the surface of the steel sheet before plating, the plating / plating is performed while maintaining the shape after galvanizing and alloying treatment. It is thought that it exists in the steel plate interface or is dispersed in the plating film. Therefore, what is necessary is just to make the oxide which exists in the steel plate surface before plating into the state disperse | distributed finely instead of film | membrane form.

  As a method for controlling the oxide state on the surface of the steel sheet before plating, there are a method for controlling the annealing conditions (temperature distribution, atmosphere) before plating, a method for performing various pretreatments on the surface of the steel sheet in advance. In the present invention, the method is not limited. For example, in the annealing process, the oxygen potential in the surface layer of the steel sheet is increased, and the surface layer portion of the steel plate is internally oxidized to form a film-like oxide on the surface of the steel sheet before plating. It is possible not to form.

In addition, for example, the steel plate is CGL having a DFF type or NOF type heating zone, and the steel plate temperature on the heating zone outlet side is set to 700 ° C. or more, so that the Fe-based scale is attached to the steel plate surface layer. There is a method in which the steel sheet surface layer is internally oxidized by becoming an oxygen supply source in the belt. When the temperature of the steel sheet on the heating zone is less than 700 ° C., the amount of Fe-based scale produced is insufficient, so that no internal oxide layer is formed during reduction annealing in the reduction zone, and film-like Si is formed on the steel sheet surface before plating. And / or an oxide of Mn is formed in a film shape. Furthermore, the CO / H ₂ O ratio (capacity ratio) of the atmospheric gas composition in the heating zone is set to 0.8 or less. Since the heating zone combustion gas such as coke gas is mixed, the gas composition ranges in a wide, of H _{2 O} by the CO is one type of unburnt gas therein to dissociation equilibrium with H _{2 O} there is an effect of reducing the H _2. Therefore, when the CO concentration increases, the oxidation of the steel sheet is suppressed. On the other hand, H ₂ O decomposes on the steel sheet surface and releases O to oxidize the steel sheet. Therefore, in order to actively oxidize the steel sheet, it is necessary to suppress the CO concentration and increase the relative amount of H ₂ O. However, when the CO / H ₂ O ratio exceeds 0.8, the reduction effect is excellent even when the DFF outlet temperature is 700 ° C. or higher, so that the oxidation amount of Fe cannot be secured, and film-like Si and / Alternatively, an oxide of Mn is formed in a film shape.

  Next, in the reduction zone, by recrystallization annealing for 15 to 600 s in a temperature range of 700 to 940 ° C., the oxide state on the steel sheet surface before plating can be made into a dispersed state instead of a film shape. Zinc plating or further alloying treatment can be performed to realize an oxide state satisfying the provisions of the present invention. The galvanizing method and alloying method are not particularly limited.

  The present invention defines the shape of the oxide present in the plating / steel interface and the plating film. It is not limited to the kind and composition of the alloy layer of the whole plating film. Moreover, in the hot dip galvanized steel sheet of the present invention, the production method of the base steel sheet is not particularly limited, and a normal pickling plate or cold rolled plate may be used. Usually, the plate thickness is 5 mm or less. Moreover, it does not specifically limit about the manufacturing apparatus of a hot dip galvanized steel plate, For example, the continuous hot dip galvanizing apparatus currently used normally may be sufficient. Further, press formability may be further improved by applying an upper layer plating mainly containing Fe or Ni or an oxide film mainly containing Zn to the hot dip galvanized steel sheet of the present invention.

The plating adhesion amount is preferably ²⁰ to 120 g / m ² per side. If it is less than 20 g / m ² , it becomes difficult to ensure corrosion resistance, and if it exceeds 120 g / m ² , the plating peel resistance deteriorates. Further, the Fe content of the plating layer is preferably 7 to 15%. If it is less than 7%, uneven alloying and flaking properties deteriorate, and if it exceeds 15%, the plating peel resistance deteriorates.

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

  A slab obtained by melting steel having chemical components shown in Table 1 was made into a cold rolled steel sheet having a thickness of 1.2 mm by hot rolling, pickling and cold rolling.

This cold-rolled steel sheet was heated with a CGL having a DFF type heating zone by appropriately changing the CO / H ₂ O ratio by introducing the heating zone exit side temperature and, if necessary, H ₂ O or CO into the heating zone. The DFF delivery side steel plate temperature was measured with a radiation thermometer. Thereafter, recrystallization annealing was performed at 850 ° C. for 20 s in a reduction zone, plating was performed at a weight per unit area of 50 g / m ^{2 in} a 460 ° C. zinc plating bath, and a part thereof was subjected to alloying treatment.

About the obtained alloyed hot-dip galvanized steel sheet and non-alloyed hot-dip galvanized steel sheet, the appearance (plating appearance) and plating peel resistance were investigated. In addition, after dissolving the galvanized layer in a non-aqueous solution by constant potential electrolysis and filtering the obtained residue with a Nuclepore filter having a diameter of 50 nm, the residue captured by the filter is observed with a scanning electron microscope. did. In this example, the galvanized layer having an area of 100 mm ² was electrochemically dissolved and filtered with a filter having an area of 254 mm ² . In observation with a scanning electron microscope, arbitrary five visual fields were observed at a magnification of 1000 times (observation visual field area: about 8000 μm ² ). The film-like substance was sequentially enlarged and observed, and elemental analysis by EDS was performed to count the number of Si and / or Mn oxides observed as an area of 4 μm ² or more.

  Appearance was judged as good appearance (symbol ◯) when there was no appearance defect such as non-plating or alloying unevenness, and when it was present, it was judged as poor appearance (symbol x).

  In an alloyed hot-dip galvanized steel sheet, suppression of plating peeling at the bent portion when the plated steel sheet is bent at an acute angle exceeding 90 ° is required as plating peeling resistance. In this example, the bent part when bent by 120 ° is peeled off from the tape, the amount of peeling per unit length is measured by the fluorescent X-ray, the Zn count is measured, and rank 1 (symbol ○) according to the following criteria: Those with 2 (symbol Δ) were evaluated as having good plating peel resistance, and those with 3 or more were evaluated as having poor plating peel resistance (symbol x).

Fluorescent X-ray Zn count number Rank 0 to less than 500: 1 (good)
Less than 500-1000: 2
Less than 1000-2000: 3
Less than 2000-3000: 4
3000 or more: 5 (poor)

Non-alloyed hot-dip galvanized steel sheet requires plating peeling resistance at the time of impact especially as plating peeling resistance. Therefore, the evaluation is performed by applying a weight of 1 kg to 1.8 kg in a ball impact test. It was dropped on a 2-inch punch, and the portion where the plated steel plate was hit was peeled off with cello tape (registered trademark), and the presence or absence of peeling of the plating layer was visually judged and evaluated as follows.
No peeling: ○
With peeling: ×

  Tables 2 and 3 show the production conditions and the results obtained.

  Table 2 shows examples of galvannealed steel sheets. As is apparent from Table 2, the hot-dip galvanized steel sheet produced by the method of the present invention is a high alloy steel containing a large amount of easily oxidizable elements such as Si, but is resistant to plating peeling during high processing. Excellent plating appearance. On the other hand, a hot-dip galvanized steel sheet manufactured outside the scope of the present invention method is inferior in plating peel resistance at the time of high processing or further inferior in plating appearance.

  Table 3 is an example of a non-alloyed hot dip galvanized steel sheet. As is apparent from Table 3, the hot-dip galvanized steel sheet produced by the method of the present invention is a high alloy steel containing a large amount of easily oxidizable elements such as Si, but is resistant to plating peeling during high processing. Excellent plating appearance. On the other hand, a hot-dip galvanized steel sheet manufactured outside the scope of the present invention method is inferior in plating peel resistance at the time of high processing or further inferior in plating appearance.

  The high-strength hot-dip galvanized steel sheet of the present invention is excellent in plating peel resistance at the time of high processing, and can be used as a surface-treated steel sheet for reducing the weight and strength of an automobile body itself.

Claims (1)

In mass%, C: 0.01 to 0.15%, Si: 0.8 to 2.0%, Mn: 1.0 to 3.0%, P: 0.025% or less, S: 0.01 %, With the balance being Fe and unavoidable impurities, and a film shape having an area of 4 μm ² or more among the oxides of Si and / or Mn present in the plating / steel interface and plating layer A high-strength hot-dip galvanized steel sheet excellent in plating peel resistance, characterized in that the number of oxides is 10 or less in total in an area corresponding to 10,000 μm ² of the galvanized steel sheet.