JPH0660376B2 - Hot-dip galvanized steel sheet with excellent workability and method for producing the same - Google Patents

Description

TECHNICAL FIELD The present invention relates to a hot-dip galvanized steel sheet in which the plated layer is formed into a Fe—Zn based alloy layer by a heat diffusion treatment after galvanizing, and a method for producing the same. .

In particular, by forming a specific Si layer on the surface of the steel sheet and heating and reducing it, changing the alloy layer morphology of the plating layer from the hierarchical type to the disordered layer type makes the plating layer highly tough A hot-dip galvanized steel sheet with excellent properties is provided on the market.

[Prior Art] Hot-dip galvanized steel sheets (hereinafter simply referred to as “alloyed sub-plates”) are suitable for painting bases of home appliances, automobiles, etc. due to their appropriate sacrificial anode action and the excellent catching effect resulting from the unevenness of the substrate. It is one of the surface-treated steel sheets that are currently widely used as rustproof steel sheets.

The surface properties of alloyed subplates include corrosion resistance, workability, weldability, and paintability. Among these, one of the most market-demanding properties is peeling of the plating layer due to processing (flaking, powder). There is a ring). In order to improve this, the steel grade, pretreatment for plating, hot dip plating conditions, alloying heating conditions, etc. are now being actively researched and developed.

However, in the prior art, especially in alloyed subplates using ordinary-grade aluminum-killed steel (hereinafter simply referred to as Al-K) as a substrate, the Fe-Zn alloy layer structure is generally At present, only the alloy layer morphology that is formed in a layered manner with a concentration gradient can be obtained, and the technology that embodies the concept of modifying the morphology to achieve high workability has not yet been found.

For example, Japanese Patent Laid-Open No. 56-13470, in which a small amount of Al in a hot dip galvanizing bath is galvanized and then alloyed, pre-galvanized steel sheet is pre-plated with Fe, Ni, etc., galvanized and alloyed. Treated JP-A-58-104163, JP-A-60-110
Although No. 859 and the like are proposed, all alloyed sub-plates obtained by these techniques have a drawback that the plating layer is easily peeled off in a powdering or flaking form even under severe press working.

The cause is in the alloy layer structure of the plating layer as described above, and the phase structure of the alloy layer generated by Fe-Zn interdiffusion at the interface of the base metal is from the side of the base iron with a high Fe diffusion rate toward the plating surface. The points are Γ, δ ₁ , and ζ, and these are generated in an orderly and hierarchical manner parallel to the ground plane. Therefore, it is considered that when a constant processing stress is applied by the processing, the stress concentration occurs in the hard and brittle Γ phase having the highest Fe content, which causes the peeling of the rooting plating layer.

[Problems to be Solved by the Invention] In the prior arts described above, after all, the alloy layer morphology formed is Fe in each phase in which Fe diffusivity is relatively parallel to the base steel sheet.
-The Zn diffusion layer has a multi-layered structure that is formed by stacking layers on top of each other, which causes concentration of processing stress, which causes cracks in the brittle alloy layer, resulting in a powdery form from the iron matrix when it cannot withstand the stress. There is a problem in practice due to peeling.

In the present invention, in order to improve the workability of the alloy layer, which the conventional technique has, the discontinuous ζ ₁ phase main component in which the ζ phases capable of stress dispersion are mixed in the generation form of the alloy layer having a hierarchical structure is mixed. Thought that it is necessary to change to, as a result of various studies, after providing a specific Si layer on the steel plate surface before applying the hot dip Zn plating, by specifying the heating reduction plate temperature and heating, The present invention has been proposed based on the knowledge that an alloy layer morphology in which the δ ₁ phase and the ζ phase are disordered can be obtained.

[Means for Solving the Problems] The present invention is based on the above-mentioned technical idea. However, it should be noted that such behavior of Si has similar operational effects even in Si in steel. It has already been proposed as a prior application of the present invention.

The structure of the present invention is shown below.

(1) An Al-enriched layer consisting of Zn-Fe-Al-Si at
1 μm thick, and a Fe-Zn alloy layer of 5 to 50 μm in which the δ ₁ phase is mainly formed and the ζ phase and the δ ₁ phase are disturbed is formed on the upper layer. steel sheet.

(2) In the Zenzimer type hot dip galvanizing line, the surface of the steel sheet to be plated is pre-plated with a Si layer of 10 to 10000Å, and the maximum sheet temperature is set to 500 to 900 ° C in order to heat and reduce the steel sheet in a hydrogen gas reducing atmosphere. After that, Al: 0.01 to 0.15% and Sb: 0.05 to 0.5% by weight of the components of the hot dip galvanizing bath are added, and the total amount of inevitable impurities such as Pb is less than 0.02% is used. A method for producing a hot-dip galvanized steel sheet having excellent workability, which comprises plating.

(3) In the Zenzimer hot-dip galvanizing line, the surface of the steel sheet to be plated is pre-plated with a Si layer in the range of 10 to 10000Å, and the maximum sheet temperature is set to 500 to 900 ° C in order to heat reduce the sheet in a hydrogen gas reducing atmosphere. After that, the components of the hot-dip galvanizing bath, in weight%, Al: 0.01-0.15%, Sb: 0.05-0.5
%, Mg: 0.01 to 0.2%, Ti: 0.01 to 0.05%, B: 0.0
A method for producing a hot-dip galvanized steel sheet having excellent workability, which comprises adding 0.1 to 0.01% and performing plating using a plating bath in which the total amount of inevitable impurities such as Pb is less than 0.02%.

The skeleton is mainly composed of δ ₁ mixed with ζ as an alloy layer form, and the generation of δ ₁ discontinuously at the steel plate interface by ζ causes the alloy layer to have high toughness, which results in high workability of the plated layer. In order to do so, a specific Si pre-plated layer is provided on the surface of the original plate before annealing, and it is heated by specifying the reducing atmosphere and the plate temperature.

[Operation] (I) Regarding thickness of Si pre-plating: Before pre-annealing, the Si pre-plating formed on the surface of the steel sheet is made of Fe-Si by the diffusion of Si in the subsequent annealing process.
Of the Fe-Si-Al-Zn system Al concentrated layer generated at the base iron interface during hot dip galvanization by forming a uniform thin film and diffusion of the Al concentrated layer during alloying treatment It is an indispensable control factor for enabling the alloy layer morphology to be disturbed by entering the δ ₁ phase due to the ζ phase in the present invention by suppressing. In addition,
The Si pre-plating means may be any known means such as a vapor deposition method, a vapor phase method or a thermal spraying method.

If the thickness of the Si pre-plating is less than 10 Å, the Fe-Si diffusion layer cannot be sufficiently formed on the surface layer of the steel sheet after annealing, so that the Fe-Si-Al formed on the base iron interface in the hot dip galvanized layer before alloying. -Zn-based Al enriched layer is formed thickly and only discontinuously. The alloying morphology of the alloy layer that occurs in the subsequent alloying treatment is Γ, δ ₁ and ζ with the Fe concentration gradient that is often seen in the past. Each of the phases of the alloy has an alloy layer structure that is formed hierarchically in parallel with the base material.
It is not preferable because it deviates from the gist of the present invention.

On the other hand, when the Si pre-plating thickness exceeds 10,000 Å, the Fe-Si diffusion reaction of the steel sheet surface layer during the annealing process is not sufficient under the high-speed line in terms of time, and the molten Zn plating layer during alloying treatment is In addition, the unreacted metal Si forms an oxide of SiOx due to a small amount of oxygen in the annealing atmosphere, which easily causes non-plating, which greatly impairs the commercial value, which is not preferable. .

Therefore, the preferable Si pre-plating thickness is 30 to 10
00Å is good.

(II) Steel plate reducing conditions after Si pre-plating The steel plate reducing plate temperature after Si pre-plating is one of the indispensable control factors that constitute the present invention together with the reducing atmosphere described later.

The plate temperature needs to be sufficiently controlled in order to accelerate the thermal diffusion of Fe-Si in the Si pre-plated layer at the steel plate interface in a short time and to promote its uniform diffusion.

If the plate temperature is less than 500 ° C., the Fe-Si diffusion reaction at the steel plate interface is not sufficient, which reduces the uniform thin film formation of the interface Al concentrated layer before and after the alloying treatment and its excessive diffusion prevention effect,
Consequently, the discontinuity of the δ ₁ phase due to the ζ phase, which is an alloy layer form having strong workability according to the present invention, cannot be sufficiently achieved, which is not preferable.

In addition, when the plate temperature exceeds 900 ° C, Fe-S at the steel plate interface
i Diffusion layer is excessively generated, which excessively suppresses the diffusion of Fe into the galvanizing layer during the alloying process, and on the contrary, the alloying process in a short time becomes insufficient, resulting in a decrease in productivity. Or
In addition, it is better to avoid it as much as possible because of the deterioration of the material strength of the steel sheet as a product.

The more preferable reduction plate temperature is 600 ~ at the maximum plate temperature.
850 ℃ is good.

In addition, when heating at the maximum reducing plate temperature as described above, it is necessary to maintain a state in which the steel sheet surface is sufficiently reduced while preventing non-plating as a heating atmosphere and suppressing the oxide formation of the pre-plated Si layer as much as possible. Especially, DP management should be carried out as follows.

In the oxidation region where DP is higher than -20 ° C in the atmosphere, the surface oxidation of the pre-plated Si layer or the Fe-Si diffusion layer formed by thermal diffusion is promoted, resulting in non-plating during hot dip galvanization or at the interface. Formation of a uniform thin film of the Al-enriched layer is hindered, and it is difficult to form at least the alloy layer structure after the alloying treatment aimed by the present invention.

From this point, it is necessary to make DP as low as possible to keep the reduction region, but considering the reducing ability and productivity of Fe or Fe-Si oxides generated on the steel plate surface, the lower limit of DP is -50 ° C.
Saturates below.

Therefore, as DP in the annealing reduction atmosphere of steel sheet,
-40 ° C is preferred.

In addition, in the present invention, it is also noted that even if non-oxidative heating is performed only when the above-mentioned reduction annealing of the Si pre-plated steel sheet is performed, the present invention is not hindered.

(III) Constituents of hot dip galvanizing bath 1) Al concentration Al is an excessive amount of Fe-Zn at the steel sheet interface during the reaction of the steel sheet in the bath.
It is an essential component for suppressing the interdiffusion reaction by the via effect of the Fe-Al-Zn ternary alloy layer, suppressing the generation of Γ phase in the subsequent alloying process, and controlling the alloy phase morphology mainly composed of ζ _1. . If Al is less than 0.01 wt%, the above-mentioned ternary alloy layer via effect does not occur, and a brittle overalloy is likely to be formed during processing, which is not preferable.

On the other hand, when Al exceeds 0.15 wt%, the via effect of the ternary alloy layer is excessively exhibited, which makes it easy to unalloy in the subsequent alloying process, which impairs the commercial value.

Therefore, the Al content in the bath is preferably 0.01 to 0.15 wt%, more preferably 0.08 to 0.03 wt%.

2) Sb concentration Sb is eutectic with Al in the bath and becomes an Al-Sb compound to segregate at the base iron interface and surface layer of the galvanized layer and randomize Fe diffusion in the alloying process like Si in steel, This is at least to suppress the formation of hierarchical alloy layers. If it is less than 0.05% by weight of Sb, its action is not sufficiently exhibited, and if it exceeds 0.5% by weight of Sb, the viscosity of the plating bath increases, and stable control becomes difficult against discontinuity of the δ ₁ alloy layer due to ζ. .

Therefore, the Sb concentration is preferably 0.05 to 0.5 wt%, more preferably 0.1 to
0.3wt% is good.

3) Mg concentration Mg is to improve the corrosion resistance as an alloyed galvanized steel sheet. If the Mg content is less than 0.01 wt%, the effect will not be fully exhibited.If the Mg content exceeds 0.2 wt%, Mg oxide will frequently occur on the surface of the hot dip galvanizing bath and will reattach to the steel sheet as scrap dross. It causes problems and impairs practicality.

Therefore, the Mg concentration is preferably 0.01 to 0.2 wt%, but preferably 0.
05-0.1wt% is good.

4) Ti concentration Ti is for improving the corrosion resistance of the galvannealed steel sheet. If Ti is less than 0.01 wt%, high corrosion resistance is not sufficient, and
When Ti exceeds 0.05 wt%, it is not preferable because it promotes formation of the interfacial alloy layer, which promotes stratification of the alloy layer after the alloying treatment.

Therefore, the Ti concentration should be 0.01-0.05 wt%, but preferably
0.01-0.03wt% is good.

5) B Concentration B is to prevent fatigue embrittlement of the galvannealed steel sheet over time.

If B is less than 0.001 wt%, the effect cannot be fully exerted, and if B exceeds 0.01 wt%, it is difficult to physically form a solid solution in the plating bath, and dross is regenerated on the steel sheet. Not practical because it adheres. Therefore, the B concentration is 0.00
1 to 0.01 wt% is preferable, but 0.003 to 0.008 wt% is preferable
Is good.

6) Total amount of unavoidable impurities The unavoidable impurities referred to in the present invention form a local battery with Zn, which is the basic component of the plating layer such as Cb and Sn including Pb,
It is an element that should be excluded from the system as much as possible because it causes deterioration of corrosion resistance.

Therefore, the total amount of the impurities is less than 0.02 wt% and preferably 0.1.
01 wt% or less is preferable.

(IV) Plating Thickness of Hot-Dip Galvanized Steel Sheet The thickness of the hot-dip galvanized steel sheet is basically a factor controlling the corrosion resistance of the hot-dip galvanized steel sheet.

If the plating thickness is less than 5 μm, the corrosion resistance after coating, which is the maximum characteristic of the alloyed subplate, is extremely deteriorated. If it exceeds 50 μm, the workability is not hindered, but too thick a film is alloyed. It takes a lot of time and impairs productivity, which is not preferable.

Therefore, the appropriate plating thickness is preferably 5 to 50 μm, and more preferably 7 to 30 μm.

The effects of the present invention will be described in more detail below with reference to examples.

[Example] A low carbon steel sheet of aluminum killed steel with a thickness of 0.6 mm and a width
914 mm cold rolled steel sheet or 3.5 mm thick and dehydrated scale hot rolled steel sheet with a width of 1200 mm is first degreased with an alkali.
After being washed with water and dried, Si specified in Table 1 by the vapor deposition method
Apply pre-plating. Immediately after heating and reducing in a 15% H ₂ + N ₂ mixed gas atmosphere in a Zenzimer hot-dip galvanizing line, the hot-dip galvanized plate was heated so that the maximum plate temperature became the maximum plate temperature specified in Table 1. As 4
After cooling to 70 ° C., it is immersed for 2 seconds in a hot dip galvanizing bath specified in Table 1 with a bath temperature of 460 ° C. After that, gas wiping is performed in the atmosphere to control the amount of deposits on the plate, and then it is heated and diffused in the alloying furnace so that the maximum plate temperature on the outlet side becomes 550 ° C. It is cooled with steam and then water-quenched and dried. .

The hot-dip galvannealed steel sheet of the present invention thus obtained exhibits excellent workability without impairing other performances as shown in Table 1, and is an epoch-making product unprecedented in the past and It turns out that it is the manufacturing method.

Effect of Si pre-plating No. 1 to No. 18 of the present invention in Table 1 and No.
Shown together with No. 19 to No. 20. Of these, the No. 3 example of the present invention was observed by SEM for the state of alloy layer formation in the cross section, and
A conceptual diagram of EPMA element distribution at that time is shown in FIG.
Further, as a comparative example, FIG. 2 shows a conceptual diagram of the result of similarly analyzing No. 19.

As is apparent from these results, the Fe-Zn alloy layer morphology changes from hierarchical to random depending on the thickness of pre-plated Si, and the phase morphology suppresses the Γ phase, which is brittle during processing, and It can be seen that the δ ₁ phase and the morphology change into a disordered form. The reason for exhibiting this alloy layer morphology is that the Si pre-plated layer at the base iron interface is
In the annealing process, Fe was converted to Fe-Si by diffusion of Fe from the base material, and further, this caused a preferential reaction of Al and Si in the bath in the reaction in the hot dip galvanizing bath, resulting in the Zn-Fe-Si at the base iron interface. It is considered that the formation of the Al concentrated layer composed of -Al in a uniform and thin film was promoted. Therefore, in the alloying process, this tight Al-enriched layer is smashed to cause diffusion of Fe into the plating layer, but the Fe diffusion site is Although it is almost the same as the preferential diffusion, it is thought that the diffusion rate of Fe is slightly delayed due to the barrier action of the Al concentrated layer, which promotes the discontinuous formation of the δ ₁ alloy layer due to the ζ phase. To be

Effect of maximum heating plate temperature before plating Example No. 21 to No. 34 of the present invention and Comparative example No. 35
~ Shown together with No. 36 As is clear from this, when the plate temperature is as low as 460 ° C., it is easy for the alloy layer after alloying to take a hierarchical form.

The reason for this is the formation of an Al-enriched layer of Zn-Fe-Al-Si that occurs at the base iron interface in the hot-dip galvanizing process before alloying. The layer is the plate temperature
It is considered that, compared with the high temperature annealing at 500 to 900 ° C, the film is thicker, but it is non-uniform and porous. Porousization of the Al-enriched layer causes excessive diffusion of Fe from the base iron in the subsequent alloying process, which leads to halving the effect of modifying the alloy layer morphology by the Si pre-plating formation according to the present invention. It is thought to be the cause.

On the other hand, when the plate temperature exceeds 900 ° C, the Fe-rich Fe-Si diffusion layer excessively increases, which causes excessive formation of the interfacial Al concentrated layer and its discontinuity during hot dip galvanization. The excessive diffusion of Fe in the alloying treatment promotes the layering of the alloy layer morphology, which is not preferable.

From the above, the preferred maximum heating plate temperature is 600 to 850 ℃
Is good.

As described above, the maximum hot plate temperature before hot dip galvanizing is important for stably ensuring the performance of the alloyed sub-plate, and the plate temperature range of the present invention is to answer this. I understand.

Effect of Each Component of Hot Dip Galvanizing Bath 1) Al and Sb are one of the basic plating bath components in the present invention.

Regarding the effect of Al, the invention examples No. 37 to No. 41
Comparative Examples No. 42 to No. 43, and Examples of the present invention relating to Sb are No. 44 to No. 50, and Comparative Examples No. 51 to N.
Shown with o.52.

If any of the component systems deviates from the concentration range of the present invention, workability and finished appearance will be impaired.

2) Regarding the effects of other additive components, Mg, Ti and B, comparative examples are given in No. 53 to No. 56 of the present invention of Mg.
No. 57 to No. 58, the present invention example of Ti is No. 5
No. 9 to No. 61 show comparative examples No. 62 to No. 63. or,
As for B, the invention examples No. 64 to No. 68 and
No. 71 to No. 72, and Comparative Examples No. 69 to No.
70.

As is apparent from these, each of these components is mainly intended to improve the overall corrosion resistance as an alloyed subplate and the fatigue fracture resistance that occurs with corrosion, and the expectation is outside the scope of the present invention. thin.

3) Further, the proper range of inevitable impurities such as Pb is shown in Examples 3 and 73 of the present invention together with Comparative Example No. 74. As is clear from this, since the impurities mainly cause a decrease in corrosion resistance, in the present invention, it is necessary to consider removing them from the plating bath system as much as possible.

4) Proper deposition amount range of alloyed sub-plate The deposition amount range referred to in the present invention should be basically determined according to the usage environment and cost, but the deposition amount is also from the overall performance level. , With restrictions.

Regarding the range of the adhesion amount, the present invention example No. 75 to N
The results are shown in Table 79 together with Comparative Examples No. 80 to No. 81.

As is clear from this, if the proper amount of adhesion of the present invention is deviated, corrosion resistance and workability are impaired, which is not practical.

[Effects of the Invention] As described above in detail with reference to the examples, the present invention focuses on Si among the steel sheet components, and the modification of the alloy layer morphology by this is an alloyed sub-plate. Based on the knowledge of the previous application that greatly improved the workability of the above, by forming a Si layer with a specific thickness in advance and then heating at a specific plate temperature, it is possible to reform the alloy layer similar to the previous application. As a result, this is an epoch-making technology that is unprecedented as a general-purpose technology that can dramatically improve the workability of alloyed sub-plates regardless of the steel type of the steel sheet, and shows remarkable effects. .

[Brief description of drawings]

FIG. 1 is a typical example of the alloy layer morphology of the embodiment of the present invention, which is a conceptual view showing the situation from the microscopic observation and EPMA element distribution state of the galvannealed steel sheet described in No. 3 of Table 1 FIG. 2 illustrates the conceptual diagram of the alloy layer form in the plating of the hot-dip galvanized steel sheet described in No. 19 of Table 1 as a comparative representative example of the prior art.

Claims (3)

[Claims] 1. An Fe-Zn layer having a Zn-Fe-Al-Si Al enriched layer of 0.1 to 1 μm formed on the steel sheet interface, and a δ ₁ phase-dominant type ζ phase and a δ ₁ phase intermixed in the upper layer. System alloy layer 5 to 50 μm
A hot-dip galvanized steel sheet with excellent workability characterized by being formed. 2. In a Sendzimer type hot dip galvanizing line, a Si layer is pre-plated on the surface of a steel sheet to be plated in an amount of 10 to 10000Å in advance, and the maximum sheet temperature is 500 to 900 for heat reduction in the hydrogen gas reducing atmosphere. After the temperature was adjusted to ℃, the components of the hot dip galvanizing bath were Al: 0.01-0.15% and Sb: 0.05-
0.5% is added, and the total amount of inevitable impurities such as Pb is 0.
A method for producing a hot-dip galvanized steel sheet with excellent workability, which comprises plating using a plating bath having a content of less than 02%. 3. In a Sendzimer type hot dip galvanizing line, a Si layer is pre-plated on the surface of a steel sheet to be plated in an amount of 10 to 10,000 Å in advance, and the maximum sheet temperature is 500 to 900 for heat reduction of the steel sheet in a hydrogen gas reducing atmosphere. After the temperature was adjusted to ℃, the components of the hot dip galvanizing bath were Al: 0.01-0.15% and Sb: 0.05-
0.5%, Mg: 0.01 to 0.2%, Ti: 0.01 to 0.05%, B:
A method for producing a hot-dip galvanized steel sheet with excellent workability, which comprises adding 0.001 to 0.01% and plating using a plating bath in which the total amount of inevitable impurities such as Pb is less than 0.02%.