JPH046258A - Galvannealed steel sheet excellent in workability and its production - Google Patents

Abstract

PURPOSE:To produce a galvannealed steel sheet excellent in workability by specifying Si content in a steel sheet, gas heating reduction sheet temp. in a plating stage, and the components of the plating bath, respectively, at the time of applying hot-dip galvanizing to a low carbon steel sheet and subjecting this steel sheet to diffusion treatment by heating. CONSTITUTION:At the time of producing a galvannealed steel sheet by applying hot-dip galvanizing to a low carbon steel sheet and subjecting this steel sheet to diffusion treatment by heating, Si content in the low carbon steel sheet is regulated to 0.05-1.0% by weight. Moreover, gas heating reduction sheet temp. in a hot-dip galvanizing stage is regulated to 500-900 deg.C, and further, the plating bath has a composition which contains, by weight, 0.01-0.15% Al and 0.05-0.5% Sb and in which 0.01-0.2% Mg, 0.01-0.05% Ti, and 0.001-0.01% B are added, if necessary, and the total content of inevitable impurities, such as Pb, is regulat ed to <0.02%. By this method, the galvannealed steel sheet in which powdering and flaking are prevented and which has high toughness and superior workability can be obtained.

Description

[Detailed Description of the Invention] [Industrial Field of Application] The present invention relates to a hot-fusion galvanized steel sheet in which the plating layer is made into an Fe-Zn alloy layer by heat diffusion treatment after galvanizing, and a method for manufacturing the same. It is. In particular, by specifying the Si in the steel of the steel sheet and the heating conditions before hot-dip galvanizing, we have created a hot-alloyed galvanized steel sheet with high toughness and excellent workability that prevents powdering and flaking by changing the form of the alloy layer. It can be offered to the market as such.

[Prior art] Melt-alloyed galvanized steel sheets (hereinafter simply referred to as alloyed steel sheets) are used as a base coating for home appliances, automobiles, etc. due to their appropriate sacrificial anode action and excellent casting effect due to the unevenness of the base material. This is one of the surface-treated steel sheets that is currently widely used as a rust-resistant steel sheet.

On the other hand, in recent years, there has been a trend toward higher strength steel sheets for the purpose of reducing the weight of automobile bodies in the steel type of this alloyed mini-plate.

Research has recently progressed into alloyed miniature plates using this high-strength steel plate as a substrate, and methods for manufacturing the same, and some are now being marketed.

In particular, the surface properties of alloyed slabs include corrosion resistance,
There are processability, weldability, paintability, etc., but one of the properties with the highest market demand is peeling of the plating layer due to processing (
flaking powdering). In order to improve this, active research and development is currently underway to optimize the steel type, plating pretreatment, hot-dip plating conditions, alloying heating conditions, etc.

Among these, the following inventions have been proposed, although the number is small, as the current status of hot-dip galvanized steel sheets using high-strength steel sheets as substrates and methods of manufacturing the same.

A method for producing dense Zn plating on Si-killed steel sheets using Sendzimer hot-dip plating is disclosed in Japanese Patent Publication No. 52-44739.
Hot-dip galvanizing of high-strength hot-rolled steel sheets with excellent workability by applying specific hot-rolling and annealing conditions to high-Mn steel slabs and making the substrate structure a composite structure of ferrite structure and low-temperature transformation phase. As a manufacturing method, JP-A-56-13459
No. 80-33318 and JP-A-80-33318 have been proposed.

Further, a method for manufacturing a high tensile strength galvanized steel sheet with excellent workability characterized by hot rolling and cold rolling from a high Mn-5i 914 slab is disclosed in JP-A-56-13437.

However, these methods end with the process of hot-dip galvanizing the steel sheet, and do not mention the subsequent alloying treatment. However, in these methods, the heat diffusion alloy layer produced generally ends up in a hierarchical alloy layer morphology depending on the amount of Fe diffusion, and such an alloy layer has a relatively low Fe content especially when subjected to compression processing. high hard and brittle r
There is a drawback that the phase (alloy layer closest to the base metal) is prone to powdering and flaking due to stress concentration, resulting in peeling of the plating.

[The invention tries to solve ii! ! ! ! ! ] In the conventional technology described above, the form of the formed alloy layer is formed by forming Fe-Zn of each phase with different Fe diffusivity relatively parallel to the base steel plate.
The diffusion layer has a hierarchical structure formed by overlapping multiple layers, which causes concentration of processing stress, causing cracks to occur in the brittle alloy layer, and when it can no longer withstand the stress, it peels off from the iron base in a powder form. However, there are practical problems.

In the present invention, in order to improve the workability of the alloy layer that conventional technology has, we have changed the formation form of the alloy layer with a hierarchical structure to δ mixed with ζ phase that can disperse stress, and non-directionality mainly composed of phases. As a result of various studies, we determined that it was necessary to change the steel sheet to The present invention has been proposed based on the discovery that it is possible to achieve non-direction.

[Means for solving the problem] The present invention is based on the above-mentioned technical idea,
Its composition is shown below. An Fe-Zn alloy layer formed by heat diffusion after hot-dip galvanizing is formed on the upper layer of a low carbon steel sheet consisting of 1tI4 component by weight% and Si: 0.05 to 1.0%. The structure is ζ
It is characterized by a δf-based type in which the 61 phase and 61 phase are mixed, the 61 phase is formed in a discontinuous state with respect to the steel sheet base, and the alloy layer is formed in a thickness of 5 to 30 Pm. Melt-alloyed galvanized steel sheet with excellent workability.

Steel composition of low carbon steel plate in weight% C: 0.05 to 0.10
%, Si: 0.05~1.0%, Mn: 0.40~
0.60%, P・0.020% or less, S: 0.030
% or less, and the total Ai+0.010% or less.

3. In a method of manufacturing a hot-dip galvanized steel sheet by subjecting a low-carbon steel sheet to hot-dip galvanizing and heating and diffusing the steel sheet, the Si content of the low-carbon steel sheet is set to 0.05% by weight.
~1.0%, the temperature of the gas heating reduction plate in the hot-dip galvanizing process is 500 to 900°C, and the components of the plating bath are A or 0.01 to 0.15% by weight, Sb: 0.05
~0.5%.

Mg+0.01-0.2%, Ti;0.
Plating is carried out using a plating bath to which B: 01 to 0.05% and B: 0.001 to 0.01% are added, and the total amount of unavoidable impurities such as PB is less than 0.02%. A method for producing hot-fusion galvanized steel sheets with excellent workability.

The essence is δ1-dominant type with ζ mixture, and δ1
In order to promote the formation of a high-toughness alloy layer that is discontinuously formed at the steel plate interface, (1) Si should be particularly specified as the steel component of the m-plate, and (2) the steel plate before hot-dip galvanizing should be The problem lies in the need to specify the heating plate temperature.

[Function] ■ Regarding Si in steel In the present invention, Si in steel is concentrated and segregated at least at the base metal interface δ, phase and its surface during the formation or growth process of the Fe-Zn alloy layer, and this 51 segregation zone is It serves as a barrier to suppress excessive diffusion of Fe into the ζ phase in the upper layer of 61, and at the same time promotes the dispersion phenomenon of 61, which facilitates the dispersion of processing stress. This is to prevent the formation of Fe over-diffused alloy layers such as phase and r-phase, and at the same time to suppress the layered growth thereof. If the Si content in the steel sheet is less than 0.05% by weight, Fe-Zn during hot-dip galvanizing
In the interfacial reaction, excessive Fe diffusion occurs from the grain boundaries preferentially within the grains of the steel sheet, which promotes the growth of a hard and brittle Fe-excess alloy layer in the subsequent alloying process, which in turn leads to concentration of processing stress. This is not preferable because the alloy layer form shown in FIG. 2, which promotes the formation of a hierarchical alloy layer with poor powdering properties, is produced. On the other hand, if the Si content exceeds 1.0% by weight, during the heating process of the steel sheet before hot-dip galvanizing, Si oxides will be formed on the surface of the steel sheet and its reduction will be insufficient, resulting in poor wettability with hot-dip zinc. In addition, during the subsequent alloying process, F due to Si decreases, resulting in non-plating.
It has a strong e-diffusion suppressing effect and requires a long time to complete the alloying treatment of the plating layer, making it not very practical. Therefore, the Si content in the steel is preferably 0.05 to 0.1% by weight, preferably 0.05 to 0.5% by weight.

In addition, to explain in more detail the formation form of the alloy layer that occurs at this appropriate Si content, the initial alloy layer that is formed in this reaction in the plating bath has at least a slightly non-directional growth direction with respect to the base surface. However, in general, a twill-like columnar 61 phase is formed with hidden spaces, and a ζ phase is formed to fill the hidden spaces, resulting in a complex alloy layer. However, in the Si distribution situation determined by CMA shown in FIG. 2, Si in the steel is scattered in this 61 phase, and this Si-enriched layer is observed at the boundary with the ζ phase.

This tendency of Si concentration in the alloy layer becomes even stronger in the subsequent alloying process, and ζ decreases and δ1 becomes more dominant, resulting in enlargement, but the disordered morphology of 61 also becomes more complex. do. What is characteristic is that at least the δ1 generated in the initial alloy layer in contact with the base metal
A fragmentation phenomenon occurs due to ζ, and ζ is generated in the fragmented part.

In other words, Si in steel is formed during the formation or growth process of the alloy layer.
This Si segregation band acts as a barrier to suppress excessive Fe diffusion into the upper layers ζ and η, which induces or promotes the disordering phenomenon of ζ and δ1, which facilitates the dispersion of processing stress; This is presumed to be one of the major causes of the difference in morphology from at least the layered alloy layer containing the r-phase or the through-alloy layer that grows to the r-phase, which is recognized in the prior art.

■ Concerning the heating plate temperature of the steel plate before hot-dip galvanizing In the present invention, the heating atmosphere for the steel plate is designed to maintain the temperature of the steel plate surface in order to improve the uniform wettability of the steel plate with the hot-dip galvanizing bath and the uniform diffusivity of the Fe4n interfacial alloying reaction. A non-oxidizing or hydrogen-reducing mixed gas atmosphere is preferable in order to suppress excessive oxidation of the gas and to purify the surface by reducing the surface gas.
It is better to keep the temperature below 0℃.

In such a heating atmosphere, the maximum heating plate temperature before hot-dip galvanizing the steel sheet is such that the decarburization and reduction reaction of Fe oxides and Fe-C carbides that are slightly generated on the surface of the steel sheet can be carried out in a short time from the viewpoint of high productivity. This is a necessary condition to complete the process and clean the surface of the steel plate.

If the maximum heating plate temperature is less than 500°C, the decarburization or reduction of carbides and oxides precipitated on the surface of the steel sheet by the atmospheric gas will not be sufficient, resulting in non-plating during subsequent hot-dip galvanizing.
Even if alloying treatment is performed, the unplated parts remain as defects, and deterioration in performance such as corrosion resistance and painted appearance is unavoidable.

On the other hand, if the sheet temperature exceeds 900° C., it becomes difficult to control the material strength of the alloyed galvanized steel sheet, which is not very practical.

Therefore, the maximum heating plate temperature of the steel plate before plating is 50
The temperature is preferably 0 to 900°C, preferably 650 to 850°C.

■ Regarding the components of the hot-dip galvanizing bath 1) AIL concentration Aj2 is the excess F at the steel plate interface during the reaction of the steel plate in the bath.
The e-Zn interdiffusion reaction is suppressed by the barrier effect of the Fe-An-Zn ternary alloy layer, and in the subsequent alloying process, it is necessary to suppress the formation of phases and control the morphology of the alloy layer mainly consisting of δ1. component.All is 0.01w
If it is less than t%, there is no barrier effect of the ternary alloy layer described above,
This is undesirable because overalloying, which is brittle during processing, is likely to be formed.

On the other hand, if the An content exceeds 0.15 wt%, the barrier effect of the ternary alloy layer will be excessively exhibited, and the alloy will be more likely to be unalloyed in the subsequent alloying process, impairing its commercial value.

Therefore, the amount of Au in the bath is preferably 0.001 to 0.15 wt%, preferably 0.08 to 0.13 wt%.

2) The Sb concentration sb eutecticizes with An in the bath, becomes a 1-Sb compound, and segregates at the base metal interface and surface layer of the galvanized layer, randomizing Fe diffusion during the alloying process like Si in steel. This is to at least suppress the formation of hierarchical alloy layers. Sb 0.0
If it is less than 5wt%, its effect will not be fully exhibited, and if Sb
If it exceeds 0.5 wt%, the viscosity of the plating bath will increase, making it difficult to control the amount of plating deposited stably.

Therefore, the sb concentration is preferably 0.05 to 0.5 wt%, preferably 0.1 to 0.3 wt%.

3) Mg concentration Mg is provided to improve corrosion resistance as an alloyed galvanized steel sheet and phosphate treatability as a base treatment for painting.

If the Mg content is less than 0.01wt%, the effect will not be fully exhibited, and if the Mg content exceeds 0.2wt%, Mg oxides will frequently appear on the hot-dip galvanizing bath surface and re-deposit on the steel plate as dross, resulting in poor appearance. This causes problems and impairs practicality.

Therefore, the vg concentration is preferably 0.01 to 0.2 wt%, preferably 0.05 to 0.1 wt%.

4) Ti concentration Ti is intended to improve the corrosion resistance of the alloyed galvanized steel sheet. If the Ti content is less than 0.01 wt%, high corrosion resistance will not be achieved sufficiently, and if the Ti content exceeds 0.05 wt%, sufficient phosphate treatment properties as a coating base treatment will not be obtained.

Therefore, the Ti concentration is preferably 0.01 to 0.05 wt%, preferably 0.01 to 0.05 wt%. O2N is preferably 2.03 wt%.

5) B concentration B is provided to prevent fatigue embrittlement of the plating layer of the alloyed galvanized steel sheet over time.

If B is less than 0.001 wt%, its effect cannot be fully exhibited, and if B exceeds 0.01 wt%, it is physically difficult to dissolve it sufficiently in the plating bath, resulting in dross. It is not practical because it re-adheres to the steel plate. Therefore,
The B concentration may be 0.001 to 0.01 wt%, but
Preferably it is 0.003 to 0.008 wt%.

6) Total amount of unavoidable impurities In the present invention, unavoidable impurities include Pb, Cd,
It is an element that should be excluded from the system as much as possible because it forms a local battery with Zn, which is a basic component of the plating layer, resulting in a decrease in corrosion resistance.

Therefore, the total amount of impurities is preferably 0.02 wt%, preferably 0.01 wt% or less.

(2) Regarding the plating thickness of the hot-melt galvanized steel sheet, the plating thickness is basically a factor that controls the corrosion resistance of the hot-melt galvanized steel sheet.

If the plating thickness is less than 5 μm, the corrosion resistance after painting, which is the most important characteristic of alloyed plain sheets, will be extremely reduced, and if it exceeds 30 μm, there will be no problem with workability, but the film will be too thick and will require alloying treatment. λ is not preferred because it takes time and reduces productivity.

Therefore, the appropriate plating thickness is preferably 5 to 30 μm, but preferably 7 to 15 μm is practical.

The effects of the present invention will be explained in more detail below based on Examples.

[Example] A low carbon steel plate with the composition specified in Table 1 with a plate thickness of 0.6
Cold rolled steel plate with a plate width of 914 mm or a plate thickness of 3.5 m
A dehydrated scaled hot-rolled steel sheet with a width of 1200 mm was first degreased with alkaline, washed with water, and dried, then subjected to the blemishing specified in Table 1 and immediately placed in a 15% H2 + N2 mixed gas atmosphere in a Sendzimer hot-dip plating line. The plate is heated so that the maximum plate temperature before hot-dip coating becomes the maximum plate temperature specified in Table 1, and the plate temperature is 480℃ as the hot-dip plated penetration plate temperature.
After cooling to 0.degree. C., it is immersed for 2 seconds in a hot-dip galvanizing bath specified in Table 1 with a bath temperature of 460 t. After that, it is gas wiped in the atmosphere to control the predetermined coating amount, and then heated and diffused in an alloying furnace so that the highest plate temperature on the outlet side reaches 550°C, cooled with air and water, and then water-quenched and dried.

As shown in Table 1, the hot-melt galvanized steel sheet of the present invention produced in this way exhibits excellent workability without impeding other properties, and is an unprecedented and innovative product. This is the manufacturing method.

(2) Effect of Si in Steel Examples of the present invention in Table 1 are shown in Nos. 1 to 16, along with comparative examples Nos. 17 to 18. Of these, no
, the state of alloy layer formation in the cross section of 10 examples of the present invention is SE
M observation, and the first EPMA element distribution at that time.
As shown in the figure. Further, as a comparative example, the results of similarly analyzing No. f7 are shown in the $2 chart.

As is clear from these results, F depends on the Si content in the steel.
The e-Zn alloy layer morphology changes from hierarchical to random,
In addition, it can be seen that the phase form changes to a form in which the r phase, which is brittle during processing, is suppressed and the ζ phase and 61 phase are mixed together. The reason for this alloy layer morphology is that the Si segregation zone on the δ1 phase side of the interface between the base metal and the δ phase and the ζ phase is clearly not affected by the EPM octane analysis. The reason why the alloy layer is mixed is because the diffusion of Fe supplied from the base iron or the δ1 phase toward the ζ phase is suppressed.
This is thought to be due to the amount of segregation.

In this way, Si in steel creates an alloy layer (“phase”) that is brittle to process.
disappears, and the ζ phase and 61 phase, which have different hardnesses, are mixed together, making it possible to disperse processing stress, which is thought to have led to overall improvement in the workability of the alloyed plate. It will be done.

■ Effect of maximum heating plate temperature before flaking Example of the present invention is No. 19 to No.
, 26 to No. 27. As is clear from this, a low sheet temperature is not very preferable because the surface of the steel sheet is in an unreduced state, and this may result in discoloration of the finished appearance of alloying or because of the layered alloy layer, resulting in a decrease in processing adhesion. on the other hand,
If the plate temperature is too high, the effect on workability is small, but Fe
There is a decrease in overall corrosion resistance, probably due to excessive diffusion into the plating layer.

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

■Effects of each component of the hot-dip galvanizing bath 1) AjZ and sb are one of the basic plating bath components in the present invention.

Regarding the effect of No. 81, the present invention example No., 28 to No. 33, comparative example No. 34 to No. 3,
5, and examples of the present invention regarding sb are shown in Nos. 36 to 36.
No. 42 is shown together with comparative examples No. 43 to No. 44.

If the concentration of any component system is outside the range of the present invention, processability and finished appearance will be impaired.

2) Regarding the effectiveness of other additive components Mg, Ti, and B, the present invention example of Mg is No. 45 to No. 5.
Comparative Example No. 1, No. 52 to No. 53, and No. 53 to Example of Ti according to the present invention, No. 54 No. Comparative Example No. 56, Comparative Example No.
, 57-No., 58. Regarding B, examples of the present invention are No. 59 to No. 62, comparative examples No. 63 to No. 62.
No. 64.

As is clear from this, the purpose of each of these components is mainly to improve the overall corrosion resistance of the alloyed plate and to improve the fatigue fracture resistance that occurs with corrosion, and if it is outside the scope of the present invention, it is not expected. thin.

3) Also, regarding the appropriate range of unavoidable impurities such as PB, the present invention example No. 61 and No. 6
5 together with Comparative Example No. 66.

As is clear from this, these impurities mainly cause a decrease in corrosion resistance (therefore, in the present invention, care must be taken to exclude them from the plating bath system as much as possible.

4) Appropriate coating weight range for alloyed plain plate The coating weight range referred to in the present invention should basically be determined depending on the usage environment and cost, but the coating weight should be determined from the overall performance level. , with restrictions.

Regarding the coating amount range, Examples of the present invention are No. 87 to 87.
No. 70 is shown together with comparative examples No. 71 to No. 72.

As is clear from this, if the coating amount is outside the appropriate coating amount of the present invention, corrosion resistance, workability, etc. will be impaired and this is not practical.

[Effects of the Invention] As mentioned above, the contents of the present invention have been explained in detail based on the examples.The present invention focuses on Si among the steel sheet components, and the modification of the alloy layer morphology by this is effective in producing an alloyed plate sheet. This is presented here as an unprecedented, groundbreaking technology that has significantly improved the processability of.

[Brief explanation of drawings]

FIG. 1(a) shows N of Table 1 as a representative example of the embodiment of the present invention.
A micrograph showing the plating cross-sectional structure of the hot-fusion galvanized steel sheet described in 3.o and 10, and (b) is a pattern diagram obtained by EPMA line analysis of the element distribution state of the cross section of the galvanized mackerel. Furthermore, (cl) is a schematic diagram showing an image of alloy layer formation in the cross section based on the results of (a) and (b). Similarly, Fig. 2 (a) is a comparative representative diagram of the conventional technology. As an example, it is a micrograph showing the plating cross-sectional structure of the galvanized steel sheet No. 17 in Table 1, and (b) is a pattern diagram obtained by EPMA line analysis of the element distribution state of the plating cross section. (C) is a schematic diagram showing an image of alloy layer formation in the cross section based on the results of (a) and (b).・ Iki x 1ρO0 (b)

Claims (1)

[Scope of Claims] 1. An Fe-Zn alloy layer formed by heating diffusion after hot-dip galvanizing on the upper layer of a low carbon steel sheet whose steel composition is Si: 0.05 to 1.0% by weight. The structure is a δ_1-dominant type with a mixture of ζ phase and δ_1 phase, and the δ_
A melt-alloyed galvanized steel sheet with excellent workability, characterized in that the formation of one phase is discontinuous with respect to the steel sheet base, and the alloy layer is formed in a thickness of 5 to 30 μm. 2 The steel composition of the low carbon steel plate is C: 0.05 to 0.05% by weight.
10%, Si: 0.05~1.0%, Mn: 0.40~
0.60%, P: 0.020% or less, S: 0.030%
The melt-alloyed galvanized steel sheet with excellent workability according to claim 1, wherein the total Al content is 0.010% or less. 3. In a method of manufacturing a hot-dip galvanized steel sheet by subjecting a low-carbon steel sheet to hot-dip galvanizing and heating and diffusing the steel sheet, the Si content of the low-carbon steel sheet is set to 0.05% by weight.
~1.0%, the temperature of the gas heating reduction plate in the hot dip galvanizing process is 500 to 900°C, and the components of the plating bath are A in weight%.
l: 0.01-0.15%, Sb: 0.05-0.5%
, Mg: 0.01 to 0.2%, Ti: 0.0% as necessary.
Plating is performed using a plating bath to which B: 01 to 0.05% and B: 0.001 to 0.01% are added, and the total amount of unavoidable impurities such as Pb is less than 0.02%. A method for producing hot-fusion galvanized steel sheets with excellent workability.