JPH05279829A - Production of high tensile strength galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To stably produce a galvannealed steel sheet excellent in film characteristics without requiring particular conditions by using a high Si added steel sheet with high tensile strength as a material to be plated. CONSTITUTION:At the time of producing a galvannealed steel sheet by using a high tensile strength steel sheet with 0.1-1.5wt.% Si content as a material to be plated and applying preliminary iron plating to the steel sheet (it is preferable that, in the case of hot rolled steel plate, preliminary iron plating is done after surface grinding) prior to the production of the galvannealed steel sheet, the coating weight M (g/cm<2>) per side of preliminary iron plating is regulated so that it satisfies the relations in 0.05X[Si]XW<=W<=M-<=0.10X[Si]XW with respect to the coating weight W(g/cm<2>) of hot dipping and the content of [Si](wt.%) in the steel, or further, hot-dipping is done by using a hot-dip galvanizing bath where effective Al concentration (wt.%) is regulated to 0.07-0.16wt.%. Then alloying treatment is performed under the conditions where alloying temp. T('C) and alloying time (t)(sec) satisfy the relations in 370+1000XC<=T<=460+1000XC and 0.08XW'<=t<=0.5XW' [where W'=(coating weight of both sides)/2].

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention uses a high Si-added high-strength steel sheet, which is extremely difficult to hot-dip galvanize or alloy, to obtain a plating film performance (plating adhesion, coating adhesion, etc.) The present invention relates to a method for stably producing an excellent high-tensile-alloyed hot-dip galvanized steel sheet without using specific conditions.

[0002]

[Prior art and its problems] "Alloyed hot dip galvanized steel sheet" in which an alloying heat treatment is performed after hot dip galvanizing to make a Fe alloy of the plating film is noted for its excellent corrosion resistance, weldability, and coating adhesion. In recent years, it has come to be used in a wide range of fields such as automobiles, building materials and home appliances.

On the other hand, looking at the recent trends in the demand field for steel sheet materials, due to environmental problems and the like, there has been a further progress toward weight reduction of products (vehicle bodies, etc.), especially in the automobile-related fields. There is an increasing demand for high-strength steel sheets that can secure higher strength than products. Among the several high-strength steel sheets, "Si-added steel sheet", which is said to be easy to secure strength, attracts attention for its relatively good formability and low manufacturing cost, and is regarded as a very promising material satisfying the above requirements. Came.

Therefore, from the viewpoint of "a lightweight corrosion-promoting material having high corrosion resistance", the demand for alloyed hot-dip galvanized steel sheet using the above-mentioned high Si-added steel as a base material will naturally increase from now on. is expected. However, Si-added steel is easily oxidized when it is subjected to hot dip plating.
Therefore, there is a problem that the plating is easily repelled, and it is very difficult to perform the plating under the same hot dip plating conditions as those for general ordinary steel materials.

Therefore, in order to solve the above-mentioned problems, when the steel sheet is annealed, the surface is heated and oxidized in a strong oxidizing atmosphere and then further reduced to increase the amount of reduced iron on the surface as much as possible, and to melt molten zinc. The method of ensuring the wettability with was sometimes adopted. However, it has been pointed out that this method causes a phenomenon in which reduced iron wraps around a conveyor roll (hearth roll) in a processing furnace, which causes a flaw on the surface of the steel sheet to be plated.

Separately from this, in order to solve the above-mentioned problems and to carry out desired plating even under normal plating conditions (plating conditions for ordinary steel), the surface of the steel sheet to be plated is preliminarily Fe. Alternatively, a proposal has been made to secure wettability with molten zinc by plating Ni (Japanese Patent Laid-Open No. 57-70268 and Japanese Patent Laid-Open No. 2-19415).
No. 6). However, even the above-mentioned proposed method has not always been a sufficient solution to the problem for a high Si-added high-strength steel sheet in which the Si addition amount is 0.1 to 1.5 wt%.

Incidentally, in the production of an alloyed hot-dip galvanized steel sheet using Si-added steel as a material to be plated, Si in the steel has a problem that the wettability of the coating is deteriorated and the alloying rate of the zinc coating adhered is slowed. There is also a "problem during alloying process". That is, the alloying rate significantly changes depending on the Si content in the steel, and when the Si content in the steel is very large, no matter how strict the alloying conditions are (increasing the alloying temperature or increasing the alloying time). (Even though) it may not alloy at all.
Also, even if the Si content in the steel is low to some extent, alloying does not occur unless the alloying temperature is made considerably higher than that in the case of ordinary steel, so chances may be limited depending on the steel type and Si content. It became necessary and made continuous operation extremely difficult. Further, it is advocated that "Fe plating is performed before the hot dip galvanizing treatment" in order to deal with such a problem, not from the viewpoint of wettability (JP-A-2-156056). , Still high Si
It was not a sufficient method for the added high-strength steel sheet.

From the above, the object of the present invention is to obtain a plating film even when a very high Si-added high-strength steel sheet having a Si content in the steel of 0.1 to 1.5 wt% is used as the material to be plated. It was to establish a means for stably producing a high-strength galvannealed steel sheet with good performance under conditions that are not different from general conditions for galvannealing treatment, without restrictions on production opportunities.

[0009]

As a result of intensive studies to achieve the above object, the present inventor was able to obtain the following findings. (a) In hot dip galvanizing of a steel sheet containing Si, which is an easily oxidizable element, although pre-plating the surface with Fe was an effective means for ensuring plating wettability, When the galvanized steel sheet plated by taking such a measure is alloyed, the desired plating film performance (plating adhesion, etc.) may not be obtained.
The above problem can be stably solved by adjusting the basis weight of Fe plating to a specific value corresponding to the basis weight of hot dip galvanizing and the Si content in the steel sheet to be plated.

(B) Furthermore, the amount of Al in the bath added for suppressing alloying during the hot dip galvanizing process is controlled within an appropriate range, and "the amount of Al in the bath is commensurate with that in the alloying treatment process". By selecting "a specific alloying temperature" and "a specific alloying time corresponding to the weight of hot dip galvanizing", the production stability of the galvannealed steel sheet can be further improved.

(C) Further, when the material to be plated is a hot rolled steel sheet, if the surface of the steel sheet is ground before the preliminary Fe plating, the alloying speed during the alloying treatment is sufficiently improved, To further improve the manufacturability of galvannealed steel sheet.

The present invention has been completed on the basis of the above-mentioned findings and the like. "Alloying melting of a high-strength steel sheet having a Si content rate [Si] in the steel of 0.1 to 1.5 wt% as a material to be plated In the method for producing a galvanized steel sheet, when the preliminary Fe plating is performed and then the hot dip galvanizing and alloying treatment is performed, the basis weight M (g / cm ² ) per side of the preliminary Fe plating is formed on the surface. Satisfies the relationship of 0.05 x [Si] x W ≤ M ≤ 0.10 x [Si] x W with respect to the zinc coating weight W (g / cm ² ) and the Si content rate [Si] (wt%) in steel Or in addition to this, hot dip galvanizing is performed using a hot dip galvanizing bath in which the Al concentration is adjusted to 0.07 to 0.16 wt% with an effective Al concentration C (wt%), and the alloying temperature is T (° C) and alloying time t (se
c) is 370 + 1000 × C ≤ T ≤ 460 + 1000, respectively
× C, 0.08 × W '≤ t ≤ 0.5 × W' (W '= double-sided areal weight / 2) By alloying under the conditions, a high-strength alloy with excellent plating film performance can be obtained. It has a major feature in that it enables stable production of hot-dip galvanized steel sheets. Furthermore, "the material to be plated is a hot-rolled high-tensile steel sheet with Si content [Si] in the steel: 0.1 to 1.5 wt%. In this case, the high-strength hot-dip galvanized steel sheet having excellent plating film performance can also be obtained by performing surface grinding treatment after pickling descaling and then treating under any of the above conditions or both conditions. It is also characterized in that it enables stable production.

In the present invention, in the steel of the steel sheet to be plated
The Si content [Si] is limited to 0.1 to 1.5 wt% because it is pointed out that the Si-containing high-strength steel sheets with [Si] concentration of 0.1 to 1.5 wt% have the above-mentioned problems. This is because the present invention aims to solve the problem.

[0014]

As described above, according to the present invention, the basis weight of the preliminary Fe plating, which also ensures the wettability, is applied to the steel sheet to be plated having a very high Si content [Si] of 0.1 to 1.5 wt%. Amount "target hot-dip coating weight" and "Si content in steel sheet"
It is a major feature that it is specified in relation to.

That is, in the past, the preliminary Fe plating was a treatment mainly performed only to secure the wettability with respect to molten zinc, but when the preliminary Fe plating is performed according to this conventional standard, the hot dip galvanizing treatment is possible. Even if it becomes, depending on the amount of Si in the steel sheet, the alloying rate during the alloying treatment may be slow and the desired alloying may not be achieved. As described above, the reason why the alloying rate varies depending on the Si content in the steel sheet is that "reduction in hot dip coating, concentration of Si on the steel sheet surface during the annealing step" that is usually found is due to the Si content in the steel sheet. It is considered that the higher the height, the more prominent it is. Because this surface-enriched Si acts to prevent the diffusion of Fe from the steel sheet to be plated into the hot-dip galvanized film during the alloying treatment, and the higher the amount of surface-enriched Si, the more alloyed the plated film becomes. This is because processing becomes difficult.

[0016] Depending on the amount of Si in the steel sheet,
Although some Fe diffusion can be expected, basically, "Since the optimum amount of Fe in the galvanized film is achieved as an alloyed hot-dip galvanized sheet, the amount of Si in the steel sheet that influences the degree of Fe diffusion is taken into consideration. Preliminarily securing the "Fe source" in the form of "preliminary Fe plating" on the steel sheet to be plated is essential for stable production of high Si-added high-strength steel sheet with an alloyed hot-dip galvanized coating of desired performance. That is the reason why the present invention was achieved.

At that time, the preliminary Fe plating basis weight M per one surface
Is less than [0.05 × [Si] × W] (g / cm ² ), the alloying treatment of the plating film becomes insufficient, and "repelling" tends to occur during hot dipping, which is not preferable. On the other hand, the preliminary Fe plating weight M per side is [0.10 x [Si] x W] (g
/ cm ² ), the workability at the interface between the “plating film” and the “steel plate” during alloying is poor and Zn-Fe with a high Fe content is present.
Since the alloy layer is formed thick, the workability of the obtained galvannealed steel sheet deteriorates, and there is a possibility that a plating film peeling phenomenon may occur at the interface between the plating and the steel sheet during processing.
In addition, it is not preferable to form the preliminary Fe plating layer to be thick, because the cost is increased and it is economical.

By the way, in the production of the alloyed hot-dip galvanized steel sheet including the step of performing preliminary Fe plating under the above conditions,
The hot dip galvanizing bath used has an Al concentration of effective Al concentration C in the bath.
It is preferably adjusted to 0.07 to 0.16 wt%. This is because when the effective Al concentration is lower than 0.07%, the formation of the Fe-Al-Zn phase, which serves as an alloying barrier in the initial stage of plating, is insufficient and alloying proceeds too much during the plating process. At times, the alloying degree of the plating film exceeds the proper range, and there is a concern that only alloyed hot-dip galvanized steel sheets having poor plating film adhesion during processing can be obtained. On the other hand, the effective Al concentration is 0.
When it is higher than 16%, on the contrary, the Fe-Al-Zn phase is strongly formed and the plating adhesion is improved, but the alloying rate during alloying treatment becomes very slow and There is a concern that it will become a problem.

Further, in this case, since the alloying temperature and the holding time during the alloying treatment have a considerable influence on the plating film performance and productivity, it is preferable to appropriately adjust them. Among these, the alloying temperature is preferably changed according to the Al concentration in the bath because the alloying start temperature during the alloying treatment changes depending on the Al concentration in the hot dip galvanizing bath. ℃), and the effective Al concentration in the bath is C (wt%): 370 + 1000 × C ≤ T ≤ 460 + 1000
It is optimal to adjust to the range of xC.

Here, the alloying temperature T is [370 + 100
If it is lower than 0 × C] (° C.), alloying does not proceed, or the progress of alloying is insufficient, and unalloying tends to occur, which is not preferable. On the other hand, T is “460 + 1000
If it is higher than × C ”(° C.), alloying tends to proceed excessively and the degree of appropriate alloying tends to be exceeded, which tends to cause a problem that the plating adhesion is lowered during processing, which is not preferable.

Further, the proper alloying time is related to the coating weight amount formed during the hot dip galvanizing process.
For example, when the basis weight of hot-dip galvanizing is large, it takes time for alloying to reach the plating surface, and therefore, it is necessary to take a long alloying holding time. Taking these points into consideration, the alloying time t (sec) is 0.08 × when half the double-sided basis weight of hot dip galvanizing, that is, the average basis-weight formed per side is W '(g / cm ² ). It can be said that the optimum range is W'≤t≤0.5 × W '.

That is, the alloying time t is [0.08 × W '] (s
If it is shorter than ec), there is a concern of "unalloying untreated", in which alloying does not proceed to the surface of the plating film.
If it is longer than 0.5 × W ′] (sec), alloying will proceed excessively and the degree of alloying will exceed the appropriate level, which may cause a problem that the plating adhesion is lowered during processing.

In recent years, in order to reduce the weight of automobile bodies, for example, high-strength steel sheets are increasingly used for hot-rolled steel sheet-applied members. There are increasing opportunities for alloying hot-dip galvanizing to improve rust resistance. The hot-dip galvanizing treatment of the hot-rolled steel sheet is usually performed after pickling and descaling. However, in the case of a high Si-added high-strength hot-rolled steel sheet, the alloying progress during the alloying treatment is much slower than in the case of performing the alloying hot-dip galvanizing treatment on a cold-rolled material having a similar Si content. Even if the preliminary Fe plating is performed under the above-mentioned conditions and then the alloying hot dipping treatment is performed, alloying tends to be insufficient.

The reason for this is not clear, but probably in the steel sheet.
Even if the Si amount is the same, it is considered that the hot-rolled steel plate and the cold-rolled steel plate have different amounts of Si concentrated on the surface, and the hot-rolled steel has a higher concentration of Si. Therefore,
In hot-rolled materials, it is recommended to grind and remove Si concentrated on the surface before hot dip galvanizing.

However, it is very difficult to specify the grinding amount because the surface Si concentrated layer is not constant and varies depending on the manufacturing conditions, but the grinding amount is generally about 0.2 to 0.6 μm. Can be said to be preferable. In other words, if the grinding amount is less than 0.2μ, the effect of grinding may be insufficient and unalloying may occur, especially for hot-rolled steel sheet with a high Si content in the steel, while if the grinding amount is more than 6.0μ. Not only is it economically disadvantageous to do so because it puts too much load on the grinding, but such strong grinding is also not preferable from the aspect of appearance because grinding marks are left on the plated surface after the galvannealing treatment. Because. As a grinding method, mechanically grinding with a brush containing abrasive grains is the most suitable from the viewpoint of productivity and economy.

Next, the effects of the present invention will be described more specifically by way of examples.

EXAMPLE A cold-rolled steel sheet and a pickling descaling hot-rolled steel sheet having each chemical composition and plate thickness as shown in Table 1 were prepared and used as a base material to be plated to produce an alloyed hot-dip galvanized steel sheet. The test was conducted.

[0027]

In the test, the hot-rolled steel sheet was subjected to surface grinding with a resin brush containing abrasive grains (grinding amount: 0).
~ 7.0μ).

Now, for each base material to be plated, first degreasing,
Electrolytic Fe plating was performed under the following plating bath and plating conditions. The amount of Fe plating deposited was changed by controlling the electrolysis time. [Fe plating bath] Ferrous sulfate: 350 g / L (liter), soda sulfate: 75 g / L. [Fe plating conditions] Liquid pH: 1.8, liquid temperature: 50 ° C, current density: 40 A / cm ² .

Next, the Fe-plated base material was subjected to "25%
H ₂ -75% N ₂ atmosphere ”, reduction annealing at 800 ° C. for 40 seconds, and then hot dip galvanizing bath (bath effective depending on test type
Immersion in Al) for 2 seconds, followed by gas wiping to control the amount of zinc plating deposited was immediately subjected to alloying treatment under various conditions.

With respect to the alloyed hot-dip galvanized steel sheet thus produced, the appearance was first evaluated, and then it was investigated whether or not the alloying had proceeded to the plated surface.
Further, the plating adhesion (powdering property) was evaluated as the performance of the plating film, and the plating adhesion was evaluated as follows. That is, after performing a drop bead test under the following conditions, the tape was peeled off the bead side, and the plating adhesion was evaluated by the blackened state of the tape. [Bead test conditions] Bead shape: triangular bead (tip R = 0.5 mm), indentation depth: 5 mm, pressing load: 500 kgf, drawing speed: 200 mm / min, lubrication: anti-rust oil coating.

The results are shown in Table 2 together with the plating conditions. In Table 2, the evaluation results of the appearance of the finished plating are indicated by ◯: alloying progresses uniformly up to the surface, Δ: untreated or partially alloying progresses, ×: no alloying progresses at all (passed Level is "○" or higher).

Further, the evaluation results of the plating adhesion are indicated by ○: almost no black discoloration, △: slightly black discoloration, ×: all black discoloration (pass level is “△” or higher).

[0034]

[Table 2]

From the results shown in Table 2, it can be seen that the basis weight M (g / cm ² ) per surface of the preliminary Fe plating is W (g / cm ² ) of the hot dip galvanization formed on that surface and the A good hot-dip galvanized appearance can be obtained only when 0.05 x [Si] x W ≤ M ≤ 0.10 x [Si] x W is satisfied in relation to the Si content rate [Si] (wt%).
It can be seen that plating adhesion (powdering property) can be obtained.

FIG. 1 is a graph showing the relationship between the Al concentration C in the hot dip galvanizing bath and the alloying temperature T as seen from the appearance of the plating. 37
It is understood that when the relation of “0 + 1000C” is satisfied, a good plating appearance can be stably obtained.

FIG. 2 is a graph showing the relationship between the Al concentration C in the hot-dip galvanizing bath and the alloying temperature T in terms of plating adhesion (powdering property). From the above, it is understood that good plating adhesion can be stably ensured if the relationship of “T ≦ 460 + 1000 C” is satisfied. Therefore, it is appropriate to adjust the alloying temperature T in the range of 370 + 1000C ≤ T ≤ 460 + 1000C.

Further, FIG. 3 is a graph showing the relationship between the hot-dip galvanized area weight W ′ per one side and the alloying time t as seen from the appearance of plating. In order to obtain a stable plating appearance, “t ≧ 0.08
It is understood that it is preferable that the relationship of W '"is satisfied.
On the other hand, FIG. 4 is a graph showing the relationship between the hot-dip galvanized area weight W ′ and the alloying time t as seen from the plating adhesion (powdering property). To ensure stable plating adhesion, "t≤0.5
It is understood that it is preferable that the relationship of W '"is satisfied.
Therefore, it is appropriate to adjust the alloying time t within the range of 0.08 W'≤t ≤0.5 W '.

By the way, FIG. 5 shows the relationship between the amount of surface grinding and the plating appearance when a hot-rolled steel sheet is used as the base material to be plated. It can be seen from FIG. 5 that the surface of the hot-rolled steel sheet has a good plating appearance by grinding, but the grinding amount is preferably 0.2 μm or more. However,
If the grinding amount exceeds 6.0 μ, grinding marks will remain on the plating appearance, so the surface grinding amount of hot rolled steel sheet is 0.2
It can be said that a value of up to 6.0 μ is preferable.

[0040]

[Summary of Effects] As described above, according to the present invention, even when a high Si-added high-strength steel sheet, which has been very difficult to manufacture an alloyed hot-dip galvanized steel sheet, is used as a material to be plated,
Industrially extremely useful effects are brought about, such as the continuous production of stable galvannealed steel sheets that is stable under operating conditions that do not change so much as compared with general materials.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the appearance of plating, the Al concentration in a hot dip galvanizing bath, and the alloying temperature.

FIG. 2 is a graph showing the relationship between plating adhesion, Al concentration in hot dip galvanizing bath, and alloying temperature.

FIG. 3 is a graph summarizing the relationship between the appearance of plating, the coating weight of hot dip galvanizing on one side, and the alloying time.

FIG. 4 is a graph showing the relationship between plating adhesion, hot dip galvanization areal weight, and alloying time.

FIG. 5 is a graph showing the relationship between the amount of surface grinding and the appearance of plating when a hot-rolled steel sheet is used as a base material to be plated.

Claims (3)

[Claims] 1. A method for producing an alloyed hot-dip galvanized steel sheet using a high-strength steel sheet having a Si content [Si] in the steel of 0.1 to 1.5 wt% as a material to be plated. When performing the alloying treatment, the basis weight M (g / cm ² ) per side of the preliminary Fe plating, the basis weight W (g / cm ² ) of the hot dip galvanization formed on the surface and the Si content in the steel Rate [Si]
A method for producing a high-strength hot-dip galvanized steel sheet, which comprises adjusting to satisfy a relation of 0.05 × [Si] × W ≦ M ≦ 0.10 × [Si] × W with respect to (wt%). 2. The effective Al concentration C (wt%) is 0.07-0.
After hot-dip galvanizing in a 16 wt% hot dip galvanizing bath, the alloying temperature T (° C.) and alloying time t (sec) are 370 + 1000 × C ≦ T ≦ 460 + 1000, respectively.
2. The high-strength alloyed molten zinc according to claim 1, wherein the alloying treatment is carried out under the condition that the relationship of xC, 0.08xW '≤ t ≤ 0.5 x W' (W '= double-sided areal weight / 2) is satisfied. Manufacturing method of plated steel sheet. 3. The method for producing a high-strength hot-dip galvanized steel sheet according to claim 1, wherein the material to be plated is a hot-rolled high-strength steel sheet which has been surface-ground after pickling and descaling.