JPH06136503A - Production of galvannealed steel sheet - Google Patents

Abstract

PURPOSE:To attain the improvement of the plating adhesion and formability of a galvannealed steel sheet by introducing solid solution carbon into the surface of a steel sheet having a specified compsn. and regulating the Al concn. in a plating bath. CONSTITUTION:The compsn. of the steel is constituted of a one contg., by weight <=0.01% C, <=0.2% Si, 0.05 to 1.0% Mn, <=0.1% P, <=0.02% S, 0.005 to 0.08% solAl, <=0.006% N and either or both of Ti and Nb so that the effective Ti content, C content and Nb content expressed by the formula I can satisfy the inequality II, and the balance Fe with inevitable impurities. The call rolled steel sheet of the steel having the same compsn. is annealed in a carburizing atmosphere, and at the time of defining the solid solution carbon content (ppm) from the surface of the steel sheet at 30mum in the thickness direction as A, and the Al concn. in a galvanizing bath as B, 1<AB<100 is satisfied. In this way, the galvannealed steel sheet excellent in plating adhesion and formability can be obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a galvannealed steel sheet having excellent galling adhesion.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets are widely used to enhance the rust preventive power of automobile bodies because they have excellent corrosion resistance, as well as excellent paintability and weldability. In order to improve fuel economy of automobiles, further improvement in moldability and higher strength are demanded. Among them, for improving the formability, an ultra low carbon steel plate is used, and IF (Interstitial Free) steel obtained by adding Ti, Nb or the like is widely used as a base plate. That is, solid solution carbon harmful to formability is fixed as a carbide of Ti or Nb to improve formability.

However, when the solid solution carbon disappears, the crystal grain boundaries of the base iron are purified, and as a result, the reactivity with the molten zinc during the plating of the molten zinc becomes extremely high, and the Zn--Fe alloy layer develops in this portion. However, a so-called outburst structure is locally formed. This outburst structure promotes the growth of the hard and brittle Γ layer in the subsequent alloying heating, and reduces the plating adhesion.

Therefore, in order to solve such a problem, T
Various studies have been conducted on the optimization of the concentration of addition of i or Nb alone, the optimization of the concentration balance of compound addition, and the addition of other elements, but this inevitably leads to a decrease in formability. .

[0005]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention As a result of intensive studies conducted by the present inventors to solve the above-mentioned problems in alloyed molten zinc plated steel sheets, solid solution carbon is introduced to the surface of the steel sheet before plating. As a result, the present inventors have found that the above outburst structure is suppressed, and as a result, an alloyed molten zinc plated steel sheet having excellent plating adhesion and formability can be obtained, leading to the present invention. It is a thing.

That is, it is an object of the present invention to provide a method for producing a galvannealed steel sheet having excellent alloy adhesion and formability.

[0007]

The method for producing a galvannealed steel sheet according to the present invention is a method for galvannealing a cold-rolled steel sheet and then performing galvanizing and alloying treatment on the galvannealed steel sheet. In the manufacturing method of
0.01% or less, Si 0.2% or less, Mn 0.05-1.
0%, P 0.1% or less, S 0.02% or less, sol A
1 0.005 to 0.08%, and N 0.006% or less, and further contains one or both of Ti and Nb, and the following formula Ti ^* = total Ti-{(48/32) S + (48/14)
The effective Ti amount, the C amount, and the Nb amount represented by N} are expressed by the following formula 1 <(Ti ^* / 48 + Nb / 93) / (C / 12) <4.
5, cold-rolled steel sheet consisting of balance iron and unavoidable impurities was annealed in a carburizing atmosphere, and the amount of solid solution carbon up to 30 μm in the thickness direction from the surface of the steel sheet (pp
m) and B are Al concentrations (%) in the molten zinc plating bath, the amount of solute carbon up to 30 μm in the thickness direction from the surface of the steel sheet is 1 <AB <100.

Further, according to the present invention, the cold-rolled steel sheet used may contain B in the range of 0.003% or less in addition to the above elements. Generally, in continuous molten zinc plating, a small amount of Al is added to the plating bath in order to improve the plating adhesion of non-alloyed zinc plated steel sheet which is not alloyed. That is, Al is added to the plating bath to preferentially form the Fe—Al alloy layer at the interface between the plating layer and the base metal during plating, thereby suppressing the formation of a hard Fe—Zn alloy layer. is there. When an alloyed molten zinc plated steel sheet is manufactured using such a plating bath, Fe of the base iron and Zn of the plated layer are diffused through the Fe-Al alloy layer at the interface during the alloying treatment. As a result, alloying will proceed. Here, when IF steel is used as the base steel plate, since the crystal grain boundaries of the base iron are purified, Fe-Al
Zinc that has diffused through the alloy layer undergoes a vigorous alloying reaction at the grain boundaries to generate an outburst structure.

In order to reduce the alloying reactivity of zinc at the grain boundaries, the present inventors have previously introduced solid solution carbon to the surface of the steel sheet before plating, so that the outburst at the grain boundaries is reduced. It was found that the generation of tissue was suppressed. Furthermore, the present inventors also have an effect that the Al concentration in the plating bath influences the diffusion reaction of Fe and Zn through the Fe-Al alloy layer, and the higher the Al concentration in the plating bath, the more outbu.
It was found that the generation of rst tissue tends to be suppressed.

However, even if the Al concentration in the bath is increased, when IF steel is used as the original plate, it is difficult to suppress the formation of the outburst structure, and the alloying rate is significantly reduced. , Productivity is greatly reduced. As described above, in order to suppress the formation of the outburst structure, it is necessary to properly set the conditions of the concentration of solute carbon on the surface of the steel sheet and the concentration of Al in the bath. That is, A in the bath
When the l concentration is low, the concentration of solute carbon on the surface of the steel sheet is increased, and conversely, when the Al concentration in the bath is high, the concentration of solute carbon on the surface of the steel sheet is lowered to cause excessive carburization. Cost-up can be avoided.

The present inventors examined in detail the conditions of both the concentration of solute carbon on the surface of the steel sheet and the concentration of Al in the bath necessary for suppressing the formation of the outburst structure, and the results shown in FIG. Obtained. In FIG. 1, the horizontal axis represents A in the molten zinc plating bath.
l concentration (%), vertical axis is 30 μm in the thickness direction from the steel plate surface
The amount of solute carbon (ppm) up to, the o indicates that no outburst structure was generated, and the x indicates that an outburst structure was generated. From this result, it is necessary to satisfy the following conditions in order to suppress the generation of outburst tissue.

1 <AB where A is the amount of dissolved carbon (ppm) up to 30 μm in the thickness direction from the surface of the steel sheet, and B is Al in the molten zinc plating bath.
The concentration (%). In addition, due to the increase in the amount of solute carbon, out
From the economical point of view, AB <100 because the effect of suppressing the generation of burst tissue is only saturated even if AB is made too high. After all, according to the invention, 1 <AB <100.

Next, the chemical composition of the cold rolled steel sheet used in the present invention will be described. C needs to increase the amount of Ti and Nb that fix C as the amount of C increases, which leads to an increase in manufacturing cost.
The smaller the amount of C, the more preferable, and in the present invention, it is 0.0.
1% or less. If the added amount of Si is too large, the wettability between the steel plate and the molten zinc may be deteriorated and the plating may not be locally applied, so the content of Si is set to 0.2% or less.

Mn is an element added to prevent hot embrittlement, but if it is less than 0.05%, its effect cannot be effectively obtained, while 1.0% is added. If it exceeds the above range, the ductility is deteriorated.
~ 1.0%. P segregates at the grain boundaries and easily causes secondary work embrittlement of the steel sheet, so P is made 0.1% or less.

Since S combines with Ti to form TiS, an increase in the content thereof increases the amount of Ti necessary for fixing C and N. Therefore, in the present invention, S is 0.02% or less. Al is added for the purpose of deoxidizing molten steel, but when the content of sol Al is less than 0.005%, the above purpose is not achieved, and 0.08%
When it exceeds, the deoxidizing effect is saturated, so the Al content is 0.
005 to 0.08%.

Since N combines with Ti to form TiN, an increase in its content increases the amount of Ti necessary to fix N. Therefore, in the present invention,
N is 0.006% or less. Ti and Nb are both
C and N are fixed, which has the effect of increasing the formability of the steel sheet. Therefore, according to the present invention, the following formula Ti ^* = total Ti-{(48/32) S + (48/14)
The effective Ti amount, the C amount, and the Nb amount represented by N} are expressed by the following formula 1 <(Ti ^* / 48 + Nb / 93) / (C / 12) <4.
It is necessary to add Ti or Nb or both of them so as to satisfy 5. When the value of the above formula is 1 or less, C and N cannot be fixed sufficiently, and the formability is deteriorated. On the other hand, when the value of the above formula is 4.5 or more, the effect of improving the formability is saturated, and the solid solution Ti and N fix the C that has entered during the annealing in the atmosphere. Grain boundary segregation is less likely to occur, which is not preferable.

In the present invention, the cold rolled steel sheet used may contain B in addition to the above alloy elements.
B has the effect of supplementing the secondary working brittleness resistance of the steel sheet.
Even if the addition amount is excessive, the above effect is saturated, so the addition amount is set to 0.003% or less. In the method of the present invention, annealing is performed in a carburizing atmosphere in which the carbon potential is controlled by carbon monoxide gas or the like.

[0018]

The present invention will be described below with reference to examples.
The present invention is not limited to these examples. A cold-rolled steel sheet having the chemical composition shown in Steel No. 1 in Table 1 was annealed in a continuous zinc plating line under various conditions of carburizing atmosphere, and then plated with a plating bath having various Al concentrations. Then, two types of plated steel sheets, one that was cooled as it was and one that was subsequently alloyed, were manufactured. The effective Ti content of this steel sheet is 0.0344%.
Is. Also, (Ti ^* / 48 + Nb / 93) / (C /
The value of 12) is 4.09.

Of the above two types of plated steel plates, the former is
The latter was used to investigate the outburst texture of the plating layer, and the latter was used to measure the thickness of the Γ layer. The amount of deposit of the alloyed plated steel sheet was adjusted to 60 g / m ² , and the Fe concentration in the plated layer was adjusted to 12%. The thickness of the Γ layer depends on the strictness of the molding conditions, but in order to secure the plating adhesion,
Generally, it is necessary to suppress the thickness to about 0.5 μm or less. The obtained results are shown in Table 2. Incidentally, the plating surface was uniformly plated over the entire surface, and no local non-plating portion was observed. In addition, the elongation value of the obtained barbed steel sheet by the tensile test exceeded 40%, showing good ductility.

When the amount of solute carbon up to 30 μm in the thickness direction from the surface of the steel sheet and the Al concentration in the molten zinc plating bath satisfy the above formula, no outburst structure is observed in the as-plated state. In the alloyed steel sheet, the Γ layer thickness in the plating layer is suppressed to 0.5 μm or less. Next, cold-rolled steel sheets having the chemical compositions shown in Steel Nos. 2 and 3 in Table 1 were used in the same manner as in cold-rolled steel sheet 1 in Table 2 so that the Al concentration in the bath was 0.10% and the amount of solute carbon was 50 to 0 ppm. I'm stuck. With regard to the cold-rolled steel sheets from Steel No. 2, since the Si content in the steel was large, the plating surface was not completely plated, the non-plated portion was locally observed, and it was rejected as a product. Regarding the cold-rolled steel sheets from Steel No. 3, the elongation value, which is one of the mechanical properties, was a low value of 35 to 37% because of the large amount of Mn in the steel, and the ductility was insufficient.

The cold-rolled steel sheet from the addition of B shown in Steel No. 4 in Table 1 is cold-rolled steel sheet 1 as shown in Table 2 as cold-rolled steel sheet 4.
It has almost the same data as the results obtained in. Effective Ti amount is 0.0337%, (Ti ^* / 48 + Nb / 9
The value of 3) / (C / 12) is 4.21.

[0022]

[Table 1]

[0023]

[Table 2]

The amount of solute carbon from the surface of the steel sheet to 30 μm in the thickness direction was controlled by changing the carbon monoxide gas concentration in the carburizing atmosphere gas (5% hydrogen, balance nitrogen gas). That is, in heating at 800 ° C. for 30 seconds, the above carbon monoxide gas concentrations are 0.05 and 0.05, respectively, in order to make the solid solution carbon amount 5, 10, 20, 30, 40 and 50 ppm.
It was set to 1, 0.2, 0.3, 0.4 and 0.5%.

[0025]

As described above, according to the method of the present invention,
In a method for producing an alloyed molten zinc plated steel sheet, in which a Ti and / or Nb-added ultra-low carbon cold rolled steel sheet is annealed, and then hot-dip galvanizing and alloying treatment are performed, a predetermined amount of a predetermined amount is applied to the surface of the steel sheet before the plating. By introducing solute carbon and appropriately adjusting the Al concentration in the plating bath, it is possible to obtain a galvannealed steel sheet having excellent galling adhesion and formability. .

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the Al concentration in the plating bath and the amount of solute carbon up to 30 μm in the thickness direction from the surface of the steel sheet, where ○ indicates that no outburst structure was generated,
× indicates that outburst tissue was generated.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification number Internal reference number FI Technical display location // C21D 9/46 J (72) Inventor Mitsuru Kitamura 2222 Ikeda, Ikeda, Onoe-machi, Kakogawa-shi, Hyogo Address 1 Inside the Kobe Steel, Ltd. Steel Technology Research Laboratory (72) Inventor Tatsuya Asai 2222 Ikeda, Ikeda, Ikeda, Onoue-cho, Kakogawa City, Hyogo Prefecture Address 1 Inside the Kobe Steel Works Steel Technology Research Laboratory

Claims (2)

[Claims] 1. A method for producing an alloyed hot-dip galvanized steel sheet, which comprises subjecting a cold-rolled steel sheet to annealing and then hot-dip galvanizing and alloying treatment, in a weight percentage of C 0.01% or less, Si 0 0.2% or less, Mn 0.05 to 1.0%, P 0.1% or less, S 0.02% or less, sol Al 0.005 to 0.08%, and N 0.006% or less , And also contains one or both of Ti and Nb, and has the following formula: Ti ^* = total Ti − {(48/32) S + (48/14)
The effective Ti amount, the C amount, and the Nb amount represented by N} are expressed by the following formula 1 <(Ti ^* / 48 + Nb / 93) / (C / 12) <4.
5, cold-rolled steel sheet consisting of the balance iron and unavoidable impurities was annealed in a carburizing atmosphere, and A was dissolved carbon amount up to 30 μm from the surface of the steel sheet in the thickness direction (pp
m) and B, where Al is the Al concentration (%) in the molten zinc plating bath, the amount of solute carbon up to 30 μm in the thickness direction from the surface of the steel plate is 1 <AB <100. Method for producing steel plate with chemical-fused zinc. 2. A method for producing an alloyed molten zinc plated steel sheet, which comprises performing hot dip galvanizing and alloying treatment after annealing a cold rolled steel sheet, in a weight% of C 0.01% or less, Si 0 0.2% or less, Mn 0.05 to 1.0%, P 0.1% or less, S 0.02% or less, sol Al 0.005 to 0.08%, B 0.003% or less, and N 0. 0.006% or less, and either or both of Ti and Nb, and the following formula Ti ^* = total Ti-{(48/32) S + (48/14)
The effective Ti amount, the C amount, and the Nb amount represented by N} are expressed by the following formula 1 <(Ti ^* / 48 + Nb / 93) / (C / 12) <4.
5, cold-rolled steel sheet consisting of balance iron and unavoidable impurities was annealed in a carburizing atmosphere, and the amount of solid solution carbon up to 30 μm in the thickness direction from the surface of the steel sheet (pp
m), where B is the Al concentration (%) in the molten zinc plating bath, the amount of solute carbon up to 30 μm in the thickness direction from the surface of the steel plate is 1 <AB <100. Method for producing steel plate with chemical-fused zinc.