JPH08104925A - Method for producing high-strength hot-dip galvanized steel sheet with excellent plating properties - Google Patents

Abstract

(57) [Summary] [Purpose] To provide a method for producing a high-strength hot-dip galvanized steel sheet and a high-strength hot-dip galvanized steel sheet that do not cause non-plating during hot-dip galvanizing even if Si is contained at 0.1% or more. [Constitution] C: 0.1% or less, Si: 0.5% or less, Mn: 2.0
% Or less, P: 0.03 to 0.10% of the steel slab is hot-rolled, and then cold-rolled at a reduction ratio in the range of 40% or more and {-80 × Si (%) + 90}% or less, and 900 ° C. Below and 750 ℃ or more in continuous hot dip galvanizing equipment, {-2.5 × rolling reduction (%)
After recrystallization annealing in the temperature range of +1025} ° C or less, hot dip galvanizing and alloying treatment are performed.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-dip galvanized steel sheet having a weight of 45 kg / mm ² or more and a method for producing a high-strength galvannealed steel sheet mainly for automobile materials.

[0002]

2. Description of the Related Art Alloyed hot-dip galvanized steel sheets are widely used for automobile materials such as inner plates, outer plates and structural parts of automobile bodies because of their excellent corrosion resistance after painting.
There are various methods for producing alloyed hot-dip galvanized steel sheets,
Cost, in-line annealing method for production efficiency, that is, annealing is performed to combine the annealing for obtaining the material and the surface Fe oxide to perform annealing, and then hot dip galvanizing by dipping in a hot dip zinc bath, After that, pull up to control the amount of adhesion, and heat to alloy to perform galvanization.
The mainstream is Zn-Fe alloy plating.

On the other hand, high tensile strength of steel materials is required to reduce the weight of automobile bodies. In particular, there is a strong need for the improvement of fuel efficiency of automobiles due to CAFE (Global Environmental Issues). Therefore, high strength alloyed hot-dip galvanized steel sheet is required. High-strength hot-dip galvanized steel sheets are also in demand for automobiles, home appliances, building materials, etc. There are various methods for increasing the tensile strength, such as solid solution strengthening and microstructural strengthening. In any method, C, Si, Mn, P, Cr, Ni,
Although additional elements such as Mo, Ti, and Nb are required, it was found that Si of 0.1 wt% (hereinafter referred to as%) or more significantly impairs the plating property. In particular, when hot dip galvanizing a high-strength steel sheet containing 0.1% or more of Si and 1.0% or more of one or more of Mn, Cr, P, Mo, Cu and Ni in total,
Zinc plating did not cover the entire surface, and so-called non-plating occurred with the steel sheet material exposed.

As a countermeasure for non-plating, it is possible to appropriately control the bath temperature of the hot dip galvanizing and the Al concentration of the hot dip galvanizing bath, but even in that case, no non-plating occurs if Si is contained at 0.1% or more. It's not easy to do. In addition, it is not easy to change the bath temperature and the Al concentration under normal operating conditions, which is a major operational problem. As a countermeasure against this, Japanese Patent Application Laid-Open No. 55-122865 discloses a technology for preventing surface concentration of Si, Mn, etc. by high-temperature initial oxidation treatment in an atmosphere.
57-79160 discloses a technology for preventing surface concentration of Si, Mn, etc. by Fe-based pre-plating. However, it is difficult for the former to control the amount of oxidation, and for the latter, practically, a large amount of Fe is deposited, that is, usually 5 g / m ² or more, and Si is 0.5%.
If it exceeds, about 10 g / m ² is required, and both equipment cost and running cost are high.

Therefore, there is a need for an inexpensive and easy surface thickening preventing treatment technique which is an alternative to these methods without changing the conditions of the hot dip galvanizing bath.

[0006]

The non-plating during hot dip galvanizing is a fatal defect, and if the non-plating occurs even in a very small amount, it cannot be a product at all. The present invention makes Si 0.1
% Or more, it is an object to provide a method for producing a high-strength hot-dip galvanized steel sheet and a high-tensile-alloyed hot-dip galvanized steel sheet that have excellent plating properties without causing non-plating during hot-dip galvanizing. Is.

[0007]

According to the present invention, C: 0.1% or less, Si: 0.5% or less, Mn: 2.0% or less, P: 0.03 to 0.10.
% Of the steel billet is hot-rolled, and then cold-rolled at a reduction ratio represented by the following formula (1), at a hot dip galvanizing facility at 900 ° C or lower and by the following formula (2). A method for producing a high-strength hot-dip galvanized steel sheet having excellent plating properties, which comprises hot-dip galvanizing after annealing at a recrystallization annealing temperature represented, and the present invention is an all-radiant tube method for the recrystallization annealing. The present invention is a method for producing a high-strength hot-dip galvanized steel sheet having excellent plating properties, which comprises subjecting the high-strength hot-dip galvanized steel sheet obtained above to an alloying treatment.

Note 40% ≦ reduction ratio ≦ {−80 × Si (%) + 90}% ……… (1) 750 ° C ≦ recrystallization annealing temperature ≦ {−2.5 × reduction ratio (%) + 1025} ° C. (2)

[0009]

[Function] As a result of investigating the cause of non-plating of the above high-strength steel sheet, the present inventors have found that oxides such as SiO ₂ , MnO and Cr ₂ O _{3 on the} surface of the steel sheet, FeSiO 2 and MnSiO 2 in the case of compound addition It was reconfirmed that this is because the complex oxide of (3) is concentrated on the surface of the steel sheet and covers the steel sheet. In particular, Si was an element which surface-concentrated when added in an amount of 0.1% or more and particularly impairs the plating property.

[0010] Mn thickens the surface, though not so much as Si, and also hinders the plating property. It is said that the surface thickening reduces the wettability with molten zinc, resulting in non-plating, and the prevention of thickening is also in line with this idea. By the way, recrystallization annealing of cold-rolled steel sheet is usually performed with H of several%.
_It is performed in an atmosphere that is reducing for Fe, including ₂ . However, when Si, Mn, Cr, etc. are added to increase the tensile strength, these elements are easily oxidizable, and are oxidizable in a normal recrystallization annealing atmosphere. Therefore, the surface is selectively oxidized to form an oxide, and these components in the bulk of the steel diffuse toward the surface, so that a large amount of surface concentrated layer is formed.
This surface concentrated layer hinders the plating property.

Therefore, it is important to suppress this surface thickening, but this is a natural phenomenon thermodynamically, and as a conventional measure, at most, the amount of Si, Mn, and Cr added is restricted. It was However, the present inventors have found for the first time that the surface concentration changes and the platability changes as a result, even though the components in the steel are the same and the conditions after annealing are the same.

Then, as a result of diligent examination of the cause, it was found that the main cause was the cold rolling process after hot rolling, and the present invention was accomplished. By the way, until now, the main objectives have been to obtain the material (mechanical properties) of the components in the steel, the cold rolling reduction, the annealing temperature and other conditions. In that case, Si is added because the material is easy to obtain and the cost is low, and the amount increases to nearly 0.5%. In order to obtain a high r value, the cold rolling reduction rate becomes high and the annealing temperature also becomes high. As a result, although the material was obtained, the plating property was poor.

In the present invention, for the first time, it becomes possible to secure the plating property without using Fe-based pre-plating or initial oxidation treatment. Selective oxidation on the surface of Si, Mn, Cr, etc. in the ordinary recrystallization annealing is thermodynamically unavoidable. However, according to the study by the present inventors, the amount of surface enrichment can be reduced by adding the regulation of the cold rolling reduction as a function of the amount of Si in the steel together with the regulation of the content of the components in the steel. It has been found that the plateability can be improved.

The amount of Si is limited to 0.5% or less. This is because as the Si content of the original plate increases, the surface concentration after annealing increases and the plating properties deteriorate. However, merely limiting the Si amount is not sufficient, and it is necessary to limit the cold rolling reduction to {-80 × Si (%) + 90}% or less as a function of the Si amount.
The reason for this is that even if the amount of Si is the same, if the cold rolling reduction ratio is high, the amount of surface concentration increases and the plating property deteriorates.
That is, in order to increase the cold rolling reduction, it is necessary to reduce the amount of Si added.

Although the cause of the cold rolling reduction on the amount of surface thickening has not been determined, the strain is imparted to the steel by cold rolling and the strain does not completely disappear during annealing and at the time of completion of annealing. It is presumed that Si diffuses through this strain and concentrates on the surface. In other words, it is considered that diffusion through strain is overwhelmingly faster than normal diffusion without strain. Even during this annealing, the amount of strain remaining (number) is proportional to the cold rolling reduction, so the lower the cold rolling reduction, the same Si.
It is considered that the mechanism is such that the amount of surface thickening decreases even with the addition amount.

The lower limit of the cold rolling reduction is 40% or more. The reason is that if it is less than 40%, recrystallization during annealing is difficult to occur and a good crystal orientation is difficult to obtain. In summary, FIG. 1 shows a graph of a region diagram (hatched portion) of the allowable cold rolling reduction rate based on the Si content. The amount of Mn added is limited to 2.0% or less. The reason for this is that if Mn exceeds 2.0%, the plating property is impeded by the surface concentration of Mn itself. Mn does not become a limited function of cold rolling reduction. The reason is the same Si
The effect of cold rolling reduction on the surface enrichment after annealing was investigated using steel sheets of Mn content and Mn content.The cold rolling reduction and the Si surface enrichment amount are almost directly proportional. This is because it has been found that the degree of concentration has a small slope, although a slight positive correlation is recognized.

FIG. 2 shows the relationship between the cold rolling reduction and the steel and steel surface enrichment of Si and Mn after annealing. Surface concentration is GDS
It was determined by the integrated value of (Grim glow emission spectrum). The fact that the surface concentration of Mn is hardly affected by the cold rolling reduction is
It is considered that the diffusion mechanism of Si is different. In other words, Mn also contributes to diffusion due to strain, but rather it is thought that the main component is diffusion via grain boundaries.

In fact, according to the surface observation with an SEM (scanning electron microscope) after annealing, a composite oxide mainly containing Mn on the grain boundary and Si mainly on the grain surface (internal grain surface) is observed. Things are often found. C is limited to 0.1% or less.
The reason is that it is desired to increase the strength in order to obtain the strength, but it is preferably lower in order to obtain the material characteristics. Especially high r
To obtain a value, C is preferably 0.01% or less.

P is an effective additive element for obtaining strength, and since it hardly deteriorates the plating property, it is made 0.03% or more. However, if it is too large, the amount of segregation at grain boundaries increases and embrittlement occurs, so the content is made 0.10% or less. These steel sheets may be subjected to recrystallization annealing in a usual CGL (continuous hot dip galvanizing equipment) and then hot dip galvanizing.

Even if the same steel sheet is used, the amount of surface enrichment changes even if the annealing temperature changes. Naturally, the lower the temperature, the smaller the amount of surface thickening, but it was also found that the effect of the annealing temperature on the surface thickening also depends on the cold rolling reduction. That is, even if the annealing temperature is the same, if the cold rolling reduction is low, the amount of surface enrichment is suppressed. Therefore, it is necessary to lower the annealing temperature when the rolling reduction is high, and the temperature can be increased when the rolling reduction is low. FIG. 3 shows a graph of an allowable region diagram (hatched portion) of the cold rolling reduction and the annealing temperature.

That is, it is limited to the range of 900 ° C. or lower and 750 ° C. ≦ annealing temperature ≦ {−2.5 × reduction rate (%) + 1025} ° C. The reason why the temperature is limited to 750 ° C or higher is for recrystallization, and the temperature of 900 ° C or lower is not necessary for the temperature higher than that. After the annealing, the steel sheet is cooled, and it is preferable to plate by immersing the steel sheet in a zinc plating bath having a bath temperature of 450 to 490 ° C. at a plate temperature of 400 to 550 ° C. The temperature of the penetration plate is preferably 400 ° C or higher in order to accelerate the reaction between the steel plate and the plating bath.
The temperature of 0 ° C. or lower is preferable because the temperature of the Zn bath increases if the plate temperature is too high. The galvanizing bath temperature is
A temperature of 450 ° C or higher is preferred to accelerate the reaction with the plate, and a temperature of 490 ° C or lower is preferred because the amount of Fe eluted into the zinc bath increases and the amount of dross increases. Is.

When producing an alloyed hot-dip galvanized steel sheet, heating is performed immediately after hot-dip galvanizing to cause a diffusion reaction between the hot-dip zinc and Fe in the steel sheet to form an alloyed hot-dip galvanized sheet. In the case of normal hot-dip galvanizing, Al is contained in the zinc bath in the range of 0.1 to 0.2%, but in the case of alloying hot-dip galvanizing, a slightly lower Al concentration is used to promote alloying. Recommended.

In a normal CGL (continuous type hot dip galvanizing apparatus), there are two types of heating methods during annealing: NOF (non-oxidizing furnace) and all radiant tube type (reducing atmosphere). The present invention is particularly suitable for all-radiant tube type lines. Of course, the effect of the present invention is not originally affected by the heating method, but in the case of NOF, the surface of the steel sheet is oxidized regardless of whether it is preferred or not.
Fe oxide is generated. The name of NOF is Non Oxidizing
Although it is a Furnace (non-oxidizing furnace), since it directly heats the surface of the steel sheet with the direct fire of the burner and its exhaust gas, the actual situation is as described above even if it is partially reducing. . It is a well-known fact that the produced Fe oxide has a surface concentration suppressing effect on Si and Mn. However, its effect is extremely unstable and difficult to control. On the other hand, in the case of the all-radiant tube type, such disturbance does not occur. Therefore, the present invention is effective even in the NOF type, but is particularly effective in the all radiant type line.

Regarding the components in the steel, Cr, B, S, Ti, Nb, Al, O and N are appropriately used to control the material and grain boundary characteristics.
Etc. may be added or the addition amount thereof may be limited.

[0025]

[Examples] Steel slabs having various compositions shown in Table 1 were hot-rolled at a finishing temperature of 860 to 910 ° C, and then the reduction ratio was changed to obtain cold-rolled sheets having a thickness of 0.8 mm. Next, except for Example 8 and Comparative Example 10, after recrystallization annealing with all-radiant tube type CGL, the coating adhesion amount was 30 to 60 g / m ² / one side hot dip galvanizing.

For the plating property, the presence or absence of non-plating was judged by visual observation, an optical microscope, a laser microscope and SEM / EDX (scanning electron microscope / energy dispersive X-ray analysis). The evaluation standard of the plating property is as follows. ○ means no plating under normal conditions. △ indicates that there is slight non-plating under normal conditions, and other measures such as high bath temperature are required.

X indicates non-plating. Not a product.

[0028]

[Table 1]

From Table 1, it can be seen that the production methods within the scope of the present invention have good plating properties, no non-plating, and high strength.

[0030]

INDUSTRIAL APPLICABILITY The present invention can reduce the surface enrichment of Si etc. by defining the component limitation and the cold rolling reduction as a function of Si content and the recrystallization temperature as a function of the reduction. It has become possible to manufacture high-strength hot-dip galvanized steel sheets and high-tensile-alloyed hot-dip galvanized steel sheets having excellent plating properties.

[Brief description of drawings]

FIG. 1 is a graph showing a region diagram of an allowable cold rolling reduction rate based on Si content.

FIG. 2 is a graph showing the relationship between the cold rolling reduction rate and the surface concentration of Si and Mn after annealing evaluated by GDS.

FIG. 3 is a graph showing an allowable annealing temperature region diagram based on cold rolling reduction.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C23C 2/28

Claims (3)

[Claims] 1. C: 0.1% or less, Si: 0.5% or less, Mn:
After hot rolling a steel slab containing 2.0% or less and P: 0.03 to 0.10%, it is cold rolled at a rolling reduction represented by the following formula (1), and 900 ° C in a continuous hot dip galvanizing facility. A method for producing a high-strength hot-dip galvanized steel sheet having excellent plating properties, which comprises hot-dip galvanizing after annealing at a recrystallization annealing temperature represented by the following formula (2). Note 40% ≤ rolling reduction ≤ {-80 x Si (%) + 90}% …………… (1) 750 ℃ ≤ recrystallization annealing temperature ≤ {-2.5 × rolling reduction (%) + 1025} ℃ (2) 2. The method for producing a high-strength hot-dip galvanized steel sheet having excellent plating properties according to claim 1, wherein the recrystallization annealing is performed by an all-radiant tube method. 3. A method for producing a high-strength hot-dip galvanized steel sheet having excellent plating properties, which comprises subjecting the high-strength hot-dip galvanized steel sheet obtained in claim 1 to an alloying treatment.