JPH0466653A - Manufacture of hot-dip galvanized steel sheet for high working excellent in surface property - Google Patents

Abstract

PURPOSE:To provide a steel sheet with high strength, excellent deep drawability and good surface properties by subjecting a extra low carbon Ti-Nb steel to hot rolling at a high temp., coiling this steel at a low temp., executing high cold rolling and subjecting it to high temp. reduction annealing in a hot-dip galvanizing apparatus. CONSTITUTION:The compsn. of the stock of a steel sheet is formed of, by weight, <=0.01% C, >0.1 to 0.5% Si, 0.5 to 2.2% Mn, 0.03 to 0.15% P, <=0.015% S, 0.01 to 0.1% solAl, <=0.005% N, 0.0002 to 0.002% B, (48/12%C+48/14%N+48/32%S) to 0.1% Ti, 0.01 to 0.1% Nb and the balance Fe with impurities. This steel slab is finished with hot rolling at 920 to 1050 deg.C and is coiled at 400 to 600 deg.C. The slab is subjected to acid pickling and is thereafter cold-rolled at >=70% draft. Next, the slab is charged to a continuous hot-dip galvanizing apparatus, is subjected to reduction annealing at 800 deg.C to the Ac3 point or below and is subjected to hot-dip galvanizing in the stage of cooling. Then the slab is held under heating at 450 to 600 deg.C for >=1 sec to form a galvannealed layer.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for producing a high-tensile galvanized steel sheet with excellent surface properties and deep drawability.

(Prior art and its problems) In recent years, high-strength steel sheets have come to be widely used as steel sheets for automobiles from the viewpoints of safety, reduction in vehicle weight, and reduction in material usage. Among these high-strength steel sheets for automobiles, for example, in order to prevent corrosion by salts contained in road snow melting agents during cold seasons, high-strength steel sheets with an alloyed hot-dip galvanized layer that has excellent corrosion resistance are used. There is a growing need for tensile cold rolled steel sheets.

For this reason, there has been a strong demand for mass production of high-strength steel sheets that have excellent workability and corrosion resistance.

As a general method of imparting corrosion resistance to steel sheets.

There are methods of adding elements such as Cu and Cr to increase the corrosion resistance of steel, and methods of applying metal plating to the surface of the steel sheet, but the former is not very effective in severe corrosive environments such as salt damage. Can not.

Therefore, in such a severe corrosive environment, the latter metal plating, especially hot-dip galvanizing, which can be easily and inexpensively coated with a thick coating to improve corrosion resistance, is effective. Furthermore, in consideration of coating film adhesion and weldability, alloying treatment is generally performed after hot-dip galvanizing.

However, the alloyed hot-dip galvanized steel sheets obtained by these methods commonly deteriorate in press workability, especially deep drawability, as the strength increases and the coating layer thickness increases, making it difficult to perform difficult forming processes. There was nothing.

Furthermore, compared to soft steel sheets, high-tensile strength alloyed hot-dip galvanized steel sheets contain extremely large amounts of reinforcing elements such as Si, Mn, and P to ensure high strength. especially.

Steel sheets containing solid solution strengthening elements Si and Mn have surface defects such as dotted plating and scale patterns, and from the viewpoint of appearance and corrosion resistance, there is no product that has a high yield rate. Ta.

(Knowledge related to problem solving) The present invention has excellent rust prevention and plating adhesion that do not cause powdering, which are required for automotive steel sheets, high strength, and r value, which is an index of press formability. The objective of the present inventors is to produce a steel plate with a hardness of 1.4 or more, which does not cause secondary processing cracks during press working, and has excellent surface properties. The results of various studies on the composition and manufacturing method of rolled steel sheets.

Predetermined solid solution strengthening elements Si, An, P, and B are added to ultra-low carbon Ti and Nb composite addition steel, and the hot rolling finishing temperature is set high and the winding temperature is set low. We have found that a high tensile strength galvanized steel sheet having the above properties can be obtained by performing high temperature reduction annealing in a hot-dip coating apparatus.

(Structure of the invention) The present invention The base steel plate is in weight%, C: 0.01% or less, Si: More than 0.1% and less than 0.5%.

Mn: 0.5-2.2%.

P: 0.03-0.15%, s: 0.01% or less.

soQ, AQ: 0.01-0.1%, N: 0.
005% or less, B: 0.0002 to 0.002% Ti: (48/12%C + 48/14%N + 4
8/32%S)~o, i%Nb: 0.01~0.1
%, with the remainder consisting of Fe and unavoidable impurities.
i, Nb composite addition ultra low carbon steel slab 920~105
After hot rolling at 0°C, rolling at a temperature of 400 to 600°C and pickling, cold rolling is performed at a rolling reduction of 70% or more, and the resulting cold rolled steel sheet is then inline annealed. charged into continuous hot-dip galvanizing equipment, 800℃~Ac,
Reduction annealing is performed at a temperature below the transformation point, hot-dip galvanizing is performed during the cooling process, and then 450 to 600
Provided is a method for manufacturing a high-strength hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer, which is heated and held in a temperature range of 0.degree. C. for 1 sec or more, and has excellent surface properties and good press formability.

Next, the reasons for limiting the composition in the present invention will be explained.

C: In this invention, C is fixed as a carbide such as TiC or NbC. The lower the content, the more advantageous it is for deep drawability and ductility, and the amount of Ti and Nb added can also be reduced. The reason for setting the upper limit to 0.01% is that Ti must be added if C increases further.
This is because the amount of carbon and Nb increases significantly, leading to an increase in cost. Therefore, the amount of C is set to 0.01% or less.

Si: Si is an effective element for increasing the strength of steel sheets, but if it exceeds 0.5%, it will extremely deteriorate plating adhesion and cause surface texture defects such as non-plating.
The upper limit is set to 0.5%. On the other hand, if it is 0.1% or less, it does not significantly contribute to increasing the strength of the steel sheet.

Mn: Mn is an effective element for increasing the strength of steel sheets, but if it exceeds 2.2%, the A3 transformation point will drop significantly and become close to the recrystallization temperature, causing α→γ transformation of the metal structure during annealing. is promoted. For this reason, the (111) recrystallization texture favorable to the r value formed in the recovery-recrystallization process is damaged. Furthermore, during annealing, a hardened structure may be produced during the cooling process. The reason why the upper limit is set to 2.2% is that the metallurgical factors mentioned above significantly deteriorate the elongation and r value of the steel plate. On the other hand, if it is less than 0.5%, it does not significantly contribute to increasing the strength of the steel sheet8.

The amount of Mn is 0.5 to 2.2%.

FDP also increases the strength of steel sheets like Si and Mn, but
If it exceeds 0.15%, not only will secondary processing cracks occur significantly during press working, but also alloying in a continuous hot-dip galvanizing apparatus will become difficult.

The content shall be 0.15% or less.

Moreover, if it is less than 0.03%, it will not work effectively to increase the tensile strength of the steel plate. Therefore, the amount of P is 0.03 to 0.1
5%.

S: S combines with Mn to form nonmetallic inclusions, which tends to cause defects such as cracks during press molding.

Furthermore, since S forms TiS, the amount of Ti added required to fix C increases as the amount of S increases, which is unfavorable in terms of cost. Therefore, the upper limit is set to 0.015%.

AQ: AQ is added for deoxidation and remains at 0.01% or more, but if it exceeds 0.1% it will adversely affect the surface quality, so the upper limit for sOΩ, AQ is set at 0.1%.

N: N is an impurity element in steel, but due to T1, TiN
is fixed as , improving the r value. However, if this N content exceeds o, oos%, the amount of Ti added required for stabilization will increase, which is unfavorable in terms of cost. therefore,
The upper limit is set to o, oos%.

Ti: Ti is added to fix S, N, and C and develop a recrystallized texture (111), and the lower limit is (4g/12%c + 48/14%N + 48/
32%S). The reason why the upper limit is set to 0.1% is that the effect will be saturated even if it is added in excess of this amount.

Nb: Nb forms coarse Ti and Nb composite chamber nitrides during hot rolling using stable Ti carbonitrides as cores.

Therefore, since the (111) recrystallization texture is further developed, the r value is improved. Furthermore, the addition of Nb also improves in-plane anisotropy. However, if the amount added is less than 0.01%, no improvement in the r value or in-plane anisotropy of the r value can be seen, so it is recommended to set the amount to be 0°01% or more, and to set the upper limit to 0.1%. This is because even if it is added in excess of this, the effect will be saturated.

B: B preferentially segregates at the grain boundaries in the steel sheet than the aforementioned P, and P
This suppresses deterioration of press formability due to grain boundary brittleness, and the effective lower limit is 0.0002%. On the other hand, if its content is too high, grain growth will be inhibited and the r value of the steel sheet will be reduced.
Since B reduces elongation, the upper limit of B addition is set at 0.002%.

In addition, the manufacturing conditions for the raw material steel of the steel plate of the present invention are that molten steel adjusted to the above chemical composition range is continuously cast, and the resulting slab is sent directly without cooling, or - once cooled and then reheated as a cold piece. Hot rolling is then performed.

The hot rolling finishing temperature is one of the main manufacturing conditions of the present invention, and hot rolling finishing in a high temperature range of 920°C or higher is
It is effective in improving the surface quality of products without deteriorating deep drawability.

Generally, the finishing temperature of a steel plate with deep drawability is Ar,
Below the transformation point, a hot rolling texture, which is unfavorable to the (111) recrystallization texture, is produced.

Ar is usually set at a temperature above the transformation point, but in the method of the present invention, it is carried out at a temperature above 920°C.

At 920° C. or higher, the bite scale of the hot rolled sheet is reduced without deteriorating the r value, the surface roughness of the steel sheet after pickling and cold rolling is reduced, and the surface properties of the product are improved. Also,
The reason why the upper limit temperature is set at 1050°C is because if the hot-rolling finishing temperature is higher than this, the grain size of the hot-rolled sheet becomes large.
I) This is because the development of recrystallized texture is suppressed and the r value decreases.

The upper limit of the winding temperature is set at 600°C since pickling properties of the scale deteriorate if it is too high, and the lower limit is set at 400°C to prevent defects in the shape of the plate after winding.

During cold rolling, the surface of the steel sheet is cleaned by ordinary pickling and then rolling is performed. At that time, the rolling reduction ratio was 1.4
In order to ensure this, it is set at 70% or more.

The obtained cold-rolled sheet is charged into an in-line annealing type continuous hot-dip galvanizing apparatus and subjected to reduction annealing in a temperature range of 800° C. or higher and Ac and lower than the transformation point. The regulation of this reduction temperature, like the above-mentioned hot-rolling finishing temperature, is a manufacturing condition that forms the essence of the present invention. At a reduction temperature of less than 800°C, the wetting adhesion of molten zinc decreases, resulting in surface texture defects in the product. Since non-plating occurs, the lower limit temperature is set to 800°C. The reason for this is not necessarily clear, but solid solution strengthening elements such as P, S
It is presumed that this is because the oxide film of i was formed and remained during the hot-dip galvanizing process. The reason why the upper limit temperature was set below the Ac3 transformation point was due to the α→γ transformation (111)
This is to suppress deterioration of the r value due to disappearance of recrystallized texture.

After the reduction treatment, it is continuously cooled and subjected to hot-dip galvanizing treatment, and then heated and held in a temperature range of 450°C to 600°C for 1 second or more. Plating that is heated and maintained in this temperature range of 450'C to 600°C has a very small amount of plating.

This is because layers can be obtained.

In this way, according to the present invention, an alloyed hot-dip galvanized high-strength steel sheet with excellent deep drawability and good surface properties is manufactured.

(Specific disclosure of invention) Next, the present invention will be specifically explained with reference to Examples.

Table 1 shows the chemical composition of the test materials, and the example of the present invention has Nα1
-9, and the comparative example has Nα10-14.

The composition is adjusted in a converter and melted, and the thickness is 2 by continuous casting.
It was made into a 50mm slab, followed by hot rolling, and 2.3
．． 2.7.3.2.4.0.5.2mm hot rolled plate,
Next, pickling 11. Perform cold rolling. Obtained thickness 0.8
A cold-rolled steel sheet having a diameter of 1.0 mm was charged into an in-line annealing type continuous hot-dip galvanizing apparatus to produce a high-tensile galvanized steel sheet.

Figure 1.2 is a diagram showing the results of investigating the influence of hot rolling finishing temperature on the Na 2 steel in Table 1. The relationship between cold-rolled sheet r values is shown. As seen in this figure, 9
Below 20℃, the number of bite scales is large, 1200
When the temperature exceeds 0.degree. C., the r value becomes low. Figure 2 shows the relationship between hot rolling finishing temperature and surface roughness. From this figure, it can be seen that when hot rolling is completed at a temperature of 920° C. or higher, both the surface after pickling and the surface after cold rolling are significantly smooth.

Figure 3 shows a cold-rolled steel sheet made from the same steel with a 50%
The relationship between the reduction annealing temperature, plating wettability, and the properties of the plating surface when reduction annealing is performed in an H2 + 50% N2 atmosphere is shown. According to this, it can be seen that good surface properties can be obtained when annealing is performed at 800° C. or higher.

Table 2 shows the hot rolling finishing temperature, winding temperature, cold rolling rate, reduction temperature, manufacturing conditions, and product characteristics.

As seen in the results in Table 2, product 1-
■~1-■ and 2~9 have high strength of 35 kgf/l1m2 or more, excellent deep drawability with an r value of 1.4 or more, and secondary work cracking resistance with a vertical cracking limit temperature of 40°C. It can be seen that the alloyed hot-dip galvanized steel sheet has a good surface quality as below, and also has a good surface quality with no surface roughness or unplated surface.

Even if the raw material steel of the method of the present invention is within the composition range, the hot rolling finishing temperature and reduction temperature are outside the range specified by the present invention.Nα
Although 1-■ exhibits high strength, it has problems such as surface roughness and surface texture defects due to non-plating.

Similarly, NQI-■, whose hot-rolling finishing temperature, cold rolling rate, and reduction temperature are outside the range specified in the present invention, has a small r value of 1.4 or less, which improves deep drawability and improves surface roughness and unplatedness. There is a problem with the surface texture.

NQIO1lIl in which the amount of C is greater than the range specified in the present invention
Nα12 when the n amount is outside the specified range, Nα when the B amount is outside the specified range
No. 14 has a low r value of 1.4 or less and has a problem in deep drawability.

In addition, Nα13 in which the amount of P is larger than the range specified in the present invention has a longitudinal cracking limit temperature of 0, which is an evaluation value of secondary work cracking resistance.
℃, and there is a problem that secondary processing cracks occur during deep drawing.

If Nα11 has a Si content larger than the range specified by the present invention, unplating occurs even if the manufacturing conditions are within the range specified by the present invention, and there are problems with the surface quality.

(Effect of the invention) The alloyed hot-dip galvanized steel sheet according to the present invention weighs 35 kg.
It has a high strength of f/mm2 or more and an r value of 1.4 or more,
Not only does it have excellent deep drawing formability, but it also has very good surface properties, and the invention is highly effective when used as steel sheets for automobiles.

[Brief explanation of drawings]

Figure 1 is a graph showing the effect of hot rolling finishing temperature on the r value and the number of scales biting into a hot rolled sheet, and Figure 2 is a graph showing the effect of hot rolling finishing temperature on the surface roughness of a steel sheet after pickling and cold rolling. FIG. 3 is a graph showing the influence of finishing temperature, and FIG. 3 is a graph showing the influence of reduction temperature before plating on molten zinc wet adhesion.

Claims (1)

[Claims] The base steel sheet has the following weight percentages: C: 0.01% or less, Si: more than 0.1% and 0.5% or less, Mn: 0.5 to 2.2%, P: 0 .03-0.15% S: 0.015% or less, sol. Al: 0.01-0.1%, N: 0.005% or less, B: 0.0002-0.002% Ti: (48/12%C + 48/14%N + 48/32
%S) to 0.1%Nb: 0.01 to 0.1%, with the balance consisting of Fe and inevitable impurities.
i, Nb composite addition ultra low carbon steel slab 920~1050
After finishing the hot rolling at 400 to 600°C and pickling, cold rolling is performed at a rolling reduction of 70% or more, and the resulting cold rolled steel sheet is then subjected to in-line annealing. Charged to continuous hot-dip galvanizing equipment and heated to 800℃~Ac_3
Reduction annealing is performed at a temperature below the transformation point, hot-dip galvanizing is performed during the cooling process, and then 450 to 600
A method for producing a high-strength hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer with excellent surface properties and good press formability, which comprises heating and holding in a temperature range of 1 sec or more at .degree.