JPH0627312B2 - Method for producing hot dip galvanized steel sheet for processing - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to the production of hot-dip galvanized steel sheets for processing, and more specifically, ordinary Al containing no carbonitride-forming elements such as Ti and Nb. The present invention relates to a method for producing a hot-dip galvanized steel sheet having excellent press workability by using a killed hot-rolled steel sheet as an original sheet and without performing cold rolling.

(Prior Art) In recent years, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets have come to be often used for vehicle bodies such as automobiles or structural members thereof. In these applications, since the shape is complicated and the steel sheet undergoes severe processing during press working, a hot-dip galvanized steel sheet having excellent formability is required.

Conventionally, as a method for producing a hot-dip galvanized steel sheet to be used for such applications, after subjecting the hot-rolled steel strip to cold rolling, as it is or after subjecting to recrystallization annealing, a continuous hot-dip galvanizing line (hereinafter The so-called “galvanizing line”) is used for dip plating, and a so-called cold-rolled steel plate is used as a base plate to manufacture a steel plate.

However, recently, there has been an increasing demand from the consumer side for cost reduction, and a hot-dip galvanized steel sheet which is excellent in workability and is inexpensive is demanded. Therefore, instead of using the cold-rolled steel sheet as the original sheet, it is pickled after hot rolling, without cold rolling or subsequent recrystallization annealing, a method of directly threading the galvanizing line, that is, A method of reducing manufacturing cost by omitting a part of the manufacturing process has been studied and partially put into practical use.

However, conventionally, hot-rolled hot-dip galvanized steel sheets obtained by directly passing hot-rolled steel sheets to a galvanizing line without cold rolling have a relatively thick sheet thickness of 3.2 mm or more. Alternatively, the hot-rolled hot-dip galvanized steel sheet, which has a thin plate thickness and is excellent in workability, has not been produced so far because it is only used for applications where workability is not so severe.

Therefore, various attempts have been made to improve the production method of the hot-rolled hot-dip galvanized steel sheet having such a small thickness and excellent workability, but an effective method has not been found yet. An example of the conventional method will be described below.

(Problems to be Solved by the Invention) Generally, in order to produce a hot-dip galvanized steel sheet, in a galvanizing line, first, heating and soaking in an oxidizing atmosphere,
Next, in order to increase the adhesion of the plating layer, the hot dip zinc temperature
After being kept at about (460 ° C.) in a reducing atmosphere, it is immersed in a hot dip galvanizing bath. In this case, in the heating and soaking process, about 700 to about 7 for the purpose of recrystallization annealing or softening.
It is usually maintained at 850 ° C.

However, normal A containing 0.03 to 0.05% of C
In the case of 1-killed hot-rolled steel sheet, if the soaking is carried out at a temperature of around 700 ° C. before the hot dip galvanizing treatment as described above, the tensile properties are deteriorated by this soaking, and desired properties cannot be obtained. There's a problem.

Further, in order to improve the workability of the hot rolled steel sheet, a method of winding an Al killed steel at high temperature is generally used. For example, in JP-A-54-71717, Al / N (weight ratio) is 10
A method has been proposed in which a conventional Al-killed steel having the above is wound at a temperature of 600 ° C. or higher. However, when the hot-rolled steel sheet obtained by this manufacturing method is subjected to normal soaking (about 700 ° C.) in the galvanizing line as described above to perform a plating treatment, the yield point increases and the elongation increases, as will be described later. As a result, it becomes difficult to secure a low yield point (23 kgf / mm ² or less) and a high elongation (43% or more, sheet thickness 2.0 mm) that are comparable to those of so-called hot-rolled steel sheets for drawing.

As described above, deterioration of the press workability (yield point, elongation) due to the reheating treatment during the hot dip galvanizing treatment of the hot rolled steel sheet as described above is mainly due to the fact that C is re-dissolved through the reheating-cooling process.
This is because it remains as a solid solution. In order to prevent this, for example, as shown in Japanese Examined Patent Publication No. 54-26974, carbon is reduced by degassing or the like, and carbonitriding of Ti, Nb, etc. having an atomic equivalent of C or more is carried out. If steel such as a material-forming element is rolled at a high temperature after hot rolling, TiC and NbC precipitated in the hot rolling to winding process do not re-dissolve in the reheating process before and after hot dip galvanizing.
It is possible to suppress the deterioration of the press workability. However, degassing or addition of Ti, Nb, etc. leads to an increase in manufacturing cost, which is not economically preferable.

As described above, in particular, without containing carbonitride forming elements such as Ti and Nb, without subjecting to a long-term degassing treatment, ordinary Al-killed steel was hot-rolled and then cold-rolled without hot rolling. In the present situation, a method that can positively suppress the deterioration of the material due to the reheating treatment before plating has not been found yet.

The present invention has been made in view of such circumstances,
Normal A that does not contain special carbonitride forming elements such as Ti and Nb
Even when the 1-killed steel is hot-rolled, wound, and directly subjected to continuous hot-dip galvanizing without cold rolling,
An object of the present invention is to provide a method capable of producing a hot-dip galvanized steel sheet having a low yield point and a high elongation comparable to those of a conventional hot-rolled steel sheet for drawing and excellent in press workability.

(Means for Solving Problems) In order to achieve the above object, the present inventor has focused on the relationship between the workability and hot rolling of hot-dip galvanized steel sheet,
An experimental study was conducted on the re-dissolution behavior of C during reheating during hot dip galvanizing when the coiling was carried out at a higher temperature than above. By changing the precipitation morphology, the above C
It has been found that the re-dissolution behavior of is different and has a great influence on the properties of the obtained product.

Therefore, as a result of investigating the relationship between the winding temperature and the reheating conditions during the hot dip galvanizing treatment and the yield point and elongation of the product, after the hot rolling using the ordinary Al-killed steel for mild steel, it was higher than the conventional value of 690. After coiling at a high temperature of ℃ or more to agglomerate cementite, it is passed through a continuous hot dip galvanizing line without cold rolling, and reheated at a low soaking temperature of 580 ° C or less. Then, the inventors have found that the intended purpose can be achieved, and the present invention has been made here.

That is, the manufacturing method of the hot-dip galvanized steel sheet for processing according to the present invention is C: 0.015 to 0.060%, Mn: 0.0
5 to 0.45% and solAl: 0.005 to 0.10%
After hot-rolling a steel containing the balance of Fe and unavoidable impurities, the steel is coiled at a steel strip temperature of 690 ° C. or higher,
Then, without cold rolling, the temperature is 460 ° C or higher and 580 ° C in advance.
It is characterized in that hot-dip galvanizing is performed after preheating to the following steel strip temperatures.

The present invention will be described in more detail below.

First, the action of the chemical components of steel and the reason for limiting the components in the method of the present invention will be described.

(B) C C is the most important element in achieving the object of the present invention, and by controlling the amount of C, solid solution in the case where the above-mentioned reheating in the galvanizing line → cooling process is performed. The main aim is to avoid the inconvenience caused by C.

Therefore, the present inventor investigated the relationship between the C content and the mechanical properties of the hot-dip galvanized steel sheet by experiments in order to find the limit of the appropriate C content in the steel.

In the experiment, 7 levels of C content (0.002%, 0.009%
%, 0.015%, 0.025%, 0.045%, 0.
Al-killed steel having 053% and 0.070%) was melted and continuously cast into a slab, which was hot-rolled and wound into a coil at 710 ° C. Next, this hot-rolled steel sheet was subjected to hot dip galvanizing treatment in a galvanizing line. In this case, the soaking temperature before galvanization is all 55
It was set to 0 ° C.

A JIS No. 5 test piece was sampled in the L direction from the obtained hot-dip galvanized steel sheet, and a tensile test was performed. The relationship between the C content of each test piece and the yield point and elongation is shown in FIG. In the figure, the range of the arrow direction is the general yield point Y.Y. of the hot-rolled steel sheet for drawing targeted by the present invention. P (23 kgf / mm ² or less) and elongation El
(43% or more).

From FIG. 1, when the amount of C is 0.009% or more and less than 0.015% and exceeds 0.060%, even when manufactured under the winding and hot dip galvanizing conditions within the scope of the present invention. Although the target characteristics were not obtained, the C content was 0.
If 015 to 0.060%, the yield point is 23 kgf / mm ²
Hereafter, the elongation is 43% and the target characteristics are obtained. Further, it is generally known that the lower the content of C, the softer the steel and the higher the ductility, but in consideration of this point and the above results, the upper limit of the C content is 0.060%. To do.

On the other hand, as for the lower limit of the C amount, as shown in FIG.
When the amount is near 0.009%, there is a region where the yield point rises sharply and elongation decreases, and since the cementite size is small in this region, even with the winding and galvanizing conditions according to the present invention, It is considered difficult to suppress re-dissolution of C due to hot dip galvanizing treatment. Therefore, C
The lower limit of the amount is 0.015%. The amount of C is 0.0
In the case of the lowest value of 02%, it is considered that during the hot dip galvanizing treatment, even if C is solid-dissolved, it does not contribute to the extent that the material deteriorates, and therefore it brings excellent characteristics, but reduces C to this level. Therefore, the time required for the degassing process during steelmaking becomes extremely long, which leads to an increase in manufacturing cost and a decrease in productivity, which is not preferable.

(B) Mn Mn has the effect of fixing free sulfur in the steel and improving the strength of the steel. However, if the Mn content is less than 0.05%, hot brittleness may occur, while if it exceeds 0.45%, the ductility deteriorates and it becomes difficult to secure the target elongation. Therefore, the amount of Mn is made into the range of 0.05 to 0.45%.

(C) solAl Al acts as a deoxidizer during the smelting of steel, and at least 0.005% or more is necessary for that purpose. But,
If it exceeds 0.10%, surface defects of the steel sheet called sliver defects are likely to occur. Therefore, the amount of solAl is 0.0
The range is from 05 to 0.10%.

The steel containing the above components may be accompanied with impurities such as P, S, Si, N, etc., but the amount thereof is allowable as long as the effect of the present invention is not impaired.

Next, the hot rolling conditions and hot dip galvanizing conditions in the method of the present invention will be described.

(D) Hot rolling conditions In the present invention, hot rolling conditions, particularly the winding temperature is an important factor, and as described above, by changing the precipitation form of cementite after hot rolling, reheating during hot dip galvanizing treatment ( It is considered that the re-dissolution behavior of C due to (preheating) is different, and it has a great influence on the properties of the obtained product.

Therefore, the present inventor determines the appropriate winding temperature by
The relationship between the winding temperature and the mechanical properties of hot-dip galvanized steel sheets was investigated by experiments.

In the experiment, C: 0.045%, Mn: 0.20% and sol
Al: melted Al killed steel containing 0.035%,
A slab is made by continuous casting, which is hot-rolled.
Standard winding temperature (630 ℃, 650 ℃, 670 ℃, 69
It was wound into a coil at 0 ° C, 710 ° C, 720 ° C.
The obtained hot-rolled steel sheet was pickled, and then hot-dip galvanized in a galvanizing line. In this case, the soaking temperature before galvanizing was about 550 ° C.

A JIS No. 5 test piece was sampled in the L direction from the obtained hot-dip galvanized steel sheet, and a tensile test was performed. The relationship between the winding temperature of each steel sheet and the yield strength and elongation is shown in FIG. In the figure, the range in the direction of the arrow means the same contents as in the case of FIG.

From FIG. 2, it can be seen that the higher the winding temperature, the lower the yield point and the higher the elongation, and that the target tensile properties can be obtained especially when the winding temperature is 690 ° C. or higher. This is because the coiling takes place around the A ₁ transformation point at a higher temperature than before, so that cementite is lumped, coarsened, and the interfacial area is reduced. It is considered that this can be suppressed to some extent.

Therefore, in order to obtain the above effect, the coiling temperature needs to be at least 690 ° C. or higher, and is preferably higher. However, if the coil is wound at an excessively high temperature, the winding shape of the coil may be deformed or the coil cooling time may be increased, resulting in a decrease in productivity.
It shall be 40 ° C or lower.

The finishing temperature is preferably at least the _{Ar 3} transformation point. However, when the high temperature winding as in the present invention is performed, even if the finishing temperature is slightly lower than the _{Ar 3} transformation point, the ferrite particles are wound at the time of winding. Recrystallizes and in this case does not significantly affect the yield point and elongation. Therefore, the finishing temperature may be about 750 ° C. or higher.

(E) hot-dip galvanizing conditions hot-dip galvanizing treatment in the present invention, the bulk, coarsening by hot rolling, cementite hardened to cause re-solid solution by reheating, further hot-dip galvanizing conditions, especially soaking conditions It is intended to prevent the cementite from being re-dissolved as much as possible by controlling the above, and to avoid the inconvenience caused by the solute C.

Therefore, the present inventor investigated the influence of the soaking temperature in the galvanizing line on the mechanical properties of the hot-dip galvanized steel sheet when the hot-rolled steel sheet (coil) having the above cementite in the dispersed state is used as the material. .

In the experiment, C: 0.045%, Mn: 0.21% and sol
Steel containing Al: 0.035% is melted, slab is formed by continuous casting, and then hot rolling is performed to 710 ° C.
It was wound into a coil. The coil thus obtained was subjected to 6 levels of soaking temperature (470 ° C, 500 ° C,
After uniform heating at 550 ° C., 600 ° C., 650 ° C., 700 ° C.), hot dip galvanizing treatment was performed.

A JIS No. 5 test piece was sampled in the L direction from the obtained hot-dip galvanized steel sheet, and a tensile test was performed. Fig. 3 shows the relationship between the soaking temperature of each test piece in the galvanizing line and the yield point and elongation. In the figure, the range in the direction of the arrow means the same contents as in the case of FIG.

From FIG. 3, the yield point decreases and the elongation increases as the soaking temperature in the galvanizing line decreases. Especially when the soaking temperature is 580 ° C. or lower, the tensile properties targeted by the present invention are obtained. This suppresses the re-dissolution of cementite by soaking a steel sheet having cementite that is lumpy and coarsened by hot rolling at a temperature much lower than the conventional temperature (about 700 ° C) in a galvanizing line. Therefore, it is considered that the low yield point and the high elongation could be secured.

Therefore, in the present invention, the soaking temperature needs to be 580 ° C. or lower, and a lower temperature is desirable. However, at a temperature lower than 460 ° C. which is the melting temperature of zinc, the plating adhesion is deteriorated, and soaking at a minimum of 460 ° C. or higher is unavoidable.

It has been confirmed that the soaking time in the galvanizing line has almost no effect on the yield point and elongation of the hot-dip galvanized steel sheet within the range of normal operation. This is determined by the plate passing speed of the plating line, and the soaking time is about 10 to 50 seconds, as shown in the general heat cycle of FIG.

Furthermore, it is optional whether or not temper rolling is performed in-line or off-line of the galvanizing line for the purpose of preventing stretcher strain or correcting the shape. Since there is a concern that points may increase, the elongation rate is preferably 2% or less.

(F) Other conditions Regarding the pickling treatment after hot rolling and before hot dip galvanizing treatment,
There is no particular effect or influence on the mechanical properties of the hot-dip galvanized steel sheet obtained by the present invention, and therefore the conditions are not particularly limited.

Next, an embodiment of the present invention will be described. Needless to say, the present invention is not limited to this embodiment,
In addition to the various basic researches and experimental examples described above, other modes are possible.

(Example) A steel having the chemical composition (wt%) shown in Table 1 was melted by a usual method, and after the steel was taken out of the converter, it was continuously cast into a slab. Then, hot rolling was performed to a plate thickness of 2 mm, and wound at the winding temperature shown in Table 2. The finishing temperature was 880 to 915 ° C.

After pickling the obtained hot-rolled coil, it was subjected to a soaking treatment at a soaking temperature shown in Table 2 in a galvanizing line, a hot dip galvanizing treatment, and a temper rolling with an elongation of 1.0%. .

Table 2 also shows various properties of the obtained hot-dip galvanized steel sheet. In the table, the tensile properties are the results of performing a tensile test by collecting JIS No. 5 test pieces from the steel sheet in the L direction, and the aging index was 8% prestrain, and the aging treatment was performed at 100 ° C. for 1 hour. It represents the amount of increase in yield point after that and shows aging resistance.

As is clear from Table 2, Steel No. A which is an example of the present invention
-1 and No. B-1 are both soft and excellent in ductility, and also have excellent aging resistance.

On the other hand, Comparative Examples Steel No. A-2 and No. B-2 have high soaking temperature in the galvanizing line, and Steel No. B-
Since No. 3 has a low winding temperature, it is considered that the amount of residual solid solution C of the hot-dip galvanized steel sheet is large, and as a result, all of them have a high yield point, a low elongation, and further have poor aging resistance. There is. Further, among the comparative examples, Steel No. C has a small amount of C, and similarly, the amount of residual solid solution C is large and the desired characteristics cannot be obtained. Further, Steel No. D has an excessively large amount of C, and No. In D, the Mn content is too large and the elongation is insufficient, so that the desired properties cannot be obtained in either case.

(Effects of the Invention) As described in detail above, according to the present invention, Al-killed steel whose composition is adjusted without adding carbonitride forming elements such as Ti and Nb is subjected to hot rolling and then cold rolling. When hot-dip galvanizing without applying, the coiling temperature in hot rolling is higher than before and the reheating (preheating) temperature in the galvanizing line is lower than before, so material deterioration due to reheating treatment is effective It is possible to produce a hot-dip galvanized steel sheet which has a low yield point and a high elongation comparable to those of a conventional hot-rolled steel sheet for drawing and which is excellent in press workability. In addition, since it can be manufactured without adding carbonitride forming elements such as Ti and Nb or degassing for a long time, and further without requiring cold rolling, it is economical and has a great effect of improving productivity.

[Brief description of drawings]

FIG. 1 is a diagram showing the relationship between the C content of the hot dip galvanized steel sheet and the yield point and elongation, and FIG. 2 is a diagram showing the relationship between the winding temperature in hot rolling and the yield point and elongation of the hot dip galvanized steel sheet. FIG. 3 is a diagram showing the relationship between the soaking temperature in the galvanizing line and the yield point and elongation of the hot-dip galvanized steel sheet, and FIG. 4 is a diagram showing a general heat cycle in the galvanizing line.

Claims (1)

[Claims] 1. In weight% (hereinafter the same), C: 0.015
0.060%, Mn: 0.05-0.45% and solA
1: Steel containing 0.005 to 0.10%, the balance being Fe and inevitable impurities, after hot rolling, steel strip temperature 690 ° C.
As described above, the hot-dip galvanizing for working is characterized in that the hot-dip galvanizing is performed after pre-heating to a steel strip temperature of 460 ° C. or more and 580 ° C. or less without winding in a coil shape and then cold rolling. Steel plate manufacturing method.