JPH0627311B2 - Method for producing alloyed hot-dip galvanized steel sheet for processing - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to the production of alloyed hot-dip galvanized steel sheet for working, and more specifically, it is usually the case where carbonitride forming elements such as Ti and Nb are not added. Al killed hot rolled steel plate of
The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet excellent in press workability 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 or an alloyed hot-dip galvanized steel sheet having excellent formability is required.

Conventionally, as a method for producing an alloyed hot-dip galvanized steel sheet to be used for such an application, a hot-rolled steel strip is subjected to cold rolling, and as it is or after recrystallization annealing, a continuous alloyed hot-dip zinc is applied. A usual method is a method for producing a steel sheet using a so-called cold-rolled steel sheet as a base sheet, which is carried out through a plating line (hereinafter referred to as “zinc plating line”) and subjected to immersion plating and alloying treatment.

However, recently, there has been an increasing demand from the consumer side for cost reduction, and a hot-dip galvanized steel sheet and an alloyed hot-dip galvanized steel sheet which are excellent in workability and are inexpensive are demanded. Therefore, instead of using the cold-rolled steel sheet as the original plate, it is pickled after hot rolling, without performing 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, although various improvements have been attempted for the manufacturing method of the hot-rolled raw sheet hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet having such a thin plate thickness and excellent workability, an effective method is still found. Not not. An example of a conventional method for producing a galvannealed steel sheet will be described below.

(Problems to be Solved by the Invention) Generally, in order to manufacture an alloyed hot-dip galvanized steel sheet, as shown in a general heat cycle in FIG. Heated and then hot-dip zinc temperature (46
After being kept at about 0 ° 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 850 for the purpose of recrystallization annealing or softening.
It is usually kept at ° C. Further, the steel strip is reheated to 500 to 700 ° C. for the alloying treatment after the immersion plating.

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, it is common to wind the Al-killed steel at a high temperature to raise the heating and soaking temperature in the galvanizing line to a high temperature.
No. 4-71717 proposes a method of winding normal Al-killed steel having an Al / N (weight ratio) of 10 or more at a temperature of 600 ° C. or more. However, when the hot-rolled steel sheet obtained by this manufacturing method is subjected to normal soaking (about 700 ° C.)-Zinc plating-alloying treatment (about 600 ° C.) in the galvanizing line as described above, it will be described later. Moreover, the yield point rises and the elongation decreases. Therefore, it becomes difficult to secure so-called high elongation (43% or more, plate thickness 2.0 mm).

As described above, the deterioration of the press workability (yield point, elongation) due to the reheating treatment during the galvanizing treatment of the hot rolled steel sheet as described above is mainly due to the fact that C is re-dissolved-solid through the reheating-cooling process. This is because it remains because it remains molten. 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. When steel such as a material-forming element is hot rolled 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, alloying without forming carbonitride-forming elements such as Ti and Nb, without performing long-term degassing treatment, and after hot rolling of ordinary Al-killed steel without cold rolling When hot-dip galvanizing is applied, a method capable of positively suppressing deterioration of material due to reheating treatment in a galvanizing line has not yet been found.

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 then directly subjected to continuous galvannealing without cold rolling, it has a low yield point and high elongation comparable to those of conventional hot-rolled steel sheets for drawing. It is an object of the present invention to provide a method capable of producing a galvannealed steel sheet exhibiting excellent press workability.

(Means for Solving Problems) In order to achieve the above-mentioned object, the present inventor has focused on the relationship between the workability and high-temperature coiling of hot-dip galvannealed steel sheet, and at higher temperatures. An experimental study was conducted on the re-dissolution behavior of C in the reheating in the galvanizing line when the coiling was carried out.
It was found that the re-dissolution behavior of the above C differs by changing the precipitation form of cementite after hot rolling, which 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 in the galvanizing line and the yield point and elongation of the product,
After hot rolling using normal Al-killed steel for mild steel, it is coiled at a high temperature of 690 ° C or higher, which is higher than before, to agglomerate cementite, and then passes through a galvanizing line without cold rolling. It has been found that the intended purpose can be achieved by plate-forming and then reheating at a low temperature of 580 ° C. or less, and the present invention is made here.

That is, the manufacturing method of the alloying hot-dip galvanized steel sheet for processing which concerns on this invention is C: 0.015-0.060%, Mn:
0.05-0.45% and solAl: 0.005-0.
A steel containing 100% and the balance of Fe and unavoidable impurities is hot-rolled, wound into a coil at a steel strip temperature of 690 ° C or higher, and then cold-rolled in advance to 460 ° C or higher and 580 ° C or higher. It is characterized in that after preheating to a steel strip temperature of ℃ or less, hot dip galvanizing is performed, and further, alloying treatment of the plated layer is performed at a steel strip temperature of 460 ° C or more and 580 ° C or less.

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 galvannealed steel sheet by an experiment in order to find the limit of the appropriate C content in the steel.

In the experiment, 7 levels of C amount (0.002%, 0.011
%, 0.017%, 0.027%, 0.045%, 0.
055%, 0.070%) Al killed steel 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 alloying hot dip galvanizing treatment in a galvanizing line. In this case, the soaking temperature before galvanization and the alloying temperature (both are steel strip temperatures) are 540 to
The temperature was 560 ° C and 530 ° C.

JIS from the obtained galvannealed steel sheet in the L direction
A No. 5 test piece was sampled and a tensile test was conducted. C of each test piece
Fig. 2 shows the relationship among the amount, yield point and elongation. 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. The table shows P (23 kgf / mm ² or less) and elongation El (43% or more).

From Fig. 2, the case where the amount of C is 0.002% and 0.017%
When the content is 0.055% or more, the yield point and elongation are as low as those of the hot-rolled steel sheet for drawing. On the other hand, C
When the amount is 0.011% and 0.07%, the yield point is high and the elongation is low.

In addition, it is generally known that the lower the content of C, the softer the steel and the higher the ductility. However, according to the above results and the research results described later, in the alloyed hot-dip galvanized steel sheet, C is re-dissolved in the normal Al-killed steel by the reheating treatment before the galvanization and during the alloying treatment, It is considered that a considerable amount of C remains as a solid solution due to the subsequent rapid cooling, and these solid solution C increase the yield point of the steel sheet and reduce the elongation. It is considered that the yield point is lowered and the elongation is not increased simply by reducing the C content in the steel. That is, when the C content is near 0.01%,
Since the size of cementite is small, when it is reheated in a galvanizing line, even if it is manufactured under the hot rolling and alloying hot dip galvanizing conditions according to the present invention described later, it is possible to suppress the re-dissolution of C. It is thought to be difficult.

On the other hand, when the C content is 0.002% or less, it is considered that even if solid-soluted by the alloying zinc plating treatment, it does not contribute to the extent of degrading the material, and therefore, it has excellent properties, but the C content up to this level In order to reduce the above, degassing treatment for a long time is required at the time of steel making, which leads to an increase in manufacturing cost and a decrease in productivity, which is not economically preferable.

From the above points, the C content is in the range of 0.015 to 0.060%.

(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 the alloying hot dip galvanizing treatment conditions in the method of the present invention will be described.

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

Therefore, in order to awaken the proper winding temperature, the present inventor
The relationship between the coiling temperature and the mechanical properties of the galvannealed steel sheet was investigated by experiments.

In the experiment, C: 0.038%, Mn: 0.20% and sol
Al: melted Al killed steel containing 0.038%,
A slab is made by continuous casting, which is hot-rolled.
Standard winding temperature (630 ℃, 650 ℃, 670 ℃, 69
Coiled at 0 ° C, 710 ° C, 720 ° C),
The obtained hot-rolled steel sheet was pickled and then subjected to galvannealing treatment in a galvanizing line. In this case, the soaking temperature before plating and the alloying treatment temperature after plating are each 55
The temperature was 0 ° C and 540 ° C.

JIS from the obtained galvannealed steel sheet in the L direction
A No. 5 test piece was sampled and a tensile test was conducted. 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. 3, it can be seen that the higher the winding temperature, the lower the throat yield point and the tendency to increase the elongation. Especially, when the winding temperature is 690 ° C. or higher, the target tensile properties can be obtained. This is because the coil is wound in the shape of a coil near the A ₁ transformation point, which is higher in temperature than before, and the cementite becomes lumpy and coarse,
It is considered that the decrease in the interfacial area can suppress the re-solidification due to the reheating treatment in the galvanizing line 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 or the increase in the coil cooling time causes a decrease in productivity, so the temperature is preferably 740 ° C. or lower.

The finishing temperature is preferably higher than 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 during 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) alloying hot dip galvanizing conditions The alloying hot dip galvanizing treatment in the present invention is a lump in hot rolling,
Cementite that has been coarsened and made difficult to re-dissolve by soaking before plating or reheating during alloying treatment,
Further, the aim is to prevent the inconvenience caused by the solid solution C by preventing the cementite from forming a solid solution as much as possible by restricting the alloying hot-dip plating conditions, particularly the soaking condition and the alloying treatment condition. .

Therefore, the present inventor has a soaking temperature before galvanizing in a galvanizing line that affects the mechanical properties of hot-dip galvannealed steel sheet when hot-rolled steel sheet (coil) having the above cementite in a dispersed state is used as a material. And the influence of alloying treatment temperature was investigated.

In real rights, C: 0.038%, Mn: 0.21% and sol
Steel containing Al: 0.038% is melted, slab is formed by continuous casting, and then hot rolling is performed at 710 ° C.
It was wound into a coil. The obtained coil was subjected to galvannealing under various conditions shown in Table 1.

JIS from the obtained galvannealed steel sheet in the L direction
A No. 5 test piece was sampled and a tensile test was conducted. The relationship between the alloying hot dip galvanizing treatment conditions of each steel sheet and the yield point 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.

(A) and (b) of FIG. 4 show the relationship between the soaking temperature before galvanizing and the alloying treatment temperature, and the yield point and elongation, respectively. In either case, when the reheating temperature becomes low. The yield point is falling and the growth is rising. In particular, when both the soaking temperature before galvanizing and the alloying treatment temperature are 580 ° C. or less, the target characteristics are obtained.

Further, FIG. 4 (c) shows the relationship between the maximum reheating temperature in the galvanizing line and the yield point and elongation. From this, the yield point and elongation of the galvannealed steel sheet are It can be seen that there is a good correlation with the higher reheating temperature of the pre-soaking temperature or the alloying treatment temperature, whichever is higher.

Therefore, in order to obtain the low yield point and high elongation targeted by the present invention, it is necessary to set both the soaking temperature before galvanizing and the alloying treatment temperature to 580 ° C or lower.
This is because by reheating a steel sheet having cementite that has been lumped and coarsened by hot rolling at a lower temperature than before in a galvanizing line, it is possible to suppress re-dissolution of cementite, and the target properties are It is considered that it was obtained. As described above, it is preferable that both the soaking temperature before galvanizing and the alloying treatment temperature are low, but if the solidification temperature of zinc is lower than 460 ° C., the adhesion of the plating is deteriorated and the non-alloying of the plating layer is caused. Therefore, both are 460
Reheating above ℃ is unavoidable.

Note that the soaking time during reheating of the galvanizing line is the third
As shown in the drawing, the reheating temperature range according to the present invention has almost no effect on the yield point and the elongation, and thus is not particularly limited.

Further, it is optional whether temper rolling is performed in the galvanizing line or off-line for the purpose of preventing stretcher strain or correcting the shape. It is preferable to carry out in the range.

(F) Other conditions After hot rolling, the pickling treatment before galvanizing has no particular effect or influence on the mechanical properties of the alloyed hot-dip galvanized steel sheet obtained by the present invention, so the special conditions are Not 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 studies and experimental examples described, other embodiments are possible.

(Example) Steel having the chemical composition (wt%) shown in Table 2 was melted by a conventional 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 3. The finishing temperature was 885 to 910 ° C.

The obtained hot rolled coil was pickled, then subjected to galvannealing treatment in the galvanizing line under the conditions shown in Table 3 and temper rolling with an elongation of 1.0%.

The characteristics of the obtained galvannealed steel sheet are described in the third
Also listed in the table. In the table, the tensile property is J in the L direction from the steel sheet.
The IS5 test piece was sampled and the result of a tensile test was carried out. The aging index represents the amount of increase in the yield point after aging treatment with prestrain of 8% and heating at 100 ° C for 1 hour. It shows the manufacturing.

As is clear from Table 3, Steel No. A-, which is an example of the present invention.
1 and B-1 are both soft and have excellent ductility, and also have excellent aging resistance.

On the other hand, Comparative Examples Steel Nos. A-2 and No. B-2 have low coiling temperatures, and Steel Nos. B-3 and No. B-4 have soaking temperature before galvanization and alloying, respectively. Because the processing temperature is high,
It is considered that the amount of residual C after the alloying hot-dip galvanizing treatment is large, and as a result, all of them have a high yield point, a low elongation, and further have poor aging resistance. Further, among the comparative examples, Steel No. C has an excessively low amount of C, and similarly, the amount of residual solid solution C is too large to obtain the desired properties, and Steel No. D has C.
Since the amount of Mn is too large and the amount of Mn of No. E is too large, the strength is high, and the yield point is high and the elongation is low.

(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 treatment was carried out without hot rolling, the coiling temperature in hot rolling was set higher than before, and the soaking temperature before plating and the alloying temperature in the galvanizing line were set lower than before. It is possible to effectively suppress material deterioration due to heat treatment, and it is possible to manufacture an alloyed 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 having excellent breath formability. Moreover, since it can be manufactured without the need for rough rolling or long-term degassing under the formation of carbonitrides such as Ti and Nb, and further without the need for cold rolling, it is economical and improves productivity and effects. large.

[Brief description of drawings]

FIG. 1 is a diagram showing a general thermal cycle in a galvanizing line, FIG. 2 is a diagram showing the relationship between the C content of a galvannealed steel sheet and the yield point and elongation, and FIG. 3 is a diagram of hot rolling. FIG. 4 is a diagram showing the relationship between the take-off temperature and the yield point and elongation of the galvannealed steel sheet, and FIG. 4 is a diagram showing the relationship between the galvannealing treatment conditions and the yield point and elongation of the galvannealed steel sheet. ,
(a) shows the case of soaking temperature before galvanizing, (b) shows the case of alloying treatment temperature, and (c) shows the case of maximum reheating temperature.

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.
In the above, the material was wound into a coil shape, then, without cold rolling, preheated to a steel strip temperature of 460 ° C. or higher and 580 ° C. or lower, then hot-dip galvanized, and further 460 ° C. or higher and 580
A method for producing an alloyed hot-dip galvanized steel sheet for processing, which comprises subjecting a plated layer to an alloying treatment at a steel strip temperature of ℃ or less.