JPH02310354A - Manufacture of hot-dip galvanized steel sheet having excellent workability - Google Patents

Abstract

PURPOSE:To manufacture the hot-dip galvanized steel sheet having excellent workability at low cost by subjecting a steel contg. specified amounts of C, Mn and S to hot rolling at a specified temp., coiling it, heating the steel to a specified temp. as it is and hot-dip galvanizing the steel. CONSTITUTION:A steel contg., by weight, 0.02 to 0.08% C, 0.60 to 1.60% Mn and <=0.009% S is hot-rolled at the temp. of the Ar3 point or below and is coiled at <=600 deg.C coiling temp. into a coil shape. The hot rolled sheet is hot-dip galvanized without subjecting to cold rolling. At this time, the heating temp. for the steel strip before hot-dip galvanized is regulated to 650 to 750 deg.C. In this way, the hot-dip galvanized steel sheet having excellent workability of high strength-flanging properties and excellent tensile properties can be obtd. without executing cold rolling.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to the production of hot-dip galvanized steel sheets, which are produced by hot-dip galvanizing a hot-rolled steel sheet as an original sheet without cold rolling. In detail, the tensile strength is 38 to 50
kgf/mm2 hot dip galvanized steel sheet (e.g.
(equivalent to hot-rolled steel sheets for automobile structures in JIS)
The present invention relates to a method for producing a hot-dip galvanized steel sheet with excellent workability, which is suitable when higher press workability, specifically, low yield point, high elongation, and stretch flangeability are required.

(Prior Art) In recent years, hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets have been increasingly used for vehicle bodies such as automobiles and their structural members. In these applications, since the shape is complex and the steel sheet undergoes severe processing during press working, a hot-dip galvanized steel sheet with excellent formability is required.

Conventionally, the method for producing alloyed hot-dip galvanized steel sheets for such uses is to cold-roll a hot-rolled steel strip, either directly or after recrystallization annealing, and then continuously alloyed hot-dip galvanized steel sheet. A common method is to produce a steel sheet using a so-called cold-rolled steel sheet as a base sheet, in which the sheet is passed through a plating line (hereinafter referred to as a "galvanizing line") and subjected to dip plating and alloying treatment.

However, recently, demands for cost reduction have become stronger from customers, and hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that have excellent workability and are inexpensive are in demand. For this reason, instead of using a cold rolled steel sheet as the original sheet, it is pickled after hot rolling, but without cold rolling or subsequent recrystallization annealing.
A method of passing the sheet directly to the galvanizing line, that is, a method of reducing manufacturing costs by omitting part of the manufacturing process, has been studied and has been put into practical use in some cases.

However, conventional hot-rolled hot-dip galvanized steel sheets, which are obtained by passing hot-rolled steel sheets directly through a galvanizing line without cold rolling, are relatively thick steel sheets with a thickness of 3.2+++m or more. Alternatively, they are used only in applications where workability is not so demanding, and hot-rolled original hot-dip galvanized steel sheets that are thin and have excellent workability have not been manufactured to date.

Therefore, various improvements have been made to the manufacturing methods of hot-rolled hot-rolled hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets, which are thin and have excellent workability, but no effective method has yet been found. Not yet.

(Problems to be Solved by the Invention) Generally, in order to manufacture a hot-dip galvanized steel sheet, it is first heated and soaked in an oxidizing atmosphere in a galvanizing line.
Next, the temperature of the molten zinc (
After being maintained in a reducing atmosphere at about 460'C), it is immersed in a hot-dip galvanizing bath. In this case, in the heating and soaking process, for the purpose of recrystallization annealing or softening, the
It is customary to hold the temperature at 850'C. Furthermore, when performing alloying treatment for the purpose of improving paint adhesion of the product, the steel strip is further heated to about 500 to 700°C after hot-dip galvanizing. The above-mentioned hot-dip plating line is designed for cold-rolled steel sheets and includes a heating line for the target steel sheets, so it is a hot-rolled steel sheet that does not have any processed structure remaining and therefore does not need to be annealed. Even so, the temperature will inevitably rise during equipment operation.

Furthermore, from a special point of view, it is necessary to preheat the zinc to a temperature higher than the melting temperature (approximately 460°C) in order to ensure the adhesion of hot-dip plating, and it is also good when performing alloying treatment. In order to obtain good paint adhesion and workability of the plating layer, the iron concentration in the galvanizing must be controlled to an appropriate value, and for this purpose it is necessary to heat the steel strip to a temperature of approximately 550°C or higher. In any case, reheating the original plate has become an inevitable process.

However, it contains about 0.02 to 0.15% of C.

When the above heat treatment is applied to steel that does not contain carbide-forming elements such as Ti and Nb in an atomic weight greater than or equal to the atomic weight of C, the cementite that has sufficiently precipitated during the slow cooling process after hot rolling and coiling is removed. A phenomenon occurs in which C re-dissolves as a solid solution as the temperature rises. The C re-dissolved in this way cannot be sufficiently precipitated as cementite due to rapid cooling in the latter half of the hot-dip galvanizing line, and at the product stage, before passing through the hot-dip galvanizing line (hereinafter referred to as "hot-rolled"). ) compared to
The amount of C dissolved in steel increases. Therefore, when comparing the properties of as-hot-rolled steel and after hot-dip galvanizing, the yield point tends to increase and the elongation tends to decrease. Naturally, the magnitude of the change in tensile properties as described above depends on the amount of C in the steel and the heating temperature in the plating line. C in the steel is precipitated with TiC, NbC, etc. during the hot rolling stage by the carbide forming elements of
If these carbides do not dissolve into solid solution again at the heating temperature in the plating line, the change in tensile properties before and after plating will be small, but in this case, since there is almost no solid solution of C in the steel, the strength of the grain boundaries will decrease. As a result, brittle fracture (intergranular fracture) occurs when an impact load is applied after molding or when deformation is performed at low temperatures. It is not preferable to reduce the amount of C because there is a risk that so-called "vertical cracks" may occur.

On the other hand, for hot-dip galvanized steel sheets having a tensile strength of approximately 38 to 50 kgf/+nm, stretch flangeability is one of the most important properties when press-formed into automobile suspension parts, etc. Methods for improving this have different uses, but for example,
For No. 20, steel containing 0.010 to 0.120% of C was used, and Ar.

After intentional hot rolling, by performing three stages of cooling,
It has been shown to improve stretch flangeability (hole expandability). However, this proposal relates to hot-rolled steel sheets, and does not take into account the change in material quality caused by passing through the hot-dip galvanizing line as described above. For example, when a hot rolled steel sheet is used as the original sheet, there is no need to perform recrystallization annealing, so the stretch flangeability is In addition, it is unclear how the tensile properties change.

The present invention has been made in order to solve the problems of the prior art described above, and provides a method for producing hot-dip galvanized steel sheets having high stretch flangeability and excellent tensile properties without cold rolling. The purpose is to

(Means for Solving the Problem) In order to achieve the above object, the present inventors first investigated the composition of steel and found that the stretch flangeability was significantly improved especially when C was 0.08% or less. I found that it can be improved. However, as mentioned above, when hot-rolled steel sheets are used as base sheets, there is no need to perform recrystallization annealing, so soaking before plating is usually carried out at a low temperature of about 550 to 600 degrees Celsius, and under these conditions, the above-mentioned It has been found that when hot-dip galvanizing is applied to a steel sheet with a C content of 0.08% or less, in addition to stretch flangeability, tensile properties, another important property, are significantly degraded.

Therefore, as a result of further intensive research into the composition of steel, manufacturing process conditions, etc., we have reduced the amount of C compared to conventional methods, and developed appropriate hot rolling conditions (especially coiling temperature) and heating temperature on the hot-dip galvanizing line. It was discovered that a hot-dip galvanized steel sheet having both high stretch flangeability and excellent tensile properties could be obtained by a combination of the following, and the present invention was thus made.

That is, the method for producing a hot-dip galvanized steel sheet with excellent workability according to the present invention includes c:o, o2~o, 08%, Mn:
Steel containing 0.60 to 1.60% and S:O, OO 9% or less is hot rolled at a temperature of Ar3 or higher, then wound into a coil at a coiling temperature of 600°C or lower, and then cold rolled. When performing hot-dip galvanizing without galvanizing, the heating temperature of the steel strip before hot-dip galvanizing is 650°C or more and 750°C or less.

The present invention will be explained in more detail below.

(Function) First, the important elements of the present invention, such as the amounts of C and Mn, the winding temperature, and the hot-dip galvanizing conditions, will be explained based on experimental results.

In the experiment, C: 0.01-0.12%, Mn: 0.68
A steel containing ~1.90% and S:O,009% or less was produced and cast into a mold to form a slab. These slabs were hot rolled at 3 or more Ar points to a thickness of 2.00 mm, and wound up at a temperature of 400 to 650°C. When hot-dip galvanizing the obtained hot-rolled steel sheet, the heating temperature before hot-dip galvanizing was varied in the range of 500 to 800°C.

JISS No. test pieces and hole expansion test samples were taken from each steel sheet as hot-rolled and after hot-dip galvanizing in the rolling direction, and stretch flangeability and tensile properties were evaluated. In addition, in the hole expansion test, after punching a hole with a diameter of 8 m + s, the tip angle was 6.
Expand the hole using a 0° conical punch, check the diameter (D, mm) when the largest crack penetrates the plate thickness, determine the hole expansion limit (λ) as shown below, and use this value to form a stretch flange. The gender was evaluated.

λ=-X 100 (%) (1) Amount of C First, the amount of C, which is an important constituent factor in the present invention, will be explained.

An example of the above experimental results is shown in FIGS. 1 and 2.

In addition, in the case of these figures, the heating before hot-dip galvanizing is 65
The temperature was 0 to 700°C.

FIG. 1 shows the hole expansion limits of steel plates having approximately the same strength. From FIG. 1, it can be seen that the yield limit improves as the amount of C decreases. As shown in the micrograph in Figure 3 (coiling temperature: 500°C), this occurs between the ferrite and the second phase because the hard second phase existing at the grain boundaries becomes smaller and fewer due to the decrease in the amount of C. This is thought to be because the voids formed by the cracks were small and the brackets were difficult to connect, which suppressed the occurrence and growth of cracks in the hole expansion test.

Therefore, in order to obtain excellent stretch flangeability, C
The amount needs to be below 0.08%, preferably below 0.06%.

However, if the amount of C is too low, C in Figure 2: 0°01%
As shown in the example of steel, when comparing the same -Mn content and the same strength, itemmo, c:o,
The tensile strength and elongation balance are lower than the 0.05% and 0.05% lines. This is because the C content is low, so all of the C in cementite dissolves into solid solution during heating during hot-dip galvanizing, and furthermore, during the subsequent rapid cooling, the supersaturation degree of C in ferrite is low, and precipitation as cementite is slow, resulting in It is thought that this is because the amount of solid solute C in the steel increases and the elongation decreases.

Therefore, in order to suppress such a decrease in elongation, the amount of C needs to be 0.02% or more.

Therefore, the amount of C is set in the range of 0.02 to 0.08%.

(2) Amount of Mn Mn is effective as a strengthening element for steel. In the present invention, since the main reinforcing elements are C and Mn, 0.60% or more of Mn is required to obtain the desired strength (38 kgf/+u+2).

On the other hand, as can be seen from Figure 4 showing the above experimental results, the hole expansion limit hardly decreases in the range where the Mn content is 1.60% or less. It is possible to strengthen steel without damaging it. As shown in the micrograph in Figure 3, this is due to Mn
:1.92% steel has a large amount of second phase, which is thought to reduce the hole expansion limit.

Therefore, in order to obtain excellent stretch flangeability, M
It is necessary that n is 1.60% or less.

Therefore, the Mn content is set in the range of 0.60 to 1.60%.

(3) Hot-dip galvanizing conditions Also, Figure 4 shows that the hole expansion limit is higher when the winding temperature is 550°C, and in order to obtain a high hole expansion limit, the winding temperature must be low. It is preferable.

On the other hand, FIG. 4, which shows an example of the above experimental results, shows the one-winding temperature (
(hereinafter abbreviated as rcTJ) and pre-hot-dip galvanizing heating temperature (hereinafter referred to as "pre-plating heating temperature"). From the same figure, when the pre-plating heating temperature is lower than 650°C and higher than 750°C, the elongation is greatly reduced and the yield point is high compared to the as-hot-rolled steel. However, the heating temperature before plating is 650℃ or more and 750℃
In the case of the following range, all steel types have higher elongation and lower yield point than the above-mentioned cases in which the steel was heated outside this temperature range, approaching the characteristic values of as-hot-rolled steel. The cause of this is not necessarily clear, but the heating temperature before plating is 650 to 75
This is considered to be because at 0° C., the amount of re-solid solution of C in cementite during heating was appropriate, so that precipitation of cementite progressed during subsequent rapid cooling.

Therefore, the pre-plating heating temperature is in the range of 650 to 750°C. Note that other conditions for hot-dip galvanizing are not particularly limited.

(4) Winding temperature On the other hand, for the 5th winding temperature CT, C in Figure 5: 0.05
%, Mn:0.80% As shown in the example of steel, the influence of the pre-plating heating temperature differs depending on the CT. In other words, when the pre-plating heating temperature is 700°C, the lower the CT, the closer the characteristic value is to the as-hot-rolled property.In this case, the lower the CT, the better the balance between strength and elongation.Therefore, the lower the CT, the better the balance between strength and elongation. In order to obtain the balance of , CT is 60
The temperature is preferably 0° C. or lower, and in this case, higher strength can be obtained with the same component composition. In other words, in order to obtain the same strength, a smaller amount of Mn is required, which is advantageous in terms of manufacturing cost.

(5) Others Next, other factors constituting the present invention will be explained.

■SS As is well known, when the S content is high, MnS inclusions increase and the hole expansion rate decreases. Therefore, it is preferable that it be as low as possible, but in the present invention, 0.00
If it is 9% or less, the negative effect is small, so 0.009
The upper limit is %.

■Hot rolling end temperature - When hot rolling is performed in the austenite and ferrite region, the ferrite undergoes processing, and this area remains as processed ferrite or becomes coarse grained due to heating before plating.
In either case, the stretch flangeability deteriorates, so the hot rolling end temperature is set to be equal to or higher than the Arm point.

In addition, in the present invention, the above points are essential components, and other points are not particularly limited, but for example, Si and AQ may be added for the purpose of increasing the strength of steel or deoxidizing during steel refining. In addition, since there is also the influence of P mixed as an unavoidable impurity, these will be explained below.

Si: The content of Si is desirably 0.2% or less. If the content exceeds 0.2%, red scale may occur during the hot rolling stage, and the red scale pattern remains even after pickling.
Striped patterns appear on the plating surface, deteriorating the surface appearance.
Significantly reduce product value. Furthermore, when red scale occurs, the plating adhesion of the scale-generated portion deteriorates, so from this point of view as well, it is desirable to suppress the Si content as much as possible.

AQ: AQ is an element added as a deoxidizing agent during steel refining, but if the amount added is less than 0.005%, deoxidizing is insufficient, and on the contrary, if it exceeds 0.10% In some cases, the deoxidizing effect is saturated and it is disadvantageous in terms of manufacturing costs, so 0.005%
In view of the above, it is desirable that the content be 0.10% or less.

P: P is effective as a reinforcing element, but its content is 0.0
If it exceeds 3%, the alloying rate will drop significantly when performing alloying treatment after hot-dip galvanizing, so 0.0
It is desirable that it be 3% or less.

N: If N is rolled at a low temperature of 600°C or less as in the present invention, it will form a solid solution in the steel and may deteriorate the aging resistance. Particularly from this point of view, a lower N content is advantageous. 0 because there is
．． 0050% or less b).

Others: The alloying treatment after hot-dip galvanizing is not particularly limited because it has almost no effect on tensile properties and stretch flangeability in the normal treatment temperature range (500 to 700°C).

Next, one embodiment of the present invention will be described. It goes without saying that the present invention is not limited to this example, and other embodiments are possible in addition to the various basic research and experimental examples described above.

(Example) Steel having the chemical composition (wt%) shown in Table 1 was melted by a conventional method, and after being tapped from a converter, it was made into a slab by continuous casting.
Next, the material was hot rolled to a thickness of 2+am at the hot rolling end temperature shown in Table 2, and wound into a coil at the winding temperature shown in Table 2. After pickling the obtained hot-rolled coil, it was heat-treated in a galvanizing line at the pre-plating heating temperature shown in Table 2, hot-dip galvanized, and temper-rolled with an elongation rate of 1.0%. did.

Various properties (tensile properties, elongation, stretch flangeability) of the obtained hot-dip galvanized steel sheets are also listed in Table 2.

In addition, in the table, the tensile properties are JISS in the rolling direction from the steel plate.
These are the results of a tensile test performed on a No. 1 test piece, and the stretch flangeability was evaluated using the hole expansion test method described above.

As is clear from Table 2, the examples of the present invention, Nα1, Ma6, and H9, all have a good balance between strength and ductility, and have a high hole expansion limit.

On the other hand, the comparative example Nα2 has a too low C content, so its elongation is inferior to that of the invention example &1.

Furthermore, since q, Mn, and S of Comparative Examples Nα3 to &5 are too high, the hole expansion limit is lower than that of Inventive Example Nα1.

In addition, the hot rolling end temperature of Nn 7 as a comparative example was too low, and Nn
Since the winding temperature in Sample No. 8 was too high, the balance between strength and elongation was inferior to Invention Examples & No. 6.

In addition, in Comparative Example Nα10, the heating temperature before plating was too high, and Nα10
Since α11 is too low, the balance between tensile strength and elongation is poor compared to the invention example Nα9.

[Blank 1 (Effects of the Invention) As detailed above, according to the present invention, a hot-dip galvanized steel sheet having high stretch flangeability and excellent tensile properties can be
Because it can be manufactured without cold rolling,
It is advantageous in terms of manufacturing cost and can withstand severe stretch flanging and drawing processes, so it can be applied to these uses and brings about industrially advantageous effects.

[Brief explanation of drawings]

Figure 1 is a diagram showing the effect of C content on the expansion limit of punched holes. Figure 2 is a diagram comparing the tensile strength and elongation balance of steels with different C and Mn contents. It shows the difference due to the difference in C content between the case where the tensile strength is constant and the case where the tensile strength is constant. Figure 3 (a), (b), and (C') are micrographs (1000x magnification) of the metal structure (microstructure) in the cross section in the rolling direction.
Figure 4 shows the influence of the amounts of C and Mn, and the string-like part surrounded by the black outline represents structures other than ferrite (pearlite, bainite, martensite, etc.). FIG. 5 is a diagram showing the effect of the amount of Mn on the spreading limit, and FIG. 5 is a diagram showing the effect of the heating temperature in the hot-dip galvanizing line on the tensile properties. Patent applicant Takashi Nakamura, Patent attorney representing Kobe Steel, Ltd. Figure 1 C quantity (A and 2?

Claims (1)

[Claims] In weight% (the same applies hereinafter), C: 0.02 to 0.08%,
After hot rolling a steel containing Mn: 0.60 to 1.60% and S: 0.009% or less at a temperature of Ar_3 or higher, 60
When winding into a coil at a winding temperature of 0°C or lower and then hot-dip galvanizing without cold rolling, the heating temperature of the steel strip before hot-dip galvanizing is 650°C or higher and 750°C or lower. A method for producing hot-dip galvanized steel sheets with excellent workability.