JPH01319660A - Manufacture of alloying hot dip galvanized steel sheet for superdrawing - Google Patents

Abstract

PURPOSE:To improve the productivity of a galvanized steel sheet by hot-rolling a steel containing specific weight percentages of C, S, and N and also containing Ti in an amount satisfying specific conditions, coiling the resulting hot rolled strip at a specific temp., and then carrying out treatments before and after hot dip galvanizing under respectively specified conditions. CONSTITUTION:C (0.001-0.09% by weight), <=0.015% S, 0.001-0.004% N, and further Ti in an amount simultaneously satisfying the conditions represented by inequalities I, II, III are incorporated to a steel. This steel is hot-rolled and then coiled at <=620 deg.C steel-strip temp., which is subjected to hot dip galvanizing, without cold rolling. At this time, soaking treatment before hot dip galvanizing and the alloying treatment of the plating layer after hot dip galvanizing are carried out under the conditions satisfying inequalities IV, V, respectively. By this method, the productivity of the galvanized steel sheet can be improved.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet with excellent press workability. The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet with excellent formability and longitudinal cracking resistance by hot-dip galvanizing without rolling.

(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, because the shape is complex, the steel sheet undergoes severe processing during press forming, so hot-dip galvanized steel sheets or alloyed hot-dip galvanized steel sheets with excellent formability are 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, demand for cost reduction has 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 omitting part of the manufacturing process and reducing the manufacturing cost I~, has been studied and has been put into practical use in some cases.

However, conventionally, hot-rolled hot-dip galvanized steel sheets obtained by passing hot-rolled steel sheets directly to a galvanizing line without cold rolling are relatively thick steel sheets with a thickness of 3.2 mm or more. In other words, hot-rolled hot-dip galvanized steel sheets with a thin plate thickness and excellent workability have not been manufactured to date.

Various attempts have been made to improve 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) In general, in order to manufacture alloyed hot-dip galvanized steel sheets, as shown in Figure 1, a general thermal cycle, the galvanizing line is first heated and soaked in an oxidizing atmosphere. Then, the temperature of molten zinc (46
After being maintained in a reducing atmosphere at a temperature of about 0° C., it is immersed in a hot-dip galvanizing bath. In this case, in the heating and soaking process,
The recrystallization annealing acid is approximately 700 to 85 for the purpose of softening.
It is customary to be held at 0°C. Furthermore, when performing alloying treatment for the purpose of improving paint adhesion of the product, the steel strip is further heated to approximately 500 to 700"C after hot-dip galvanizing. The equipment is designed for rolled steel sheets and includes a heating line for the target steel sheet, so even if it is a hot rolled steel sheet that has no processed structure left and therefore does not need to be annealed, the equipment can be used. "Operating"
Second, the temperature will inevitably rise.

In addition, even when viewed from a special point of view, the melting temperature of zinc (approximately 4-60'C
) It is necessary to preheat the zinc plating to a temperature higher than For this reason, it is necessary to heat the steel strip to a temperature of about 550° C. or higher, and in any case, reheating the original sheet is an inevitable process.

However, for example, when C is around 0.005 to 0.05%, T
If a guild hot-rolled steel sheet is heat-treated as described above, the cementite 1~, which is present in the steel during the slow cooling process after hot rolling and coiling, will increase. The phenomenon of solid solution re-dissolving occurs depending on the temperature. Steel that has undergone this re-dissolution of carbon undergoes considerable rapid cooling during the process of passing through the hot-dip plating line, especially in the later stages, so that the carbon that has been re-dissolved in solid solution is sufficiently precipitated again. This is not easy, and most of the re-dissolved carbon remains in the steel as a solid solution. Therefore, when comparing the properties of a steel sheet after hot rolling and coiling and a steel sheet after hot dipping, the yield strength of the latter increases and the elongation significantly decreases. At the same time, the latter's statute of limitations index increases,
Mechanical properties deteriorate due to aging. As a result of the comprehensive influence of these causes, a problem arises in that the formability of steel is greatly reduced.

In order to solve these problems, it is conceivable to control the C content in the steel to an extremely low level and to fix the residual C by adding a carbide-forming element such as 1. Nb. C in the steel sheet obtained in this way is TlC,N at the stage of hot rolled steel sheet.
These carbides are precipitated as bC, and these carbides are hardly re-dissolved even in the heating and soaking process of the hot-dip plating line. Therefore, deterioration of the material quality and workability after passing through the hot-dip plating line is prevented. However, in the case of steel without solid solution C, the strength of the grain boundaries is weakened, resulting in brittle fracture when an impact load is applied after forming the door-C or when deformation is performed at low temperatures. The so-called “
This is particularly problematic when this type of steel plate is used as a strength member. Further, even if a hot rolled steel sheet has excellent longitudinal cracking resistance, when hot-dip 1111 button plating is applied, the longitudinal cracking resistance may be significantly deteriorated depending on the heating temperature of the steel strip in the galvanizing line.

Conventionally, as mentioned above, C in steel was suppressed as much as possible, and Ti, N
Fix C in the steel by using b, etc. to improve the workability of hot-rolled steel sheets.
Various methods have been proposed.

For example, JP-A No. 49-134.509, No. 6]--7
There are No. 3836 and No. 50-14.1.51.7, etc., but these are all related to the workability of hot rolled steel sheets, and they do not deal with the change in material quality due to the hot-dip galvanizing line passing as described above. No consideration was given.

When manufacturing hot-dip galvanized steel sheets using hot-rolled steel sheets as base sheets as described above, hot-dip galvanizing on a galvanizing line may cause deterioration of tensile properties (increase in yield point, decrease in elongation), and In order to suppress this, it is necessary to reduce C dissolved in the steel at the product stage, and to reduce the longitudinal cracking resistance by applying hot-dip galvanizing to the steel on the galvanizing line. There were problems, and no measures were proposed to solve them.

In contrast, at the 1988 Spring Conference of the Iron and Steel Institute of Japan, the present inventors explained that in order to obtain good longitudinal cracking resistance for hot-rolled unalloyed hot-dip galvanized steel sheets using ultra-low CtR, It was reported that optimizing the amount of solid solution C or adding B is effective.

Furthermore, the present inventors have proposed a patent application filed in 1983 as a means to achieve this.
In No. 3-1-4.38, r of the content of C, N, S, Tj! By stipulating the amount of Tl that is effective for fixing A, especially the amount of Tl in steel, and achieving a component balance, it is possible to effectively prevent the deterioration of tensile properties due to hot-dip galvanizing. The coiling temperature after hot rolling is 620.
It was proposed that the longitudinal cracking resistance could be significantly improved by setting the temperature to 460° C. or lower and setting the maximum heating temperature in the galvanizing line to 460° C. or higher and 730° C. or lower.

However, in actual operations, as mentioned above, recrystallization annealing of cold-rolled steel sheets is often performed in galvanizing lines, and in particular, galvanizing lines containing Tl or Nb, which are commonly used as cold-rolled steel sheets for deep drawing, are used in galvanizing lines. Low carbon steel has a high recrystallization temperature;
High temperature annealing of 750°C or higher is performed. Therefore, when a small lot of hot-rolled steel sheets is processed spot-wise between these coils where high-temperature annealing is performed, the temperature is similarly raised to a high temperature range. The temperature may be raised to exceed the northern limit of the maximum heating temperature in the proposed galvanizing line.

In this case, of course, even if hot-rolled steel sheets from spora 1 to small lot 1 are processed between cold-rolled steel sheets.

There is no problem if the process is carried out at 700'C or so for annealed material, but in order to reduce product delivery times and in-process inventory, it is advantageous to have no constraints on processing opportunities as mentioned above. .

However, when a hot-rolled steel sheet is heated to such a high temperature range (0730° C.) on a galvanizing line, the longitudinal cracking resistance may be significantly deteriorated depending on the temperature rising conditions.

The present invention has been made in order to solve the problems of the prior art described above, and is aimed at solving the problems of the prior art.
The purpose of the present invention is to provide a method for manufacturing an alloyed hot-dip galvanized steel sheet for ultra-deep drawing, which can obtain good longitudinal cracking resistance even when the steel sheet is used, and which is not subject to processing opportunities on a galvanizing line. That is.

(Means for Solving the Problem) In order to achieve the above object, the present inventors have conducted repeated research on the composition of steel, manufacturing process conditions, etc., and as a result of the above proposal, C, N, By adjusting the content of S and Ti, especially by specifying the amount of 'r1 that is effective for fixing C in steel and achieving a component balance, it is possible to effectively prevent deterioration of tensile properties due to hot-dip galvanizing. By controlling the winding temperature after hot rolling, the soaking temperature before plating on the galvanizing line, and the corresponding soaking time under such component adjustment, vertical cracking resistance can be significantly improved. However, the present invention has been made here.

That is, the method for producing an alloyed hot-dip galvanized steel sheet for ultra-deep drawing according to the present invention can be summarized as follows: C:
0.00] -0 to 00009%, S: 0, O], 5% or less, N: O, O○10 to 0.004 ) After hot rolling, steel containing essential elements so as to simultaneously satisfy the conditions given by
When the steel strip is wound into a coil at a temperature of 620'C or less and then subjected to alloying hot-dip galvanizing without cold rolling,
It is characterized in that the soaking conditions before hot-dip galvanizing and the alloying treatment conditions of the plating layer after hot-dip galvanizing satisfy the conditions given by the following equations (4) and (5).

TjS0.08% (3)Q o
gt≦(], 2300/ (T +273))-10,
5-(4) 730<T≦800 (5) However, Ti amount: T effective for fixing C in steel Amount T: Soaking temperature before hot-dip galvanizing ("C) t:
Soaking time (seconds) The present invention will be explained in more detail below.

First, in order to suppress the deterioration of tensile properties "due to alloyed hot-dip galvanizing" (hereinafter simply referred to as "due to galvanizing") in a galvanizing line, which is the first objective of the present invention, The main objective is to suppress re-solid solution of cementite 1 to 1 through heating due to preheating or alloying treatment before galvanizing as described above, and the remaining in the solid solution state after cooling, so C1N, S, 1゛The problem is adjusting the content of

Therefore, in order to determine the optimal balance of the content of the above-mentioned components contained in steel, the present inventors investigated the relationship between the balance of these components and the tensile properties before and after hot-dip galvanizing.

Steel having the chemical composition shown in Table 1 is made into a slab by continuous casting, and then hot rolled to a plate thickness of 2.0 mm.
It was finished and wound into a coil. The second temperature is 910
~920°C, and the winding temperature was 510-530'C. After pickling this hot rolled steel sheet, it was hot-dip galvanized using a galvanizing line.The soaking temperature before plating was 7.
Soaking time at 50°C is 20 seconds.

JISS from the rolling direction before and after passing through the galvanizing line
A No. 1 test piece was taken and its mechanical properties were investigated.

Here, rj is T''jN even in the slab heating stage before hot rolling,
It is known that it precipitates as T'S, and in this case,
It is considered that T'' precipitated as TlN and TjS does not fix C in the subsequent cooling process. Therefore, the amount of Ti (hereinafter referred to as TlN
) can be expressed as . therefore,
Regarding the results obtained from the experiment, the horizontal axis shows C in steel.
The value C' is obtained by subtracting the C that can be fixed as TiC by TA (that is, the balance between C and Ti in the steel is
Figure 2 shows the relationship between this C and the yield point and elongation.

[Left below] From Figure 2, we can see that for fiNαI and Nun with low C', the change in tensile properties due to galvanizing is small, while for steel No., IIT, No. It can be seen that the deterioration of tensile properties due to zinc plating can be suppressed by setting C' to 0.0030% or more.

In other words, if the amount of effective T shown by 'r1' is greater than the amount obtained by subtracting 0.003% from the amount of C in terms of atomic stoichiometry (formula (1) below), the deterioration of tensile properties due to zinc plating is suppressed. It turns out that it can be done.

However, in order to improve longitudinal cracking resistance, which is the second objective of the present invention, the present inventors used steel that satisfies the above formula (1) to reduce the winding after hot rolling. The relationship between temperature, pre-plating scorching temperature and soaking time on a galvanizing line and longitudinal cracking resistance was investigated.

In the experiment, steel having the chemical composition shown in Table 2 was melted,
Slabs are made by continuous casting and finished at a temperature of 910 to 93.
After hot rolling at 0°C to a thickness of 2.0 mm, it was wound into a coil. After pickling the hot-rolled steel sheet, it was hot-dip galvanized using a galvanizing line. Sampling of the steel sheet was conducted before and after passing through the galvanizing line, and its longitudinal cracking resistance was investigated. For the vertical cracking test, a blank of 4.5 mmφ was punched out, drawn into a flat-bottomed cylinder (drawing ratio: 2.3), and then subjected to edge removal processing using a lathe, with a final drawing ratio of 2°. A cup-shaped molded product corresponding to No. 0 was prepared, held at -130'C to O'C for 10 minutes, and then a hole was expanded using a conical punch. Three to five cup molded products were tested at each holding temperature, and the temperature at which the incidence of vertical cracking (brittle cracking) was 50% was defined as the transition temperature.

[Margin below] FIG. 3 shows the relationship between the winding temperature after hot rolling (hereinafter simply referred to as winding temperature) and the longitudinal cracking transition temperature.

Soaking temperature and soaking time before plating are 750°Cl2, respectively.
It is O seconds.

As can be seen from the figure, the coiling temperature of steel NαA is 62
At 0°C, the longitudinal cracking transition temperature hardly changes, but when the winding temperature reaches 680°C, the longitudinal cracking transition temperature increases significantly. In addition, steel NαB is a conventional AQ killed steel with a large amount of C, and if the same or higher longitudinal cracking resistance is secured, the longitudinal cracking resistance can be improved by lowering C and adding T. This level can be considered to have been suppressed, and this level is indicated by diagonal lines in the figure.

Therefore, from this experimental result, in order to obtain good longitudinal cracking resistance, which is the second objective of the present invention, the winding temperature must be 6.
It can be seen that it is necessary to keep the temperature below 20''C.

FIG. 4 shows the vertical cracking resistance when the pre-galvanizing soaking temperature and soaking time of a material with a coiling temperature of 520'C are varied. In the figure, the case where the longitudinal crack transition temperature is 1100℃ or less is marked O, and -
The case where the temperature is 100°C or higher is indicated by an X mark. As aforementioned,
The vertical cracking transition temperature of conventional steel, low C-A of about 0.04% C, Q quilt steel is -'-00' (un degree), and if it shows a longitudinal cracking transition temperature lower than this temperature, It can be said that it has almost good longitudinal cracking resistance.

From the figure, it can be seen that good longitudinal cracking resistance can be obtained when the relationship between the pre-galvanizing soaking temperature (T'C) and the soaking time (in seconds) satisfies the following formula (4). In other words, when the soaking temperature exceeds 730'C, so that formula (4) is satisfied,
After soaking for a slightly shorter time than usual (soaking temperature 730° C. or less), good longitudinal cracking resistance can be obtained by performing galvanizing and alloying treatment.

n og t≦1.2300/((T+273))-1
0,5-(4) On the other hand, if the soaking temperature exceeds 800°C, it is disadvantageous because the manufacturing cost I~ increases;
C or less is preferable.

As mentioned above, the detailed reason why the vertical cracking resistance after plating changes depending on the winding temperature and pre-plating soaking conditions is unknown, but it is thought to be as follows.

As mentioned above, in order to suppress changes in tensile properties due to reheating in the galvanizing line, C was reduced, and T
In the case of steel with addition of J, if there is too little C dissolved in the steel, the grain boundaries become purified and the grain boundary strength decreases, resulting in inferior vertical cracking resistance compared to normal steel with about 0.04% C. It is. It is thought that the decrease in the amount of solid C in the steel is caused by the precipitation of Ti as TiC. Usually, during the slough heating stage (approximately 1050 to 12006C) before hot rolling, most of TiC is in solid solution, and if this is wound into a coil shape at a high winding temperature of 670℃ or higher after hot rolling, then In the slow cooling process of
As mentioned above, this TjC does not re-dissolve as a solid solution when reheated (below 800°C) in the galvanizing line, and therefore, the C dissolved in the steel does not form a solid solution.
It is considered that the vertical cracking resistance is poor. However, the present inventors reduced C and added Ti in order to suppress the deterioration of tensile properties in the galvanizing line, and furthermore, the amount of T added was several times that of C in terms of atomic equivalent theory. Even with steel, it is possible to obtain good longitudinal cracking resistance if the coiling temperature is below 620°C.In other words, in this case, the precipitation of TiC can be suppressed to some extent, and the It is thought that the solid solution C was able to remain in the solid solution.

Further, in the case of pre-plating soaking, precipitation of Tl-based carbides occurs in a high temperature range of over 730°C. However, by shortening the soaking time in this temperature range so as to satisfy equation (4) according to the soaking temperature, the precipitation of T]-based carbides can be suppressed, so that an appropriate amount of It is thought that the amount of solid solute C is secured and good longitudinal cracking resistance is obtained.

The above is an explanation of the experimental results that led to the present invention, and the present invention was completed by further examining the details of the adjustment of chemical components based on the findings obtained thereby.

The limiting conditions of each factor constituting the present invention will be described below.

First, the reason for limiting the chemical components in the present invention will be explained.

(a) C In order not to deteriorate the formability after hot-dip galvanizing, it is important that the amount of solid solute C after galvanizing is small. The amount of solid solute C is the amount of C in steel and T, which is a carbide-forming element.
Determined by the amount of Therefore, as the amount of Tj added increases, the allowable C content also increases, but as the C content and the amount of Ti added increase, carbides increase and the ductility of the steel deteriorates. The upper limit of C content is 090
09%, and the amount of Tj added is also limited to a certain value or less as will be described later.

(b) Ti Ti is a carbide-forming element, and since this carbide is not re-dissolved in the heating and soaking process of the hot-dip plating line, Tj
It is thought that by adding , it is possible to reduce the amount of solid solution C after plating, and as a result, the deterioration of tensile properties due to galvanizing is small. However, the amount of C in the steel is 0.
If it is less than 0.030%, the change in tensile properties due to zinc plating is small, and according to the above formula (1), Ti' is less than O.

However, Ti precipitates as TjN and then 1''i S at high temperatures, and especially when TjS precipitates, the hole expansion properties of the steel sheet improve. The same amount of Tt is necessary (the following equation (2))
.

However, if the amount of T added increases, the ductility will decrease as described above. According to the research of the present inventors, Ti
It was found that if the content was ≦0.08%, no disadvantages due to a decrease in ductility would occur (the following formula (3)).

%Formula %(3) Since S forms a T] compound as mentioned above, TiC
It has the effect of lowering Tj required for the formation of . therefore,
In order to reduce the amount of Ti added from the viewpoint of reducing manufacturing costs, it is preferable to lower the amount of S and reduce the amount of Ti added. Therefore, it is preferable to limit S to 0.015% or less.

(d) Because N also forms a T3 compound like S, it is advantageous to reduce the production rate as low as possible, and therefore it can be kept at 0.004 Q% or less. preferable.

In addition to the above essential constituent elements, Mn and AQ can be added for the purpose of strengthening the steel or deoxidizing it during steel refining, and Sj, which is usually mixed as an unavoidable impurity,
Since there is also an influence of P and the like, preferred addition amounts or contents of these elements will be explained below.

(E) Mn Mn has the effect of suppressing hot brittle fracture caused by the presence of S. To obtain the effect of its addition, 0.05%
It is desirable to add more than 0.5%, but if it exceeds 0.5%, the moldability may deteriorate, so the amount of Mn added should be 0.5%.
% or less.

(f) Afl AQ is an element added as a deoxidizing agent during steel refining.
The amount added is 0.0 in order to improve the yield of
It is desirable that the ratio is 1% or more. However, if it exceeds Oi%, it causes so-called slipper flaws in the steel sheet, which is not preferable from the point of view of product cost reduction, so it is desirable to limit the amount of AI2 added to 0.1% or less.

(h) The content of "Sj S" is preferably 0.2% or less. If the content exceeds 0.2%, red scale may occur during the hot rolling process, and the red scale pattern will remain even after pickling.
Striped patterns appear on the plating surface, deteriorating the surface appearance.
Significantly reduce product value. Further, if red scale occurs, the plating adhesion of the scale-generated portion deteriorates, so from this point of view as well, it is preferable to suppress the S1 content as much as possible.

(H) P Since plating adhesion deteriorates if the P content is 0.05% or more, it is desirable that the content is 0.05% or less.

(2) Other hot rolling finishing temperatures are desirably below the Ar3 transformation point, but extremely low C
Steel recrystallizes even if it is slightly below the A r3 transformation point.
In this case, the temperature may be lower than about 850°C since it does not have a large effect on the yield point or elongation.

Further, the conditions are not particularly limited regarding the pickling treatment after hot rolling and before hot dip galvanizing treatment, since it does not particularly affect or influence the mechanical properties of the hot dip galvanized steel sheet obtained by the present invention.

In addition, regarding alloying treatment after hot-dip galvanizing, it does not particularly affect the yield point, elongation, or longitudinal cracking resistance within the normal treatment temperature range (500 to 700''C).
Not particularly limited.

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 (1%) shown in Table 3 was melted by a conventional method, and after being tapped from a converter, it was made into a slab by continuous casting. Next, the sheet was hot rolled to a thickness of 2 mm and wound at the winding temperature shown in Table 5. The finishing temperature is 880~
The temperature was 915°C.

After pickling the obtained hot-rolled coil, it was soaked in a galvanizing line at the soaking temperature shown in Table 5, hot-dip galvanized, and temper rolled with an elongation rate of 1.0%. .

Various properties of the obtained hot-dip galvanized steel sheet are also listed in Table 4. In the table, the tensile properties are JIS in the rolling direction from the steel plate.
A No. 5 test piece was taken and a tensile test was conducted.
Furthermore, Table 4 shows the mechanical properties of the obtained hot-dip plated steel sheets and the longitudinal cracking transition temperature determined by the longitudinal cracking test. For the vertical crack test, punch out a 145mmφ blank,
Flat-bottomed cylindrical drawing (drawing ratio: 2.3) was performed, and then a cup-shaped molded product with an equivalent final drawing ratio of 2.0 was produced using a lathe to create a cup-shaped molded product with a final drawing ratio of 2.0. 10 at °C
After holding for a minute, the hole was expanded using a conical punch. Three to five cup molded products were tested at each holding temperature, and the temperature at which the incidence of vertical cracking (brittle cracking) was 50% was defined as the transition temperature.

[Margin below] As is clear from Table 4, N which is an example of the present invention.

C-1 and E-1 have a low yield point and exhibit high elongation, as well as excellent longitudinal cracking resistance.

On the other hand, Comparative Example NαC-2 has a high winding temperature, and Comparative Example No. I! In l-2, since the soaking time is too long, Tl-based carbides precipitate in both cases, resulting in insufficient solid solution C in the steel, resulting in poor longitudinal cracking resistance.

In addition, in Comparative Example No. D, 00 or 0.0 shown in Table 3
The tensile properties deteriorate due to the soaking treatment before hot-dip galvanizing, and the resulting steel sheet has a high yield point and low elongation.

Furthermore, comparative examples NαF′ and NQG have TJ amount and C
Since the amounts are too large, the tensile properties are poor and they are not suitable for applications that require a high degree of workability.

(Effects of the Invention) As described in detail below-1, according to the present invention, the chemical components are adjusted in a well-balanced manner, and the winding temperature and the melting surface in the galvanizing line 4a) Soaking and melting before plating Since the alloying treatment conditions of the plating layer after galvanizing are controlled to appropriate conditions, the resulting alloyed hot-dip galvanized steel sheet has excellent tensile properties and solid solution C in the steel. It has excellent i[i (longitudinal cracking resistance) properties and can be manufactured stably, making it possible to apply it to more demanding applications (ultra-deep drawing) and making it possible to reduce cold rolling. Furthermore, it is economical and has a large productivity improvement because it can be manufactured without the need for hot-dip galvanizing in the same process as cold-rolled steel sheets that undergo high-temperature annealing.

[Brief explanation of the drawing]

Figure 2 shows a typical thermal cycle in a galvanizing line, and Figure 2 shows the relationship between the yield point and elongation of C5 steel, which takes into account the amount of N and S in the carbon balance. Figure 3 is a diagram showing the relationship between coiling temperature after hot rolling and vertical cracking transition temperature, and Figure 4 is a diagram showing the relationship between soaking temperature and soaking time before galvanizing and longitudinal cracking resistance in a galvanizing line. It is a figure showing a relationship. Figure 2

Claims (1)

[Claims] In weight% (the same applies hereinafter), C: 0.0010 to 0.00
90%, S: 0.015% or less, N: 0.0010-0
．． After hot rolling, a steel containing Ti and Ti as essential elements so as to simultaneously satisfy the conditions given by the following formulas (1) to (3) is coiled at a steel strip temperature of 620°C or less. When applying alloyed hot-dip galvanizing without cold rolling, the soaking conditions before hot-dip galvanizing and the alloying treatment conditions of the plating layer after hot-dip galvanizing are as follows (
4) A method for producing an alloyed hot-dip galvanized steel sheet for ultra-deep drawing, characterized by satisfying the conditions given by equations (5). Ti^*≧48/12 [C-0.0030]...(1
) Ti^*=Ti-48/14N-48/32S≧0...
・(2) Ti≦0.08%...(3) logt≦{12300/(T+273)}-10.5
...(4) 730<T≦800...(5) However, Ti^*: Effective amount of Ti to fix C in steel T: Soaking temperature before hot-dip galvanizing (℃) t : Soaking time (seconds)