JP2549539B2 - Method for producing hot dip galvanized steel sheet for ultra deep drawing - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet having excellent press workability, and more specifically, a hot-rolled steel sheet as a base sheet The present invention relates to a method for producing a hot-dip galvanized steel sheet which is hot-dip galvanized without hot rolling and has excellent formability and vertical crack resistance.

(Prior Art) In recent years, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet has been widely used for a vehicle body such as an automobile or a structural member thereof. In these applications, since the steel sheet is subjected to severe processing during press working due to its complicated shape, 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 manufacturing a steel sheet using a so-called cold-rolled steel sheet as a base sheet, which is a steel sheet which is passed through a plating line (hereinafter referred to as a “galvanizing line”) and subjected to immersion plating and alloying treatment.

However, recently, there has been an increasing demand from customers for cost reduction, and there has been a demand for inexpensive hot-dip galvanized steel sheets and alloyed hot-dip galvanized steel sheets that are excellent in workability. 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 a 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 sheet obtained by passing the hot-rolled steel sheet directly to a galvanizing line without cold rolling is a relatively thick steel sheet having a plate thickness of 3.2 mm or more, Alternatively, it is only used for applications where workability is not so severe, and hot-rolled hot-dip galvanized steel sheets having a thin plate thickness and excellent workability have not been produced so far.

Therefore, various improvements have been attempted for the manufacturing method of the hot-rolled raw material hot-dip galvanized steel sheet and the alloyed hot-dip galvanized steel sheet, which have such a thin plate thickness and excellent workability, but still an effective method has been found. Not not.

(Problems to be Solved by the Invention) Generally, in order to produce a galvannealed steel sheet,
As shown in the general heat cycle in Fig. 1, in a galvanizing line, first, heating and soaking is performed in an oxidizing atmosphere, and then the 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 soaking process, the temperature is usually maintained at about 700 to 850 ° C. for the purpose of recrystallization annealing or softening. Furthermore, when performing alloying treatment for the purpose of coating adhesion of the product, after hot dip galvanizing,
Further, the steel strip is heated to about 500 to 700 ° C. The hot-dip galvanizing line is designed for cold-rolled steel sheet,
Since it includes the temperature rising line of the target steel sheet, even if it is a hot-rolled steel sheet that originally has no work structure and therefore does not need to be annealed, it will inevitably receive a temperature rise during equipment operation.

Even from a different point of view, it is necessary to preheat above the melting temperature of zinc (about 460 ° C) to secure the adhesion of hot dip coating, which is also good when alloying treatment is performed. It is necessary to control the iron concentration during galvanization to an appropriate value in order to obtain good coating adhesion and workability of the plating layer, and for this purpose it is necessary to heat the steel strip at about 550 ° C or higher, In any case, reheating the original plate is an unavoidable process.

However, for example, when an Al-killed hot-rolled steel sheet having a C content of about 0.005 to 0.05% and containing no carbide forming elements such as Ti and Nb is subjected to the heat treatment as described above, the steel is cooled in the slow cooling process after hot rolling and winding. Then, a phenomenon occurs in which the cementite that has sufficiently precipitated is re-dissolved by increasing the temperature. The steel in which the carbon is re-dissolved is subjected to considerable rapid cooling in the process of passing through the hot dip galvanizing line, particularly in the latter half of the process, so that the re-dissolved carbon is sufficiently precipitated again. This is not easy, and most of the re-dissolved carbon remains in the steel in the form of solid solution. Therefore, when the characteristics of the steel sheet after hot rolling and winding and the steel sheet after hot dip coating are compared, the yield strength of the latter increases and the elongation decreases significantly. At the same time, the latter has a high aging index and mechanical properties deteriorate due to aging. As a result of these factors exerting a comprehensive influence, there arises a problem that the formability of steel is greatly reduced.

In order to solve such a problem, it is conceivable to control the C content in steel to an extremely low level and add a carbide forming element such as Ti or Nb to fix the residual C. C in the steel sheet thus obtained is precipitated as TiC and NbC at the stage of the hot rolled steel sheet, and these carbides hardly re-dissolve in the heating and soaking process of the hot dip coating line. Therefore, deterioration of the material and workability after passing through the hot dip plating line is prevented. However, in the case of steel in which solid solution C does not exist as described above, the strength of the crystal grain boundaries is weakened, and as a result, brittle fracture occurs when an impact load is applied after forming or deformation at low temperature is performed. The so-called "vertical crack phenomenon" may occur, which is a particular problem when this type of steel sheet is used as a strength member. Furthermore, even if the hot-rolled steel sheet has excellent vertical crack resistance, when hot-dip galvanizing is applied, the vertical crack resistance may significantly deteriorate depending on the heating temperature of the steel strip in the galvanizing line.

Conventionally, various methods have been proposed for suppressing C in steel as much as possible and fixing C in steel with Ti, Nb or the like to improve workability of a hot-rolled steel sheet. For example, JP-A-49-1345
There are 09, 61-73836, 50-141517, etc.,
These are all related to the workability of the hot rolled steel sheet, and no consideration is given to the variation of the material due to the hot-dip galvanizing line threading as described above.

As described above, when a hot-dip galvanized steel sheet is produced using a hot-rolled steel sheet as a base sheet, deterioration of tensile properties (increased yield point, reduction of elongation) due to hot-dip galvanizing in a galvanizing line, and Various problems such as deterioration of longitudinal cracking resistance due to reduction of solid solution C in steel at the product stage in order to suppress and further hot dip galvanizing such steel in a galvanizing line There was no suggestion as to how to solve these problems.

On the other hand, the inventors of the present invention firstly set the content of C in the steel to 30 ppm or less in Japanese Patent Application No. 62-11107 with respect to deterioration of tensile properties due to hot dip galvanizing in a galvanizing line. Or, the C content in steel is 90ppm or less, and Ti, Nb
It has been proposed that C be fixed by the addition of carbide forming elements such as, and the C dissolved in steel should be 30 ppm or less. Further, in Japanese Patent Application No. 62-62063, C is reduced, Ti and Nb are added,
In addition to reducing the solid solution C in the steel, it was proposed to add B in order to suppress deterioration of vertical cracking resistance due to hot dip galvanizing treatment in a galvanizing line.

The present invention is based on the findings obtained in both of the above proposals, has excellent tensile properties, further improves longitudinal crack resistance, and can be supplied to applications where more severe processing can be applied. It is an object of the present invention to provide a method capable of stably producing a steel sheet.

(Means for Solving the Problems) In order to achieve the above-mentioned object, the present inventors have made the composition of steel,
As a result of earnest research on manufacturing process conditions,
By adjusting the C, N, S, and Ti contents, and especially by defining the Ti amount effective for fixing C in steel to balance the components, deterioration of tensile properties due to hot dip galvanization is effectively prevented. It has been found that the vertical crack resistance can be remarkably improved by controlling the coiling temperature after hot rolling and the preheating temperature before plating in the galvanizing line under such a component adjustment. It was made.

That is, the method for producing a galvanized steel sheet for ultra-deep drawing according to the present invention, in short, as an essential element, C: 0.0010
~ 0.009%, S: 0.015% or less, N: 0.0010 to 0.0040%, P: 0.
05% or less, including B: 0.0005 to 0.0045% as necessary,
Furthermore, after hot-rolling steel containing Ti as an essential element so that the conditions given by the following formulas (1) to (3) are simultaneously satisfied,
When the steel strip is wound into a coil at a temperature of less than 600 ° C, and then hot-dip galvanizing is performed without cold rolling, the maximum heating temperature of the steel strip in the hot-dip galvanizing line is 460 to 730.
It is characterized in that it is set to ° C.

Ti ≦ 0.08% (3) However, Ti ^* : Ti amount effective for fixing C in steel The present invention will be described in more detail below.

First, in order to suppress the deterioration of the tensile properties “due to the galvannealing treatment” (hereinafter simply referred to as “due to galvanization”) in the galvanizing line, which is the first object of the present invention, Since the main purpose is to suppress re-dissolution of cementite by preheating before galvanizing or heating by alloying treatment and remaining in a solid solution state after cooling, the content of C, N, S, Ti Adjustment is a problem.

Therefore, the present inventors investigated the relationship between these component balances and the tensile properties before and after hot dip galvanizing in order to investigate the optimum balance of the above component contents in steel.

Steel having the chemical composition shown in Table 1 was melted, continuously cast into a slab, further hot-rolled to a plate thickness of 2.0 mm, and wound into a coil. Finishing temperature is 910-920
The coiling temperature was 570 to 590 ° C. This hot-rolled steel sheet was pickled and then hot-dip galvanized by a galvanizing line. The preheating temperature before plating is 700 ° C.

A JIS No. 5 test piece was sampled from the rolling direction before and after passing through the galvanizing line to investigate the mechanical properties.

Here, Ti is known to precipitate as TiN and TiC even in the slab heating stage before hot rolling. In this case, TiN and TiC
It is considered that Ti precipitated as does not fix C in the subsequent cooling process. Therefore, the amount of Ti effective to fix C in steel (hereinafter defined as Ti ^* ) is Can be represented by Therefore, regarding the results obtained by the experiment, the abscissa indicates the Ti from C in steel to Ti ^*.
The value C ^* (that is, the value of the balance of C and Ti in the steel with the addition of N and S) is calculated by subtracting C that can be fixed as C. And the relationship between this C ^* and the yield point and elongation is shown in FIG.

From Fig. 2, it can be seen that in steels No. I and No. II with low C ^* , changes in tensile properties due to galvanization are small, but steels with high C ^*
In No. III and No. IV, the yield point increased and the elongation decreased due to zinc plating, and it can be seen that the deterioration of tensile properties due to zinc plating can be suppressed by setting C ^* to 0.0030% or less. In other words, if the effective Ti amount represented by Ti ^* is more than the amount obtained by atomically stoichiometrically subtracting 0.003% from the C amount (following formula (1)), it is possible to suppress the deterioration of the tensile properties due to zinc plating. found.

However, Next, in order to improve the vertical cracking resistance which is the second object of the present invention, the present inventors have used a steel satisfying the above formula (1) for winding temperature after hot rolling and zinc plating. The relationship between the preheating temperature before plating on the line and the addition of B and the vertical crack resistance was investigated.

In the experiment, steel having the chemical composition shown in Table 2 was melted and made into a slab by continuous casting, and the finishing temperature was 910 to 930 ° C.
After hot rolling to a plate thickness of 2.0 mm, it was wound into a coil. This hot-rolled steel sheet was pickled and then hot-dip galvanized by a galvanizing line.

Table 3 shows the winding temperature after hot rolling and the preheating temperature before plating.

Steel plates were sampled before and after passing through the galvanizing line to investigate vertical crack resistance. As a vertical cracking test, a 145 mmφ blank was punched, flat-bottomed cylindrical drawing (drawing ratio: 2.3) was performed, and after that, it was pierced with a lathe to produce a cup-shaped molded product with a final drawing ratio of 2.0. Then-
After holding at 130 ° C to 0 ° C for 10 minutes, hole expansion processing was performed with a conical punch. Three to five cup-molded products were tested for each holding temperature, and the temperature when the vertical crack (brittle crack) occurrence rate was 50% was taken as the transition temperature.

FIG. 3 shows the relationship between the coiling temperature after hot rolling (hereinafter, simply referred to as coiling temperature) and the vertical crack transition temperature. As can be seen from the table, the longitudinal crack transition temperature hardly changes when the coiling temperature is less than 600 ° C for both steel Nos. A and B, but the coiling temperature
At both temperatures of 680 ℃, the transition temperature of vertical cracks increases significantly. Steel No. C is a conventional Al-killed steel with a large amount of C, which is a conventional steel. If vertical cracking resistance equal to or higher than this is secured, vertical cracking resistance by reducing C and adding Ti It can be considered that the deterioration of the sex could be suppressed, and this level is shown by the diagonal lines in the figure.

Therefore, from this experimental result, in order to obtain good longitudinal crack resistance which is the second object of the present invention, the winding temperature is
Below 600 ° C.

FIG. 4 shows the relationship between the preheating temperature before plating and the vertical crack transition temperature. Steel No.A and B have preheating temperature of 730 ℃ before plating
Although the transition temperature of vertical cracks hardly changes up to, the transition temperatures of both steels greatly increase when the preheating temperature before plating is 750 ° C. Similarly to the above, the vertical crack transition temperature which is the object of the present invention is shown by the shaded portion. That is, in order to achieve the object of the present invention, the preheating temperature before plating needs to be 730 ° C or lower.

As described above, the reason why the vertical crack resistance after plating changes depending on the winding temperature or the preheating temperature before plating is not clear, but it is considered as follows.

As described above, C is reduced in order to suppress the change in tensile properties due to reheating in the galvanizing line, and
In the case of a steel containing Ti, if the amount of C dissolved in the steel is too small, the grain boundary is purified and the grain boundary strength is lowered, and the vertical crack resistance is
It is inferior to the steel of about 0.04% C. It is considered that the decrease of C dissolved in the steel is caused by the precipitation of Ti as TiC. Usually, the slab heating stage before hot rolling (approx.
(1050-1200 ℃), TiC is almost solid solution, and if it is coiled at a high coiling temperature of 670 ℃ or higher after hot rolling, it precipitates as TiC in the subsequent slow cooling process. In addition, this TiC does not re-dissolve on reheating in a galvanizing line (800 ° C or lower), and therefore, the amount of C dissolved in the steel remains small and the longitudinal crack resistance is inferior. Conceivable. However, the present inventors have decided to reduce C in order to suppress the deterioration of tensile properties in a galvanizing line, add Ti, and further add Ti in an atomic equivalent stoichiometric amount to several times that of C. However, even if the coiling temperature was less than 600 ° C, good longitudinal crack resistance could be obtained, that is, in this case, precipitation of TiC could be suppressed to some extent, and It is considered that the solid solution C could be left behind.

Similarly, regarding the temperature during the reheating treatment in the plating line, if this temperature is 760 ° C or higher, even if the heating time is short, the It is considered that the solute C precipitates as TiC during this reheating and the amount of solute C in the steel decreases, so that the longitudinal crack resistance is greatly deteriorated. Therefore, even if the preheating temperature before plating is 730 ° C or lower, the reheating temperature for alloying is high when the alloying treatment is performed thereafter (for example, 760
The present inventors have confirmed that the vertical cracking resistance is deteriorated, that is, in order to obtain good vertical cracking resistance in consideration of the alloying treatment as well, it is necessary to improve the vertical cracking resistance during the alloying treatment. Including
It is necessary to keep the maximum heating temperature of the steel strip in the hot dip galvanizing line at 730 ° C or lower.

Moreover, the effect of B on the vertical cracking resistance can be seen from the results of this experiment. That is, the addition of B further improves the vertical crack resistance of the hot-dip galvanized steel sheet. This effect is considered to be a combined effect of improving the vertical cracking resistance before plating and the following effects. That is, as shown in FIGS. 3 and 4, regardless of the presence or absence of B (steel Nos. A and B), vertical cracking occurs due to the galvanizing treatment even when manufactured under the hot rolling plating conditions according to the present invention. Although the transition temperature rises, when B is added, this change amount becomes small, and therefore, excellent longitudinal cracking resistance is exhibited even after hot-dip galvanizing. Although the detailed reason for the effect of B is unknown, it is considered that B has the effect of strengthening the grain boundary strength like C, and B is further diffused to the grain boundary by reheating during the plating treatment. It seems that the grain boundary strength is increased.

The above is a description of the experimental results that led to the present invention, but the present invention has been completed by further studying 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 reasons for limiting the chemical components in the present invention will be described.

(A) C In order not to deteriorate the formability after hot dip galvanizing, it is essential that the amount of solid solution C after plating is small. The solute C content is C in steel and Ti which is a carbide forming element.
It depends on the amount of. Therefore, if the addition amount of Ti increases, the allowable content of C also increases.
If the Ti addition amount increases, carbides increase and the ductility of the steel deteriorates. Therefore, the upper limit of the C content is set to 0.009%, and the Ti addition amount is also limited to a certain value or less as described later. .

(B) Ti Ti is a carbide forming element, and since this carbide does not re-dissolve in the heating and soaking process of the hot dip galvanizing line,
It is considered that the addition of Ti can reduce the amount of solute C after plating, and as a result, the deterioration of the tensile properties due to zinc plating is small. However, the amount of C in steel is 0.
When the content is less than 30%, the change in tensile properties due to galvanization is small, and Ti ^* becomes 0 or less according to the equation (1). However, Ti precipitates as TiN and then TiS at high temperature, especially Ti
When S precipitates, the hole expansion characteristics of the steel sheet improve.
Therefore, the amount of Ti that is atomically stoichiometrically the same as N and S in steel is necessary (the following equation (2)).

However, if the addition amount of Ti is increased, the ductility is lowered as described above. According to the research conducted by the present inventors, Ti ≦
It was found that if the content is 0.08%, the inconvenience due to the decrease in ductility does not occur (the following formula (3)).

Ti ≦ 0.08% (3) (C) S S forms a Ti compound as described above, and therefore has an action of lowering Ti necessary for forming TiC. Therefore, in order to reduce the Ti addition amount from the viewpoint of reducing the manufacturing cost, S
It is preferable to lower Ti ^* and Ti ^* . Therefore,
It is preferable to control S to 0.015% or less.

(D) N N also forms a Ti compound similarly to S, so it is advantageous in terms of manufacturing cost to make it as low as possible.
It is preferably set to 0040% or less.

(E) BB B does not necessarily have to be an essential element in the present invention, but since it has a function of further improving the vertical crack resistance of the hot-dip plated steel sheet, it can be added as necessary. To obtain such an effect of adding B, at least 0.0005%
The above addition amount is desirable, but if it exceeds 0.0045%, slab surface cracking may occur at the slab stage of the continuous casting process,
Since the product cost also increases, it is desirable to limit the B addition amount to 0.0045% or less.

In addition to the above essential constituent elements, Mn and Al can be added respectively for the purpose of steel strength or deoxidation during steel refining, and the influence of Si, P, etc. which are usually mixed as unavoidable impurities Therefore, the preferable addition amount or content of these elements will be described below.

(F) Mn Mn has an effect of suppressing hot brittle fracture caused by the presence of S. In order to obtain the effect of addition, the addition amount of 0.05% or more is desirable, but if it exceeds 0.5%, the moldability may deteriorate, so the addition amount of Mn is desirably 0.5% or less.

(G) Al Al is an element added as a deoxidizer during steel refining,
From the viewpoint of improving the yield of Ti, the added amount is preferably 0.01% or more. However, if it exceeds 0.1%, it causes so-called sliver flaws of the steel sheet, which is not preferable from the viewpoint of manufacturing cost reduction. Therefore, it is desirable to limit the added amount of Al to 0.1% or less.

(H) Si The Si content is preferably 0.2% or less. If the content exceeds 0.2%, red scale may occur in the hot rolling stage, and the red scale pattern remains even after pickling.Therefore, a striped pattern emerges on the plating surface, deteriorating the surface appearance and significantly increasing the commercial value. Lower. Furthermore, if red scale occurs, the plating adhesion at the scale generation part deteriorates,
From this viewpoint as well, it is preferable to suppress the Si content as much as possible.

(I) Pp If the content of P is 0.05% or more, the plating adhesion deteriorates, so the content is made 0.05% or less.

Next, the manufacturing process factors of the steel having the above chemical composition will be described.

(G) Winding temperature, reheating temperature in galvanizing line These are, as described above, required to be the conditions capable of suppressing precipitation of TiC in order to obtain good longitudinal crack resistance.
The coiling temperature and reheating temperature in the galvanizing line shall be less than 600 ℃ and 730 ℃ or less, respectively. It should be noted that no matter how low the winding temperature is, it does not particularly hinder the present invention, and the winding temperature may be around room temperature. However, the reheating temperature in the galvanizing line is the preheating before hot dip galvanizing to ensure the adhesion of the zinc.
Since it is necessary to heat above 0 ° C), the lower limit value is 460 ° C.

(L) Others Regarding the hot rolling finishing temperature, it is desirable that the Ar ₃ transformation point or higher be used. However, in the extremely low C steel that is the object of the present invention, recrystallization occurs even if it is slightly below the Ar ₃ transformation point, in this case,
Approximately 850, as it does not significantly affect yield point and elongation
It may be at least ° C.

Further, after hot rolling, for the pickling treatment before the hot dip galvanizing treatment, since it has no particular effect or influence on the mechanical properties of the hot dip galvanized steel sheet obtained by the present invention,
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, and other embodiments are possible in addition to the various basic researches and experimental examples described above.

(Example) A steel having the chemical composition (wt%) shown in Table 4 was melted by a conventional method, and after the steel was taken out from 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 5. 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 5 in a galvanizing line, a hot dip galvanizing treatment, and a temper rolling with an elongation of 1.0%.

Table 5 also shows various properties of the obtained hot-dip galvanized steel sheet. In the table, the tensile properties are the results of a tensile test of JIS No. 5 test pieces taken from the steel sheet in the rolling direction, and Table 5 shows the mechanical properties and longitudinal cracking test of the obtained hot dip plated steel sheet. The vertical crack transition temperature obtained by As a vertical cracking test, a 145 mmφ blank was punched out, flat-bottomed cylindrical drawing (drawing ratio: 2.3) was performed, and after that, it was processed by a lathe to produce a cup-shaped molded product with a final drawing ratio of 2.0. After holding at −160 ° C. to 0 ° C. for 10 minutes, hole expansion processing was performed with a conical punch. Three to five cup-molded products were tested for each holding temperature, and the temperature when the vertical crack (brittle crack) occurrence rate was 50% was taken as the transition temperature.

As is clear from Table 5, No. C-1 which is an example of the present invention
Has a low yield point, high elongation, excellent tensile properties, and excellent longitudinal crack resistance. Further, No. E-1 of the present invention example is a steel to which B is added, which has been subjected to an alloying treatment after hot dip galvanizing, but exhibits excellent tensile properties like No. C-1 and is more excellent. It has vertical cracking resistance, indicating that the method of the present invention can be applied to the case where alloying treatment is performed.

On the other hand, in Comparative Examples No. C-2 and No. E-2, the coiling temperature was high and TiC was precipitated, so that the amount of solid solution C in the steel was decreased, so that the longitudinal crack resistance was poor. ing. Comparative example
In No. C-3 and No. E-3, since the preheating temperature before plating is high, similarly, the amount of solute C in steel is insufficient and the vertical cracking resistance is poor.

In Comparative Examples Nos. D and F, C ^* shown in Table 4 is 0.00
It is more than 30%, in other words, since the amount of Ti added is small, the tensile properties are deteriorated by the preheat treatment before hot dip galvanizing, and the yield point of the obtained hot dip galvanized steel sheet is high.
And the growth is low. Comparative Example No. I is a normal Al-killed steel, but has poor tensile properties.

Further, Comparative Examples No. G and No. H each have an excessive amount of C and an excessive amount of Ti, respectively, and therefore are inferior in tensile properties and are not suitable for applications requiring high workability.

(Effect of the invention) As described in detail above, according to the present invention, the chemical components are adjusted in a well-balanced manner, and the winding temperature and the reheating temperature during plating are controlled to appropriate conditions. The steel sheet has both excellent tensile properties and excellent vertical cracking resistance properties because it has an appropriate solid solution C in the steel, and can be stably manufactured.
It can be applied to more severe applications (for ultra-deep drawing) and can be produced without cold rolling, which is economical and has a great effect of improving productivity.

[Brief description of drawings]

Fig. 1 shows a general heat cycle in a galvanizing line, and Fig. 2 shows the relationship between C ^* (value of N and S in the balance of C and Ti in steel) and yield point and elongation. FIG. 3 is a diagram showing the relationship between the coiling temperature after hot rolling and the hot-rolled steel sheet as it is and the vertical crack transition temperature of the hot-dip galvanized steel sheet, and FIG. 4 is the preheating temperature before coating and the hot-rolled steel sheet as it is. It is a figure which shows the relationship with the vertical crack transition temperature of a hot dip galvanized steel sheet.

Claims (2)

(57) [Claims] 1. In weight% (hereinafter the same), C: 0.0010 to 0.00
90%, S: 0.015% or less, N: 0.0010 to 0.0040%, P: 0.05%
After the hot rolling, steels containing the following, and Ti as the essential elements so that the conditions given by the following formulas (1) to (3) are simultaneously satisfied, are coiled into a steel strip at a temperature of less than 600 ° C. The hot-dip galvanized steel sheet for ultra-deep drawing is characterized in that the maximum heating temperature of the steel strip in the hot-dip galvanizing line is 460 to 730 ° C when the hot-dip galvanizing is carried out and then cold-rolled. Manufacturing method. Ti ≦ 0.08% (3) However, Ti ^* : Ti amount effective for fixing the C amount in steel 2. C: 0.0010 to 0.0090%, S: 0.015% or less, N:
0.0010 to 0.0040%, P: 0.05% or less, B: 0.0005 to 0.0045%
In addition, steel that contains Ti as an essential element so as to simultaneously satisfy the conditions given by the following formulas (1) to (3) is hot-rolled, and then the steel strip is coiled at a temperature of less than 600 ° C. When applying hot dip galvanizing without winding and then cold rolling, the maximum heating temperature of the steel strip in the hot dip galvanizing line is set to 460 to 730 ° C. Production method. Ti ≦ 0.08% (3) However, Ti ^* : Ti amount effective for fixing the C amount in steel