JP2002012947A - Hot dip zinc-plated steel sheet with excellent strength- flangeability and production method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip zinc-plated steel sheet having excellent strength flange formation. SOLUTION: This high strength hot rolled steel sheet is a hot dip zinc-plated steel sheet containing, by mass, 0.04-0.1% carbon, 0.7% or less silicon, 1.3-2.3% manganese, 0.05% or less aluminum, 0.02-0.05% niobium, 0.07% or less phosphorus, 0.01% or less tin, 0.007% or less nickel and the balance substantially iron, and featuring the structure that pearlite or cementite with grain diameter of 5 μm or less is dispersed within ferrite grain boundary which is composed of a matrix of ferrite grains with the average grain diameter of 20 μm or less.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a hot-dip galvanized steel sheet having excellent stretch flangeability and used for a construction member, a machine structural member, an automobile structural member, and the like, and a method for producing the same.

[0002]

2. Description of the Related Art A hot-dip galvanized steel sheet based on a high-tensile steel sheet having a tensile strength of more than 440 MPa has both excellent rust resistance and high proof stress, and is therefore used for construction members, members for machine structural use, and automobile structures. It is widely used as an original plate for members and the like.

[0003] The galvanized steel sheets of this type include those having a hot-rolled steel sheet as a base and those having a cold-rolled steel sheet as a base.
Among them, those using a hot-rolled steel sheet as a base are advantageous in cost compared with those using a cold-rolled steel sheet as a base because a cold rolling step is omitted in manufacturing. For this reason, many techniques related to galvanized steel sheets having a hot-rolled steel sheet as a base have been disclosed.

[0004] By the way, a galvanized steel sheet having a hot rolled steel sheet as a base is required to have excellent workability, and particularly to have excellent stretch flangeability. JP-A-5-1059
No. 63 and JP-A-7-54051 disclose a method for producing a galvanized steel sheet having such excellent stretch flangeability.

[0005]

However, the manufacturing methods disclosed in JP-A-5-105963 and JP-A-7-54051 each use a run-out table on the exit side of a rolling mill in a hot rolling line. A special operation for precisely controlling the cooling pattern of the hot-rolled steel strip and controlling the winding temperature to a low temperature range of 550 ° C. or less is required. For this reason, it is extremely restricted in actual operation, and is not preferable in terms of material stability and yield.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a hot-dip galvanized steel sheet having excellent stretch flangeability, which can be manufactured without performing special operations, and a method of manufacturing the same. Is to provide.

[0007]

Means for Solving the Problems In order to achieve the above object, the present inventors have conducted intensive studies on the component composition of hot-rolled base steel sheet, hot-rolling conditions and galvanizing conditions. I got the knowledge. In other words, “Ferrite +” in which the structure of a hot-rolled steel sheet can be obtained under normal hot-rolling conditions
Even with the “pearlite” structure, the volume ratio of pearlite is suppressed by reducing the amount of C by maximizing the precipitation strengthening by NbC, and pearlite or cementite is added to ferrite grain boundaries by annealing after hot rolling. It has been found that, by forming a structure in which is finely dispersed and precipitated, a hot-dip galvanized steel sheet having both high tensile strength characteristics and excellent stretch flangeability can be realized.

[0008] The present invention has been made based on the above findings.

According to the present invention, there is provided a hot-dip galvanized steel sheet based on a high-tensile hot-rolled steel sheet.
0.04 to 0.1%, Si: 0.7% or less, Mn: 1.
3 to 2.3%, Al: 0.05% or less, Nb: 0.02
~ 0.05%, P: 0.05% or less, S: 0.01% or less, N: 0.007% or less, the balance being substantially iron, ferrite grains having an average grain size of 20 µm or less The present invention provides a hot-dip galvanized steel sheet excellent in stretch flangeability, characterized by having a structure in which pearlite or cementite having a particle size of 5 μm or less is dispersed in ferrite grain boundaries of a matrix composed of:

In this case, V: 0.02 to 0.2
It is preferable that the content be contained by mass%. In addition, Ti:
It is preferable to contain 0.02 to 0.1% by mass. Further, the galvanized film of the galvanized steel sheet is preferably alloyed.

The method for producing a hot-dip galvanized steel sheet according to the present invention comprises a hot rolling step of winding a hot-rolled steel strip into a hot-rolled coil, and a pickling step of unwinding and pickling the hot-rolled coil. And, in a continuous hot-dip galvanizing line, in a method for producing a hot-dip galvanized steel sheet having excellent stretch flangeability, comprising a plating step of galvanizing the pickled hot-rolled steel strip, wherein the hot rolling step comprises: The hot-rolled steel strip is wound up in a temperature range of 550 to 680 ° C, and in the plating step, the hot-rolled steel strip is uniformly heated and maintained in a temperature range of 720 to 850 ° C before galvanizing, and then cooled. In mass%,
C: 0.04 to 0.1%, Si: 0.7% or less, Mn:
1.3 to 2.3%, Al: 0.05% or less, Nb: 0.
02-0.05%, P: 0.05% or less, S: 0.01
%, And N: 0.007% or less, and the balance is characterized by producing a steel sheet substantially made of iron.

In this case, V: 0.02 to 0.2
It is preferable that the content be contained by mass%. In addition, Ti:
You may contain 0.02-0.1 mass%. Further, in the plating step, it is preferable to further perform an alloying treatment after galvanizing.

Hereinafter, the galvanized steel sheet according to the present invention will be described.

First, the function of each of the above components and the reasons for limiting the component ranges will be described. In addition, “%” in the following component ranges means “% by mass”.

(1) C: 0.04 to 0.1% C has a function of producing an appropriate amount of pearlite, cementite and NbC, a carbide of Nb described later, to secure necessary strength as a steel sheet. The amount is required to be 0.04% or more, but if it exceeds 0.1%, the volume ratio of pearlite or cementite increases, and the stretch flangeability may deteriorate.

(2) Si: 0.7% or less Si has a function of improving the ductility of a steel sheet. However, when the Si content exceeds 0.7%, although the ductility is further improved, the adhesion between the base steel sheet and the galvanized film may be reduced, or the surface appearance of the galvanized steel sheet may be impaired.

(3) Mn: 1.3 to 2.3% Mn has a function of securing the strength of the steel sheet. By containing Mn, the upper limit of the C content can be kept low, which contributes to suppressing an increase in the volume ratio of pearlite or cementite. Mn needs to have a content of 1.3% or more, but if the content exceeds 2.3%,
The above-mentioned pearlite or cementite is likely to be distributed in a layered form, which may lower stretch flangeability.

(4) P: 0.05% or less If the content of P exceeds 0.05%, the toughness of the welded portion may be deteriorated. Therefore, the content of P is set to 0.05% or less.

(5) S: 0.01% or less If the content of S exceeds 0.01%, the toughness of the welded portion may be deteriorated. Therefore, the content of S is set to 0.01% or less.

(6) Al: 0.05% or less, N: 0.0
7% or less Al and N have no problem as long as they are amounts contained in ordinary steel, and may be 0.05% or less and 0.07% or less, respectively.

(7) Nb: 0.5 to 3.5% Nb functions to finely precipitate ferrite grains,
Has the function of precipitating as C and strengthening the precipitation. Thereby, the required strength can be secured without increasing the C content, and it contributes to suppressing an increase in the volume ratio of pearlite or cementite. 0.02% Nb content
When it is less than the above, the function of precipitating the ferrite grains finely may not be sufficiently performed. In addition, the precipitation amount of NbC may decrease, and the precipitation strengthening may not be sufficiently performed.
On the other hand, when the Nb content exceeds 3.5%, the effect of fine precipitation of ferrite grains and precipitation strengthening by NbC is not only saturated, but also ductility may be deteriorated.

(8) V: 0.02 to 0.2% The present invention has an effect of promoting the precipitation of ferrite grains and finely dispersing pearlite or cementite at ferrite grain boundaries by containing V element. They found. The above function is particularly remarkable during cooling in the annealing treatment of a hot-rolled steel sheet, which will be described later. At the time of cooling, the solid solution C element is extruded to the ferrite grain boundaries, and promotes dispersion to the ferrite grain boundaries as pearlite or cementite. .

If the V content is less than 0.02%, the above function may not be sufficiently performed. On the other hand, 0.2
If the V content is more than 10%, the above-mentioned function may be saturated and ductility may be deteriorated.

(9) Ti: 0.02 to 0.1% Ti has a function of controlling the form of sulfide or assisting precipitation strengthening. For this reason, the fine precipitation and precipitation strengthening action of ferrite by the action of the Nb element can be enhanced by including the Ti element. When the Ti content is reduced to 0.
When the content is less than 02%, the above function becomes insufficient, and
%, The effect of the above function may be saturated and may not be achieved any more.

The base hot-rolled steel sheet of the hot-dip galvanized steel sheet according to the present invention contains each of the above-mentioned specific components and has a ferrite grain boundary of a matrix composed of ferrite grains having an average grain size of 20 μm or less, and having a grain size of 5 μm or less. A structure in which pearlite or cementite is dispersed.

The reason why the average particle size of the ferrite particles in the matrix is set to 20 μm or less is that if a matrix composed of ferrite particles having an average particle size of more than 20 μm is used, the stretch flangeability is deteriorated.

In a matrix composed of ferrite grains having an average grain size of 20 μm or less, a grain size of 5 μm
The reason why the structure is such that m or less of pearlite or cementite is dispersed can be understood from the characteristic diagram of FIG. 1 described below.

FIG. 1 shows the particle size (μm) of pearlite or cementite on the horizontal axis and the hole expansion rate (%) on the vertical axis, and shows the pearlite or cementite matrix in the ferrite particles having an average particle size of 20 μm or less. FIG. 4 is a characteristic diagram showing the results of an investigation on the effect of the grain size of the steel sheet on the stretch flangeability of a galvanized steel sheet. here,
The “hole expansion rate” is an index of stretch flangeability, and is determined by a test described below.

First, a test piece taken from the manufactured hot-dip galvanized steel sheet or alloyed hot-dip galvanized steel sheet
Punch a hole with a nominal hole diameter of 10 mm and a clearance of 12%. Next, the test piece is fixed horizontally with the side where the burr of the punched hole projects as the upper surface, and using a punch having a conical tip with a vertex angle of 60 degrees, gradually move the tip of the punch from the lower surface side of the test piece. Up vertically upward to expand the punched hole. Here, the hole diameter d (mm) at the time when the crack that has propagated from the end face of the hole penetrates the test piece plate thickness is measured. The hole expansion rate is calculated from the hole diameter increase rate 初期 (d
−10) / d｝ 100 (%).

In FIG. 1, the sample is a steel type A shown in Table 1.
To J and steel grades a to d, and were manufactured under hot rolling conditions and hot-dip galvanizing conditions shown in Table 2, respectively.

The white circles in FIG. 1 indicate steel types A, B, E,
The results when using V-free steel corresponding to G, J and V are shown, and the black circles show the results when using V-added steel corresponding to steel types C, D, F, H and I in Table 1. , Black triangles indicate the results when steel types a to d in Table 1 were used. In FIG. 1, the black circles denoted by the symbol A and the white circles denoted by the symbol I are structures in which cementite is precipitated in ferrite grains, whereas the other white circles, black circles, and black triangles are cementite or This is a structure in which pearlite is dispersed and precipitated at ferrite grain boundaries.

In FIG. 1, first, focusing on the steel type indicated by the black triangle, the hole expansion ratio is as low as 60% or less regardless of the grain size of pearlite or cementite. Was found to deteriorate. Next, paying attention to the steel type indicated by a white circle without a sign, in a region where the particle size of pearlite or cementite exceeds 5 μm, the hole expansion rate is less than 70%, and the stretch flangeability is poor. In the region where the particle diameter of cementite is 5 μm or less, the hole expansion rate is 100% or more, and it is found that the stretch flangeability is excellent. In addition, when paying attention to the steel types indicated by black circles without reference numerals, the pearlite or cementite particles are all distributed in a region of 5 μm or less, have a high hole expansion ratio of 80% or more, and exhibit excellent stretch flangeability. It has been found. On the other hand, steel types A and I in which cementite was precipitated in the ferrite grains had a low hole expansion rate of 60% or less, although the cementite particle size was 5 μm or less, and were found to be inferior in stretch flangeability.

As described above, a structure having a composition within a specific range, a matrix composed of ferrite grains having an average grain size of 20 μm or less, and pearlite or cementite having a grain size of 5 μm or less dispersed and precipitated at a ferrite grain boundary. A hot-dip galvanized steel sheet having a hot-rolled steel sheet as a base has high tensile strength and excellent stretch flangeability.

Next, a method for producing a hot-dip galvanized steel sheet having excellent stretch flangeability according to the present invention will be described.

In the production method according to the present invention, first, each of the basic components C, Si, Mn and Nb is converted into the above-mentioned components (1) to (4).
(3) contained in each component range shown in (7), and A
A slab is prepared by adjusting the components of l, N, P, and S to be equal to or less than the respective upper limits shown in the above (4) to (6) and adjusting the components so that the balance is substantially iron. In the step of preparing the slab, in addition to the above basic components of C, Si, Mn, and Nb,
Further, it is preferable to include one or two of V and Ti listed in the above (8) and (9).

The prepared slab is hot-rolled to a required thickness in a hot rolling line to form a hot-rolled steel strip, and the winding temperature is controlled by a winder within a range of 550 to 680 ° C. Winding and hot-rolled coils are obtained.

Next, the reason why the winding temperature is set in the temperature range of 550 to 680 ° C. will be described.

The reason why the winding temperature is controlled to the above-mentioned specific temperature range is to promote the precipitation of NbC to reduce the amount of C to be added for securing the strength, and to reduce the volume ratio of pearlite or cementite. It is. By promoting the precipitation of NbC, high tensile strength characteristics can be obtained and stretch flangeability can be improved. If the winding temperature is lower than 550 ° C., it becomes difficult to promote the precipitation of NbC, and the precipitation strengthening effect of NbC may be reduced. In addition, the amount of C to be added for securing strength tends to increase with this,
The volume ratio of pearlite or cementite may increase. On the other hand, when the winding temperature exceeds 680 ° C., the precipitation of NbC can be promoted, but the deposited NbC
May be easily coarsened, and the precipitation strengthening effect of NbC may be reduced.

In the present invention, hot rolling conditions other than the above-mentioned winding temperature are not particularly specified. However, the finish hot rolling is performed in a temperature range lower than the Ar ₃ transformation point, or the steel sheet after the completion of the hot rolling is removed. Unless an operation that significantly increases the particle size is performed, such as a slow cooling operation at a cooling rate of 10 ° C./sec or less,
There is no particular problem. In addition, as long as the above-mentioned winding temperature is controlled within the specified temperature range, an operation of rapidly cooling at a cooling rate of 100 to 300 ° C./sec within 1 second from the end of hot rolling, and a further reduction in the final hot rolling operation. It is also possible to perform a grain refinement operation for reducing the particle size, such as a combination of operations for increasing the particle size.

Next, the hot rolled coil obtained in the hot rolling line is unwound and pickled.

Next, in the continuous hot-dip galvanizing line, the hot-rolled steel strip after the pickling is uniformly heated and maintained in a temperature range of 720 to 850 ° C., and then subjected to an annealing treatment for cooling, followed by galvanizing. In this step, a heat treatment for alloying the galvanized film may be further performed.

By subjecting the coarse pearlite produced in the hot rolling process to solid solution again in the matrix by performing the above-mentioned annealing treatment before galvanizing, the pearlite is dispersed and precipitated as fine pearlite or cementite at the ferrite grain boundaries. Can be. As a result, the ferrite grain boundaries of the matrix composed of ferrite grains having an average grain diameter of 20 μm or less have a grain
It can be a structure in which the following pearlite or cementite is dispersed. Here, if the heating temperature at the time of the annealing treatment is set to a temperature range of less than 720 ° C., re-dissolution of coarse pearlite into the matrix becomes insufficient, and desired stretch flangeability (for example, 80% or more in hole expansion rate). Can not be obtained. On the other hand, when the heating temperature during the annealing treatment is in a temperature range exceeding 850 ° C., the pearlite reprecipitated tends to be coarsened, and the desired stretch flangeability cannot be obtained. In addition,
In the above annealing treatment, there is no great limitation on the cooling rate during cooling. However, if the cooling rate is extremely slow, for example, less than 1 ° C./sec, it is not preferable because fine dispersion of pearlite or cementite at the ferrite grain boundaries may be insufficient. Further, in the production method of the present invention, since the structure of the steel sheet is a structure in which pearlite or cementite is finely dispersed in a ferrite matrix, the heat history after cooling from a heating temperature during hot rolling to around 600 ° C. Has little effect. For this reason, the temperature condition of the hot-dip galvanizing bath in which the annealed hot-rolled steel sheet is immersed, the cooling condition after the galvanizing, and the like hardly affect the structure of the base steel sheet. This is the same even when alloying is performed after galvanizing.

In the present invention, when preparing a steel slab, either ingot making or continuous casting may be used. In the hot rolling step, continuous hot rolling may be performed by a hot rolling line in which a rough hot rolling bar is combined in the line, or the temperature may be raised to a temperature within 200 ° C. using an induction heater. No problem.

[0044]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the present invention will be described below.

(Examples) First, steel types A to J shown in Table 1 were used.
Steels of steel types a to d were tapped in a converter, and slabs were formed by continuous casting. Here, the composition of each of the steel types A to J is within the above-specified range. On the other hand, the composition of steel types a to d deviates from the specified content range of at least one type of component. Specifically, the composition of steel types a and b both has a C content less than the lower limit, and the composition of steel type c is M
The n content exceeds the upper limit, and the composition of steel type d has a Mn content exceeding the upper limit and Nb is not contained.

[0046]

[Table 1]

Next, using each of the above slabs, a hot rolled coil was produced in a hot rolling line. As the hot rolling conditions, as shown in Table 2, the heating temperature at the time of hot rolling of the slab was fixed, and the hot rolled steel strip obtained by hot rolling (thickness 2.
(3 mm). Next, the produced hot-rolled coil was unwound and pickled, and then, in a continuous hot-dip galvanizing line, a hot-dip galvanized steel sheet was manufactured by changing various heating temperatures during annealing before plating shown in Table 2. Furthermore, heat treatment was performed to heat the alloy to the alloying temperature range to alloy the galvanized film, thereby producing an alloyed hot-dip galvanized steel sheet.

Observation of the underlying structure of the manufactured hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet revealed that both had a structure in which pearlite or cementite was precipitated using ferrite as a matrix. In this microstructure observation, the average grain size of ferrite, the grain size of pearlite or cementite, and the distribution form of pearlite or cementite were examined.

The tensile strength TS (MP) of the produced hot-dip galvanized steel sheet and alloyed hot-dip galvanized steel sheet
a) was examined, and a test similar to that described with reference to FIG. 1 was performed, and the hole expansion rate (%) was examined.

The results of the above investigation are shown in Table 1 below.

[0051]

[Table 2]

As shown in Table 2, the winding temperature was 500 ° C.
The alloyed hot-dip galvanized steel sheet of Example 1 (steel type A) manufactured out of the specified range has a ferrite average grain size and a pearlite or cementite grain size within the specified range in the structure of the base hot-rolled steel sheet. However, it was found that the cementite had a distribution form in which it was precipitated in the ferrite grains, and the hole expansion rate was 50%, indicating that it had low stretch flangeability. Example 15 in which the winding temperature was 500 ° C. and the heating temperature during annealing was 600 ° C., deviating from a specific range.
The alloyed hot-dip galvanized steel sheet of (Steel Type I) also has a distribution form in which cementite is precipitated in ferrite grains, as described in Example 1 (Steel Type A) described above, and the hole expansion rate becomes 60%. It was found to have low stretch flangeability. Further, Example 4 (steel type D), Example 5 (steel type D), and Example 10 in which the heating temperature during annealing deviated from the specific range.
Each of the alloyed hot-dip galvanized steel sheets of (Steel type D) and Example 12 (Steel type F) has a distribution form in which pearlite or cementite is dispersed at ferrite grain boundaries, but all have a pearlite or cementite particle size of more than 5 μm. ,
The hole expansion rate was 40 to 65%, indicating low stretch flangeability. Further, Examples 17 to 20 manufactured using a slab having a composition not satisfying the specified component range.
Each of the alloyed hot-dip galvanized steel sheets (steel types a, b, c, and d) has a distribution form in which pearlite or cementite is dispersed at ferrite grain boundaries, but has a very low hole expansion rate of 25 to 40%. It was found to have stretch flangeability. In particular, it was found that the alloyed hot-dip galvanized steel sheet of Example 18 (steel type b) had a pearlite or cementite particle diameter of 6 μm exceeding the specified value, had a hole expansion rate of 25%, and exhibited extremely low stretch flangeability.

Examples 1 (steel type A), 4 (steel type D), 5 (steel type D), 10 (steel type D), 12 (steel type F), 15 (steel type I) and 17 to 17 20 (steel type a
Ｄd), Example 2 (Steel Type B), Example 3 (Steel Type C), Examples 6 to 9 (Steel Type D), Example 11 (Steel Type E), Example 13 (Steel Type G), Example 14 (Steel Type H) And, in the hot-dip galvanized steel sheet or the galvannealed steel sheet of Example 16 (steel type J), the pearlite or cementite is in a distribution form in which pearlite or cementite is dispersed at the ferrite grain boundary, and the particle size of pearlite or cementite is 5 μm or less. It was found that it had high tensile strength and also had high stretch flangeability with a hole expansion ratio of 80% or more. In particular, in Examples 12 to 16 (steel types C and D) and Example 19 (steel type H) manufactured using slabs containing the V element, the hole expansion rate was 10%.
It became 0% or more, and it turned out that it has very excellent stretch flangeability.

FIG. 2 is an SEM (scanning electron microscope) photograph showing the underlying structure of the galvanized steel sheet of Example 2 above. Note that a scale is added below the photograph in FIG.

In FIG. 2, a thin and light gray line indicates a grain boundary, and a black region surrounded by this line indicates ferrite grains. In addition, pearlite or cementite having a strong white color is interposed in some places along the grain boundaries. From this figure, it can be seen that the ferrite grains have an average grain size of 20 μm or less. In addition, it can be seen that pearlite or cementite has a particle size of about 3 μm, and has a distribution form dispersed in ferrite grain boundaries.

[0056]

As described above, according to the present invention, it is necessary to precisely control a cooling pattern on a run-out table in a hot rolling line and special operations such as winding a hot rolled steel sheet in a low temperature range. In addition, it is possible to provide a hot-dip galvanized steel sheet having excellent stretch flangeability and using a high-strength hot-rolled steel sheet as a base. For this reason, the manufacturing cost is significantly reduced and a high-quality galvanized steel sheet can be provided, so that the industrial value of the present invention is extremely high.

[Brief description of the drawings]

FIG. 1 is a characteristic diagram showing the results of an investigation on the effect of the particle size of pearlite or cementite on the stretch flangeability of a galvanized steel sheet in a matrix composed of ferrite particles having an average particle size of 20 μm or less.

FIG. 2 is an SEM photograph showing a base structure of the galvannealed steel sheet of Example 2.

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[Procedure amendment]

[Submission date] August 28, 2000 (2008.2.
8)

[Procedure amendment 1]

[Document name to be amended] Statement

[Correction target item name] 0020

[Correction method] Change

[Correction contents]

(6) Al: 0.05% or less, N: 0.0
07% or less Al and N are not problematic as long as they are amounts contained in ordinary steel, and may be 0.05% or less and 0.007% or less, respectively.

[Procedure amendment 2]

[Document name to be amended] Statement

[Correction target item name] 0021

[Correction method] Change

[Correction contents]

(7) Nb: 0.02 to 0.05% Nb functions to finely precipitate ferrite grains,
Has the function of precipitating as C and strengthening the precipitation. Thereby, the required strength can be secured without increasing the C content, and it contributes to suppressing an increase in the volume ratio of pearlite or cementite. 0.02% Nb content
When it is less than the above, the function of precipitating the ferrite grains finely may not be sufficiently performed. In addition, the precipitation amount of NbC may decrease, and the precipitation strengthening may not be sufficiently performed.
On the other hand, when the Nb content exceeds 0.05% , the effect of fine precipitation of ferrite grains and precipitation strengthening by NbC is not only saturated, but also ductility may be deteriorated.

[Procedure amendment 3]

[Document name to be amended] Statement

[Correction target item name] 0050

[Correction method] Change

[Correction contents]

Table 2 shows the results of the above examination.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Toshiaki Urabe 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Inside Nihon Kokan Co., Ltd. (72) Inventor Soto Kitano 1-2-1, Marunouchi, Chiyoda-ku, Tokyo No. Nippon Kokan Co., Ltd. (72) Kozo Harada, Inventor 1-1-2 Marunouchi, Chiyoda-ku, Tokyo Nippon Kokan Co., Ltd. (72) Shunsaku Node 1-2-1, Marunouchi, Chiyoda-ku, Tokyo, Japan Nippon Kokan Co., Ltd. F-term (reference) 4K037 EA01 EA05 EA15 EA16 EA18 EA19 EA23 EA25 EA27 EA31 EA32 EB09 FE02 FE03 FF02 FF03 GA05

Claims (8)

[Claims] 1. A hot-dip galvanized steel sheet based on a high-tensile hot-rolled steel sheet, wherein, by mass%, C: 0.04 to 0.1%, Si: 0.7% or less, Mn: 1.3. ~ 2.3%, Al: 0.05% or less,
Nb: 0.02 to 0.05%, P: 0.05% or less,
S: 0.01% or less, N: 0.007% or less,
The balance is substantially composed of iron, the matrix is composed of ferrite grains having an average grain size of 20 μm or less, and a structure in which pearlite or cementite having a grain size of 5 μm or less is dispersed at ferrite grain boundaries. Excellent galvanized steel sheet. 2. Further, in mass%, V: 0.02 to 0.2.
The hot-dip galvanized steel sheet according to claim 1, containing 2%. 3. The composition according to claim 2, wherein Ti: 0.02
3. The composition according to claim 1, which contains 0.1%.
2. A hot-dip galvanized steel sheet according to claim 1. 4. The galvanized steel sheet according to claim 1, wherein the galvanized film is further alloyed. 5. A hot rolling step of winding a hot-rolled steel strip into a hot-rolled coil, an pickling step of unwinding and pickling the hot-rolled coil, and A galvanizing steel sheet having excellent stretch flangeability, comprising: a galvanizing step of galvanizing the washed hot-rolled steel strip; wherein, in the hot rolling step, the hot-rolled steel sheet is heated in a temperature range of 550 to 680 ° C. In the plating step, the hot-rolled steel strip is subjected to 7
By uniformly heating and holding in a temperature range of 20 to 850 ° C. and then cooling, by mass%, C: 0.04 to 0.1%,
Si: 0.7% or less, Mn: 1.3 to 2.3%, Al:
0.05% or less, Nb: 0.02 to 0.05%, P:
0.05% or less, S: 0.01% or less, N: 0.007
% Of a hot-dip galvanized steel sheet having excellent stretch flangeability, comprising manufacturing a steel sheet containing at most 0.1% by weight and a balance substantially consisting of iron. 6. The method for producing a galvanized steel sheet according to claim 5, further comprising V: 0.02 to 0.2% by mass. 7. Ti: 0.02 to 0.1% by mass
7. The method according to claim 6, comprising: 8. The method according to claim 5, wherein in the plating step, an alloying treatment is further performed after galvanizing.