JP2001342541A - Hot dip galvanized steel sheet with high tensile strength and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a hot dip galvanized steel sheet which has a high r-value and is excellent in strain age-hardening property satisfying ΔTS: 40 MPa or more and plating properties, and provide its production method. SOLUTION: A slub contains C: 0.008% or below, Al: 0.005-0.020%, N: 0.0050-0.0250%, and further, one or more kinds of Nb, Ti and V, the contents of Si, Mn, P and S are also arranged, and N/(Al+Nb+Ti+V+B) satisfies 0.3 or more. The above slub is subjected, by turns, to rough rolling, finishing rolling at FDT800 deg.C or above, hot rolling process for rapidly cooling within 0.5s and taking up at 650 deg.C or below after finish rolling, treating process for forming internal oxide layer which is formed on hot rolling sheet, cold rolling process, annealing process for rapidly cooling to 550 deg.C or below after continuous annealing, plate treating process for forming hot dip galvanized layer, and alloying treating process, further.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-dip galvanized steel sheet suitable for use in an automobile body and a method for producing the same, and more particularly to a method of producing ductile, plated, and strain-age hardening characteristics manufactured by a continuous hot-dip galvanizing line. The present invention relates to a good high-strength hot-dip galvanized steel sheet and a method for producing the same. The steel sheet in the present invention includes a steel sheet and a steel strip. Further, the hot-dip galvanized steel sheet of the present invention can be applied as a structural member for construction and mechanical structures.

In the present invention, "excellent in strain age hardening characteristics" means that after pre-deformation with a tensile strain of 5%, a temperature of 170.degree.
When the aging treatment is performed under the condition of keeping the min, the increase in deformation stress before and after the aging treatment (hereinafter referred to as BH amount; BH amount = (yield stress after aging treatment) − (pre-deformation stress before aging treatment)) is 80.
MPa or more, and the amount of increase in tensile strength before and after the strain aging treatment (pre-deformation + aging treatment) (denoted as ΔTS; ΔTS
= (Tensile strength after aging treatment)-(tensile strength before pre-deformation)) is 40 MPa or more.

[0003]

2. Description of the Related Art In recent years, from the viewpoint of preserving the global environment,
There is a demand for improved fuel efficiency of automobiles. In addition, in order to protect occupants in the event of a collision, it is also required to improve the safety of the vehicle body, and weight reduction of the vehicle body and reinforcement of the vehicle body have been actively promoted. It is said that increasing the strength of component materials is effective in simultaneously satisfying both the weight reduction and strengthening of automobile bodies. Recently, high-tensile steel sheets have been actively used in automobile parts. Has become.

[0004] Since many automotive parts made of steel sheets are formed by press working, steel sheets for automotive parts are required to have excellent press formability. In order to realize excellent press formability, it is essential to secure high ductility in the first place. However, when high-tensile steel is used, since the strength is high, the shape freezing property deteriorates, ductility is insufficient, and problems such as cracking and necking occur during molding,
There is such a problem.

As a method for overcoming this problem, for example, in a cold rolled steel sheet for an outer panel, a very low carbon steel is used as a material.
There is known a paint bake hardening type steel sheet in which the amount of C finally remaining in a solid solution state is controlled within an appropriate range. This type of steel sheet is kept soft during press forming, secures shape freezing and ductility, and has a yield stress that utilizes the strain aging hardening phenomenon that occurs in the paint baking process at 170 ° C × 20 min after press forming. The aim is to secure the dent resistance by obtaining a rise. In this type of steel sheet, C is dissolved in the steel during press forming and is soft. On the other hand, after press forming, the solid solution C
Is fixed, and the yield stress increases.

[0006] For example, Japanese Patent Publication No. 61-45689 discloses
Due to the complex addition of Ti and Nb, JP-A-5-25549 discloses that
Disclosed are steel sheets each having improved paint bake hardenability by adding W, Cr, and Mo individually or in combination. However, since the steel sheets described in JP-B-61-45689 and JP-A-5-25549 utilize solid solution C and solid solution N in the steel, the steel sheet yields by paint baking after press forming. Although the stress rises, it cannot rise even with tensile strength, and when applied to a strength member, improvement in impact resistance after press molding cannot be expected.

In order to reduce the weight of parts, it is necessary to increase not only the yield stress due to strain aging due to paint baking treatment after press molding, but also to increase the strength characteristics when the deformation is further advanced. In other words, an increase in tensile strength after strain aging is desired. A steel sheet that is subjected to heat treatment after press forming to increase not only the yield stress but also the tensile strength,
Some have been proposed.

For example, Japanese Patent Publication No. 8-23048 discloses C:
0.02-0.13%, Si: 2.0% or less, Mn: 0.6-2.5%, so
l. Steel containing 0.10% or less and N: 0.0080-0.0250%
Reheat to 1100 ° C or higher, perform hot rolling to finish finish rolling at 850 to 950 ° C, and then cool at a cooling rate of 15 ° C / s or more.
There has been proposed a method for producing a hot-rolled steel sheet, which is cooled to a temperature of less than 0 ° C. and wound to form a composite structure mainly composed of ferrite and martensite. However, Japanese Patent Publication No. 8-23048
Although the tensile strength increases with the yield stress due to strain age hardening in the steel sheet manufactured by the technology described in the above-mentioned publication, since the coil is wound at an extremely low winding temperature of less than 150 ° C., there is a large variation in mechanical properties. There was a problem. In addition, there is a problem in that the variation in the amount of increase in the yield stress after the press forming-painting baking process is large, the hole expansion ratio (λ) is low, the stretch flange workability is reduced, and the press formability is insufficient.

Japanese Patent Application Laid-Open No. 10-183301 discloses that C:
By limiting to 0.01 to 0.12% and N: 0.0001 to 0.01%, and further adjusting the average crystal grain size to 8 μm or less, 80 MPa
A hot-rolled steel sheet having a high BH amount as described above and an AI value as low as 45 MPa or less has been proposed. However,
In the steel sheet described in JP-A-10-183301, since the texture of ferrite is randomized by austenite-ferrite transformation after finish rolling, it is difficult to obtain a high r value, and the steel sheet has sufficient deep drawability. However, there is a problem that press formability is reduced.

[0010] Also, Japanese Patent Publication No. Hei 8-23048,
Even if cold rolling and recrystallization annealing are performed using the hot-rolled steel sheet described in JP 83301 as a starting material, the same press formability and strain age hardening characteristics as those of a hot-rolled steel sheet are not necessarily obtained. This is because the microstructure changes from the hot-rolled state by cold rolling or recrystallization annealing, and strain is accumulated by cold rolling, and precipitates such as carbides and nitrides are easily formed by recrystallization annealing. This is because the amounts of solid solution C and solid solution N greatly change.

On the other hand, recently, from the viewpoint of improving corrosion resistance and occupant safety, it is soft and excellent in workability at the time of press forming, and can be made high in strength by heat treatment such as paint baking after working to increase the part strength. There is a demand for a hot-dip galvanized steel sheet having both improved properties and improved impact resistance. In response to such demands, for example, JP-A-10-310824 and JP-A-10-310847 disclose that C: 0.01 to 0.08%,
i: 0.005 to 1.0%, Mn: 0.01 to 3.0%, Al: 0.001 to
0.1%, N: 0.0002-0.01%, W, Cr, Mo
Contains a total of 0.05 to 3.0% of one or more of
Alternatively, there is disclosed an alloyed hot-dip galvanized steel sheet containing one or more of Ti, Nb, and V, and having a post-forming strength increasing heat treatment performance whose structure is mainly composed of ferrite or ferrite, and a method for producing the same. . As used herein, the post-molding strength increase heat treatment performance refers to the tensile strength after heat treatment, which is performed after heat treatment at 200 to 450 ° C. Is the performance that increases.

[0012]

SUMMARY OF THE INVENTION
In steel plates manufactured by the techniques described in Japanese Patent Application Laid-Open Nos. 10-310824 and 10-310847, it is necessary to perform the coating baking process at a temperature of 200 to 450 ° C., which is higher than the conventional (170 ° C.). However, there was a problem that productivity of the product was reduced and it became economically disadvantageous.

The present invention solves the above-mentioned problems of the prior art, has excellent ductility and a high r value, is excellent in press moldability, is excellent in strain aging hardening characteristics, and is excellent in impact resistance characteristics after parts are formed. It is an object of the present invention to propose a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having mechanical properties and excellent plating properties, and a method for stably producing such a galvanized steel sheet. The strain age hardening characteristics of the plated steel sheet of the present invention are as follows: BH amount: 80 MPa or more and ΔT
S: The target is 40MPa or more.

[0014]

Means for Solving the Problems In order to achieve the above objects, the present inventors have made intensive studies on improvement of mechanical properties and plating properties. As a result, N is used as a strengthening element, which has not been actively used in the field where high workability is conventionally required, and the large strain age hardening phenomenon developed by the action of the strengthening element is advantageously used to perform press forming. It has been found that it is possible to easily achieve both the improvement of the properties and the enhancement of the strength by the coating baking treatment after press molding.

Further, the present inventors, in order to advantageously utilize the strain age hardening phenomenon due to N, use the strain age hardening phenomenon due to N under the conditions of baking paint for automobiles or more aggressively with the heat treatment conditions after molding. Must be combined advantageously
Therefore, it is effective to control the microstructure of the steel sheet and the amount of solute N within a certain range by optimizing hot rolling conditions, cold rolling annealing conditions, cooling conditions before hot-dip galvanizing, and the like. Was found. Further, in order to stably develop the strain age hardening phenomenon due to N, in terms of composition, particularly, the Al content is set to N.
It was also found that it was important to control according to the content. In addition, the present inventors set the structure of the steel sheet to an average crystal grain size.
By using a ferrite phase of 20 μm or less, it has been found that N can be fully utilized without the conventional problem of deterioration due to aging at room temperature. Furthermore, it has been found that by using N as a strengthening element, a decrease in the r value is small and good workability with an average r value of 1.5 or more can be ensured.

It has also been found that in order to make the average crystal grain size of ferrite fine, it is preferable to make the composition containing Nb as an essential component and to perform hot rolling in a low temperature range. In addition, the present inventors have found that a decrease in plating property due to an increase in the amount of solid solution strengthening elements such as Mn, Si, and P is improved by forming an internal oxide layer on the surface layer of a steel sheet before hot-dip galvanizing. I found it.

The present invention has been completed by further study based on the above findings. That is, the first present invention is a hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on a steel sheet surface,
The steel sheet has an internal oxide on the ground iron side at the interface between the steel sheet and the hot-dip galvanized layer or the alloyed hot-dip galvanized layer, and the steel sheet has a C content of 0.008% or less in mass%,
1.0% or less, Mn: more than 1.0%, 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.005 to 0.020%, N: 0.00
50-0.0250%, Nb: 0.005-0.0045%, T
i: 0.005 to 0.070%, V: one or more selected from 0.005 to 0.10%, and the following formula (1): N / (Al + Nb + Ti + V) ≧ 0.3 (1) , N, Al, Nb, Ti, V: content of each element (% by mass)), containing 0.0015% or more of N in a solid solution state, the composition comprising the balance of Fe and unavoidable impurities, and the average crystal. It is a high tensile galvanized steel sheet excellent in strain age hardening characteristics and workability with a tensile strength of 440 MPa or more, characterized by having a structure having ferrite with a particle size of 20 μm or less,
In the first aspect of the present invention, in addition to the above composition, the following group A and group B: group A: Mo: 1.0% or less, Ni: 1.5% or less, Cu: 1.5% or less Species or two or more B group: B: 0.01% or less. One or two groups are contained, and the following equation (2) is used instead of the above equation (1). N / (Al + Nb + Ti + V + B) ≧ 0.3 (0.3) 2) (Here, N, Al, Nb, Ti, V, B: content of each element (% by mass)) may be satisfied.

Further, the present invention according to the second invention is characterized in that C: 0.
008% or less, Si: 1.0% or less, Mn: more than 1.0% 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.005-0.
020%, N: 0.0050-0.0250%, Nb: 0.00
5 to 0.45%, Ti: 0.005 to 0.070%, V: 0.005 to 0.
Contains one or more selected from 10%,
Alternatively, the following groups A and B: Group A: Mo: 1.0% or less, Ni: 1.5% or less, Cu: 1.5% or less Group B: B: 0.01% or less The following equation (2) N /
(Al + Nb + Ti + V + B) ≧ 0.3 (2) (where N, Al, Nb, Ti, V, and B: content of each element (% by mass)) A steel slab having a composition satisfying the following conditions is used.
As described above, the sheet bar is roughly rolled to form a sheet bar. The sheet bar is subjected to finish rolling at a finish-rolling exit temperature of 800 ° C. or more to form a hot-rolled sheet.After the finish rolling, cooling is started within 0.5 s. A hot rolling step of rapidly cooling at an average cooling rate of 40 ° C./s or higher and winding at a winding temperature of 650 ° C. or lower; an internal oxide layer forming step of forming an internal oxide layer on the hot rolled sheet; After pickling the hot-rolled sheet after the internal oxide layer forming step, the cold-rolling step is performed by cold rolling to form a cold-rolled sheet, and the cold-rolled sheet is subjected to 10 to 120 After performing continuous annealing for holding for s, the steel sheet is subjected to an annealing step of cooling to 550 ° C. or lower at an average cooling rate of 10 to 300 ° C./s, and a hot-dip galvanizing process is applied to the cold-rolled sheet after the annealing step to form a steel sheet surface. And a plating step of forming a hot-dip galvanized layer. This is a method for producing a high-strength hot-dip galvanized steel sheet excellent in strain aging characteristics and workability at 440 MPa or more.

Further, the present invention according to a third aspect of the present invention provides,
0.008% or less, Si: 1.0% or less, Mn: more than 1.0% 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.005-0.
020%, N: 0.0050-0.0250%, Nb: 0.005
~ 0.045%, Ti: 0.005 ~ 0.070%, V: 0.005 ~ 0.1
Contains one or more selected from 0%,
Alternatively, the following groups A and B: Group A: Mo: 1.0% or less, Ni: 1.5% or less, Cu: 1.5% or less Group B: B: 0.01% or less Formula (2) N / (Al + Nb + Ti + V + B) ≧ 0.3 (2) (where N, Al, Nb, Ti, V, and B: a steel slab having a composition satisfying the content of each element (% by mass)). A slab heating temperature: at least 1000 ° C., rough rolling is performed to form a sheet bar, and the sheet bar is subjected to finish rolling at a finish rolling exit side temperature of 800 ° C. or more to form a hot rolled sheet.
Cooling is started within s, quenched at an average cooling rate of 40 ° C./s or more, and wound at a winding temperature of 650 ° C. or less. Then, the hot-rolled sheet is pickled and then cold-rolled. A cold rolling step of forming a cold rolled sheet, an internal oxide layer forming step of forming an internal oxide layer on the cold rolled sheet, and pickling the cold rolled sheet after the internal oxide layer forming step, After performing continuous annealing at a temperature of not less than the crystal temperature and not more than 900 ° C for 10 to 120 seconds,
An annealing step of cooling at an average cooling rate of 10 to 300 ° C./s to 550 ° C. or lower, and a galvanizing step of performing a hot-dip galvanizing process on the cold-rolled sheet after the annealing process to form a hot-dip galvanized layer on the steel sheet surface. , Sequentially applied, tensile strength
This is a method for producing a high-strength hot-dip galvanized steel sheet excellent in strain aging characteristics and workability at 440 MPa or more. In the second and third aspects of the present invention, an alloying step of alloying a hot-dip galvanized layer may be further performed subsequent to the plating step. After the alloying process, the average cooling rate up to 300 ° C is 5-25 ° C /
s is preferred. Further, in the second and third aspects of the present invention, the hot-rolling step sets the slab heating temperature to 1000 ° C. or higher, completes rough rolling in a temperature range from 1000 ° C. to the Ar ₃ transformation point, and forms a sheet bar. 600 below the Ar ₃ transformation point on the sheet bar
A hot rolling process may be performed in which a hot rolled sheet is formed by performing finish rolling at a rolling reduction of 80% or more while lubricating in a temperature range of not less than ° C. to form a hot rolled sheet.

[0020]

BEST MODE FOR CARRYING OUT THE INVENTION The high-strength hot-dip galvanized steel sheet of the present invention is a hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of the steel sheet. The hot-dip galvanized steel sheet of the present invention has an internal oxide on the ground iron side at the interface between the steel sheet and the hot-dip galvanized layer or the alloyed hot-dip galvanized layer. The internal oxide is preferably formed at a depth of 2 to 10 μm from the viewpoint of suppressing the surface concentration of the plating inhibiting element. Due to the presence of the internal oxide, plating properties, particularly plating wettability, are improved. The internal oxide mainly contains oxides of Si, Mn, and Fe.

Next, the reasons for limiting the composition of the steel sheet used in the present invention will be described. Hereinafter, mass% in the composition
Is simply expressed as%. C: 0.008% or less C is preferably reduced as much as possible. The smaller the content, the more excellent the deep drawability and the advantageous the press formability. However, up to 0.008% is acceptable. Note that the solid solubility of C in the ferrite phase is lower than that of N, and the re-dissolution proceeds in the cold rolling process after the hot rolling process, so that the solid solution C in the crystal grains increases, thereby deteriorating the aging resistance at room temperature. Because it is easy to make
C is preferably set to 0.0030% or less.

Si: 1.0% or less Si is a useful element for increasing strength without lowering ductility, and is preferably contained at 0.01% or more. It is more preferably at least 0.05%. On the other hand, Si is also an element that inhibits wettability in the plating properties, and is limited to 1.0% or less. In addition, it is preferably 0.75% or less.

Mn: more than 1.0% and 2.0% or less Mn is a useful element for strengthening steel by solid solution strengthening.
It also has the effect of suppressing embrittlement due to S, and in the present invention 1.
Requires a content of more than 0%. On the other hand, a large content of Mn causes a decrease in the surface properties, ductility, and r value of the steel sheet. For this reason, Mn
Limited to 2.0% or less. In addition, it is preferably 1.50% or less.

P: 0.1% or less P is an element useful for strengthening steel by solid solution strengthening. To obtain this effect, it is desirable that P be contained in an amount of 0.01% or more. A phosphide is formed, and ductility and deep drawability decrease. Therefore, P is limited to 0.1% or less. In addition, it is preferably 0.08% or less.

S: 0.01% or less S is present as inclusions in steel, and it is preferable to reduce S as much as possible because it causes a decrease in ductility.
Up to 0.01% is acceptable. Therefore, S is limited to 0.01% or less. For applications requiring good ductility, 0.00
It is preferable that the content be 5% or less.

Nb: 0.005 to 0.045%, Ti: 0.005 to 0.07
%, V: one or more selected from 0.005 to 0.10% Nb, Ti, and V form nitrides or carbonitrides, refine crystal grains, and combine with solid solution C. NbC, TiC, VC
Is an element that contributes to improvement in workability and increase in strength. In the present invention, it is preferable that each of these elements contains 0.005% or more. On the other hand, Nb is 0.045%, Ti is 0.07%, and V is
If the content exceeds 0.10%, the ductility and the solute N decrease. Therefore, Nb is 0.005 to 0.045%, Ti is 0.005 to 0.07%,
V is preferably limited to the range of 0.005 to 0.10%.

Al: 0.005 to 0.020% Control of the Al content is particularly important in the present invention. Al
Is an element that acts as a deoxidizing agent and is effective in improving the cleanliness of steel. In the present invention, the content of 0.005% or more is required. On the other hand, since Al bonds with N, an excessive Al content reduces the amount of N in the solid solution state, making it difficult to secure solid solution N that contributes to the strain age hardening phenomenon. Further, even if solid solution N can be ensured, if Al exceeds 0.020%, the variation in strain aging hardening characteristics due to fluctuations in manufacturing conditions increases. Therefore, in the present invention, the Al content is limited to as low as 0.020% or less. In addition, from the viewpoint of improving strain age hardening characteristics, Al is 0.0
It is preferably set to 15% or less.

N: 0.0050 to 0.0250% N is an element that increases the strength of the steel sheet by solid solution strengthening and strain age hardening, and is the most important element in the present invention. In the present invention, by containing an appropriate amount of N and further controlling the manufacturing conditions, a necessary and sufficient amount of N in the solid solution state in the final product is ensured, whereby the strength by solid solution strengthening and strain age hardening is ensured. (YS, TS) Sufficiently exerts the effect of increasing the baking hardening amount (BH amount) of 80 MPa or more, and stabilizes the mechanical property requirements of the steel sheet of the present invention of increasing the tensile strength before and after coating baking treatment ΔTS40 MPa or more. Can be satisfied.

If N is less than 0.0050%, the above-mentioned effect of increasing strength is unlikely to appear stably. On the other hand, if N exceeds 0.0250%, material deterioration such as elongation becomes remarkable, the internal defect occurrence rate of the steel sheet increases, and slab cracking and the like during continuous casting occur frequently. Therefore, N is 0.0050 to 0.0250
%. Note that N is preferably in the range of 0.0070 to 0.0200% from the viewpoint of improving the stability and yield of the material and improving the strain aging hardening characteristics in consideration of the entire manufacturing process.

0.0015% or more of N in the solid solution state In order to ensure sufficient strength in the final product and to sufficiently exert strain aging hardening due to N, the N in the solid solution state in the steel is required.
(Also referred to as solute N) in an amount (concentration) of 0.0015% or more. Here, the amount of solute N is determined by subtracting the amount of precipitated N from the total amount of N in steel. As a method of analyzing the amount of precipitated N, according to the results of comparison between various analysis methods by the present inventors, it is optimal to determine the amount by the analysis method using electrolytic extraction. In this analysis method, the amount of N as a precipitate is determined by chemical analysis of a residue obtained by dissolving and extracting ground iron by constant potential electrolysis using acetyl-acetone as a solvent.

In order to obtain a higher BH amount and ΔTS, the amount of N in the solid solution is preferably 0.0020% or more, and in order to obtain a higher value, it is preferably 0.0030% or more. In order to stably maintain the solid solution N of 0.0015% or more regardless of the fluctuation of the manufacturing conditions, elements such as Al and N which strongly fix N are used.
It is necessary to limit the amounts of b, Ti, V, and B. In the present invention, the following equations (1) and (2) N / (Al + Nb + Ti + V) ≧ 0.3 (1) (where N, Al, Nb, Ti, V: content of each element (% by mass)) N / (Al + Nb + Ti + V + B) ≧ 0.3 (2) (where, N, Al, Nb, Ti, V, B: content of each element (% by mass) It is important to satisfy)). Either formula (1) or (2) is applied depending on the presence or absence of B. In addition,
Of the elements described in the formulas (1) and (2), the elements that are not contained are assumed to be 0, and the formulas (1) and (2) are calculated.

In the present invention, the following components can be contained, if necessary, in addition to the above components. Next group A and group B: Group A: one or more of Mo: 1.0% or less, Ni: 1.5% or less, Cu: 1.5% or less Group B: B: 0.01% or less 1 Group or Group 2 Group A: Mo, Ni, and Cu are all elements that increase the strength of steel by solid solution strengthening, and preferably contain one or more as necessary. Such an effect is remarkably recognized in Mo: 0.1% or more, Ni: 0.3% or more, and Cu: 0.3% or more. However, when the content exceeds 1.0% Mo, 1.5% Ni, and 1.5% Cu, the ductility decreases. Therefore, Mo: 1.0
%, Ni: 1.5%, and Cu: 1.5%, respectively.

Group B: B is an element that contributes to the refinement of the structure and the improvement in the resistance to secondary working brittleness, and can be contained as necessary. In particular, when Nb and Nb are contained in combination, a great effect of refining crystal grains is exhibited. Such effects become remarkable when the content is 0.0005% or more, and preferably 0.0010% or more. On the other hand, when the content exceeds 0.01%, precipitation of BN becomes remarkable, and the r value and the solid solution N decrease. Therefore, B is 0.01
% Is preferable. In addition, more preferably 0.00
It is between 05 and 0.0030%.

The balance Fe and inevitable impurities The balance other than the above-mentioned components is Fe and inevitable impurities. As inevitable impurities, O: 0.005% or less can be tolerated. Next, the structure of the steel sheet of the present invention will be described. The structure of the steel sheet of the present invention is a ferrite phase having an average crystal grain size of 20 μm or less.

In the present invention, as the average crystal grain size, a value calculated by a quadrature method specified in ASTM from a photograph of a cross-sectional structure,
The larger of the nominal particle sizes (see, for example, Umemoto et al .: Heat Treatment, 24 (1984), 334) determined from the cross-sectional structure photograph by the cutting method prescribed by ASTM is adopted. Although the plated steel sheet of the present invention secures a predetermined amount of solid solution N as a product, according to the results of experiments and studies conducted by the present inventors, even if the solid solution N amount is kept constant, the average crystal grain size It has been found that when the average particle size exceeds 20 μm, large variations occur in the strain aging hardening characteristics.
In addition, the deterioration of the mechanical properties when stored at room temperature becomes remarkable. Although the detailed mechanism is unknown at present, one of the causes of the variation in strain age hardening characteristics is the crystal grain size, and segregation and precipitation of alloy elements at the crystal grain boundaries, and furthermore, processing and heat treatment on these It is presumed to be related to the effect of Therefore, in order to stabilize the strain age hardening characteristics, the average crystal grain size of the ferrite phase needs to be 20 μm or less. In order to stably obtain a further increase in the BH amount and the ΔTS amount, the average crystal grain size of the ferrite is
It is preferably 15 μm or less.

The galvanized steel sheet of the present invention having the above composition and structure has a tensile strength of 440 MPa or more and an average
With a value of 1.5 or more, it is a plated steel sheet with excellent workability and excellent strain aging hardening characteristics. In the present invention, “excellent in strain age hardening characteristics” means that, as described above, after pre-deformation with a tensile strain of 5%, when subjected to aging treatment at a temperature of 170 ° C. for 20 minutes, the BH amount is 80 MPa or more. Which means that ΔTS is 40 MPa or more.

When defining the strain age hardening characteristics, the amount of pre-strain (pre-deformation) is an important factor. The present inventors have investigated the effect of the amount of pre-strain on the strain age hardening characteristics, assuming a deformation mode applied to a steel sheet for automobiles. As a result, the deformation stress in the deformation mode is extremely deep drawing. Except in the case of (1), it is possible to arrange by the amount of strain (tensile strain) equivalent to one axis. In actual parts, the amount of strain equivalent to one axis is more than 5%. It has been found that they correspond well to the strengths (YS and TS) obtained later. Based on this finding, in the present invention, the pre-deformation of the strain aging treatment was set to 5% tensile strain.

The conventional paint baking condition is 170 ° C. × 20
min is adopted as a standard. A large amount of solid solution N
When a strain of 5% or more is applied to the steel sheet of the present invention, the hardening is achieved even with a milder (lower temperature) treatment, in other words, the aging condition can be broadened. In general, in order to increase the amount of curing, it is advantageous to hold at a higher temperature and for a longer time as long as the material is not softened by excessive aging.

Specifically, in the steel sheet of the present invention, the lower limit of the heating temperature at which the hardening becomes remarkable after pre-deformation is approximately 100 ° C. On the other hand, if the heating temperature exceeds 300 ° C., the curing hardens, and on the contrary, it tends to soften slightly, and the occurrence of heat distortion and temper color becomes conspicuous. If the holding time is about 30 seconds or more when the heating temperature is about 200 ° C., almost sufficient curing can be achieved. In order to obtain even larger stable curing, the holding time is preferably 60 seconds or more. However, if the holding time exceeds 20 minutes, not only no further curing can be expected, but also the production efficiency is significantly reduced, which is disadvantageous in practical use.

From the above, according to the present invention, the aging treatment condition is the heating temperature of the conventional paint baking treatment condition of 170 ° C.
It was determined that the evaluation was performed at 20 ° C. and a holding time of 20 min. Low-temperature heating and
Even under the condition of aging treatment for a short time, large hardening is stably achieved in the steel sheet of the present invention. The method of heating is not particularly limited, and in addition to atmospheric heating using a furnace employed for normal coating baking, any of induction heating, heating using a non-oxidizing flame, laser, plasma, or the like can be preferably used.

It is necessary that the strength of automotive components be able to withstand complicated external stress loads. Therefore, not only the strength characteristics in a small strain range but also the strength characteristics in a large strain range are important for a material steel plate. . In view of this point, the present inventors set the BH amount of the steel sheet of the present invention to be used as a material for automobile parts to be 80 MPa or more and set ΔTS to be 40 MPa or more. More preferably, the BH amount is 100 MPa or more and ΔTS50 MPa or more. To increase the BH amount and the ΔTS amount, the heating temperature during the aging treatment may be set to a higher temperature and / or the holding time may be set to a longer time.

In addition, the steel sheet of the present invention is hardly subject to aging deterioration (phenomenon of increasing YS and decreasing El (elongation)) even if left undisturbed at room temperature for about one year. It has unprecedented advantages. Next, a method for producing the high-strength hot-dip galvanized steel sheet of the present invention will be described. The high-strength hot-dip galvanized steel sheet of the present invention uses a steel slab having the above composition, a hot-rolling step, an internal oxidation phase forming treatment step, a cold-rolling step, an annealing step, a hot-dip galvanizing step, or It is manufactured through an alloying process.

First, the hot rolling process will be described. Slab heating temperature: 1000 ° C or higher The slab heating temperature is set as the initial state, ensuring a necessary and sufficient amount of solid solution N, and the target value of solid solution N in the product (0.0015%
In order to satisfy the above conditions, the temperature is preferably set to 1000 ° C. or higher. If the temperature is lower than 1000 ° C., the rolling load increases, and rolling troubles are likely to occur. Note that the slab heating temperature is preferably set to 1280 ° C. or lower from the viewpoint of avoiding an increase in loss due to an increase in oxidation weight.

The steel slab heated to the above-mentioned temperature is then turned into a hot-rolled sheet by a hot-rolling step of performing hot rolling including rough rolling and finish rolling. In the present invention, the hot rolling step comprises:
Either the step A or the step B can be adopted. Step A: In step A, the steel slab is rough-rolled, subjected to finish rolling at a temperature of 800 ° C. or more on the sheet bar and finish-rolled to form a hot-rolled sheet, and then cooled within 0.5 s after finish rolling. And
Rapidly cooling at an average cooling rate of 40 ° C / s or more, winding temperature: 650 ° C
The winding step is as follows.

The slab heated under the conditions described above is turned into a sheet bar by rough rolling. The conditions for the rough rolling need not be particularly defined, but may be performed according to a conventional method. However, from the viewpoint of securing the amount of dissolved N, it is desirable to perform the treatment in as short a time as possible. Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet.

In the present invention, it is preferable that the successive sheet bars are joined between the rough rolling and the finish rolling, and the finish rolling is performed continuously. As a joining means, a pressure welding method,
It is preferable to use a laser welding method, an electron beam welding method, or the like. As a result, the existence ratio of the unsteady portion (the leading end and the trailing end of the material to be processed), which is likely to cause shape disturbance in the finish rolling and the subsequent cooling, is reduced, and the stable pressure is extended (the continuous length that can be rolled under the same conditions). ) And the stable cooling length (continuous length that can be cooled while applying tension) increase the product shape / dimensional accuracy and yield.

Further, in the conventional single-roll rolling for each sheet bar, lubricating rolling can be easily performed on thin materials and wide widths, which has been difficult to perform due to problems such as threadability and biting properties. And the life of the roll is extended. In the present invention, between the rough rolling and the finish rolling, one or both of a sheet bar edge heater for heating the width end of the sheet bar and a sheet bar heater for heating the length end of the sheet bar are used. Thus, it is preferable to uniform the temperature distribution in the width direction and the longitudinal direction of the sheet bar. Thereby, the material variation in the steel plate can be further reduced. It is preferable that the sheet bar edge heater and the sheet bar heater are of an induction heating type.

It is desirable to use the sheet bar edge heater to compensate for the temperature difference in the width direction. The heating amount at this time depends on the steel composition and the like, but is preferably set so that the temperature distribution range in the width direction on the finish-rolling exit side is approximately 20 ° C. or less. Next, the temperature difference in the longitudinal direction is compensated by the sheet bar heater. The heating amount at this time is preferably set such that the temperature at the end of the length is higher by about 20 to 40 ° C. than the temperature at the center.

Finish-rolling exit temperature: 800 ° C. or more The finish-rolling exit temperature FDT is set to 800 ° C. or more in order to make the structure of the steel sheet uniform and fine. When the FDT is lower than 800 ° C., the structure becomes non-uniform, and a partially processed structure remains. Such a residue of the processed structure can be avoided by setting the winding temperature to a high temperature. However, when the winding temperature is set to a high temperature, coarse crystal grains are generated and the amount of solute N is greatly reduced, so that it is difficult to obtain a target tensile strength of TS440 MPa or more. In order to further improve the mechanical properties, it is desirable that the FDT be 820 ° C. or higher. After finish rolling, it is desirable to cool the steel sheet early in order to refine the crystal grains and secure the amount of solute N.

Cooling after finish rolling: 0.5 after finishing rolling
In the present invention, cooling is started immediately (within 0.5 s) after finishing rolling, and the average cooling rate during cooling is set to 40 ° C./s or more. It is desirable. By satisfying this condition, Al
The high-temperature region where N is precipitated can be rapidly cooled, and N in a solid solution state can be effectively secured. If the cooling start time or cooling rate does not satisfy the above conditions, the grain growth proceeds too much to make the crystal grain size finer, and the precipitation of AlN due to the strain energy introduced by rolling is too advanced. Therefore, the possibility that the amount of solute N is deficient increases. From the viewpoint of ensuring the uniformity of the material and the shape, the cooling rate is preferably suppressed to 300 ° C./s or less.

Winding temperature: 650 ° C. or less The steel sheet strength tends to increase as the winding temperature CT decreases. In order to secure the target tensile strength of TS440 MPa or more, it is preferable that CT is 650 ° C. or less. Note that C
When T is less than 200 ° C., the shape of the steel sheet is easily disturbed, and there is a high risk of causing a problem in actual operation, and the uniformity of the material tends to be reduced. For this reason, CT is desirably 200 ° C. or higher. If more uniform material is required, the CT is preferably set to 300 ° C. or higher. The temperature is more preferably 450 ° C. or higher.

In the present invention, in finish rolling, lubricating rolling may be performed in order to reduce the hot rolling load. By performing lubricating rolling, there is an effect that the shape and material of the hot rolled sheet are made more uniform. The coefficient of friction during lubricating rolling is preferably in the range of 0.25 to 0.10.
The operation of hot rolling is further stabilized by combining lubrication rolling and continuous rolling. Step B Step B is to heat the steel slab heated as described above to 1000 ° C.
A sheet bar finished the rough rolling in a temperature range of to Ar ₃ transformation point, while lubricating subjected in a temperature range of 600 ° C. or higher than Ar ₃ transformation point to the seat bar, rolling reduction: subjected to 80% or more finish rolling This is a hot rolling process in which a hot rolled sheet is formed and then wound.

If the end temperature RDT of the rough rolling exceeds 1000 ° C., there occurs a problem that the crystal grains of the γ phase become large, while if it is less than the Ar ₃ transformation point, there is a problem that the α phase recrystallized grains become coarse. . For this reason, it is preferable that RDT be in a temperature range from 1000 ° C. to the Ar ₃ transformation point. Further, the finish rolling is preferably performed in a temperature range lower than the Ar ₃ transformation point and higher than 600 ° C. When the finish rolling temperature exceeds the Ar ₃ transformation point, there is a problem that the γ → α transformation proceeds after hot rolling and the texture is randomized, so that the r value decreases. On the other hand, when the temperature is lower than 600 ° C., the rolling load increases. I do. For this reason, the finish rolling is preferably performed in a temperature range of less than the Ar ₃ transformation point and at least 600 ° C. Further, by making the finish rolling a lubricating rolling, the rolling load is reduced, and the shape and material of the hot rolled sheet are made uniform. The coefficient of friction during lubricating rolling is preferably in the range of 0.25 to 0.10.

The rolling reduction of the finish rolling is preferably 80% or more from the viewpoint of forming a texture. If the rolling reduction is less than 80%, there is a problem that the r-value after cold rolling annealing decreases.
The hot rolled sheet after finish rolling is then wound up. At the time of winding, as the winding temperature CT decreases, processing strain accumulates and works effectively for {111} texture formation.
Is preferably 550 ° C. or lower. CT is 200 ° C
If it is less than 1, the shape of the steel sheet tends to be disordered, and there is a high risk of causing a problem in actual operation, and the uniformity of the material tends to be reduced. For this reason, it is desirable that the CT be 200 ° C. or higher. If more uniform material is required, C
T is preferably set to 300 ° C. or higher. The temperature is more preferably 450 ° C. or higher.

The hot-rolled sheet obtained in the above-mentioned step A or B is subjected to an internal oxide layer forming step and then to an acid washing and a cold rolling step, or to an acid washing and a cold rolling step. After the steps, an internal oxide layer forming step is performed. Cold Rolling Step The hot rolled sheet or the hot rolled sheet that has been subjected to the internal oxide layer forming step is usually pickled under known conditions and then subjected to cold rolling to obtain a cold rolled sheet. The conditions of the cold rolling need not be particularly limited, as long as they can be rolled to a desired size plate thickness, and may be generally known conditions. The rolling reduction is preferably 50% or more because recrystallization occurs in the subsequent annealing treatment.

Internal Oxide Layer Forming Step The internal oxide layer forming step is performed to form an internal oxide layer at a depth of 2 to 10 μm on the surface layer of a hot rolled sheet or a cold rolled sheet for the purpose of improving plating properties. . Specifically, the oxygen potential (P _O2 ) is set to 10 on the hot or cold rolled sheet (coil).
^It is preferable to form an internal oxide layer on the surface layer of the steel sheet by performing a heat treatment at 650 to 850 ° C. by batch annealing or continuous annealing in an atmosphere of ⁻³⁰ to 10 ⁻¹⁰ Pa. When the hot-rolled sheet is subjected to batch annealing, a nitrogen gas atmosphere may be used since the scale formed during hot rolling becomes an oxygen source during the internal oxide layer forming treatment. The ferrite grains may be coarsened during the internal oxide layer forming process.However, in the present invention, since Nb is contained, precipitates such as Nb carbonitride are formed during hot rolling or during the internal oxide layer forming process. The ferrite grains formed during heating do not become coarse.

Next, the cold-rolled sheet or the cold-rolled sheet which has been subjected to the internal oxide layer forming step is subjected to an annealing step. The cold rolled sheet that has undergone the internal oxide layer forming treatment step is usually subjected to pickling under known conditions before continuous annealing. Prior to the annealing step, the cold rolled sheet is preferably subjected to a generally known degreasing treatment.

In the annealing step, after continuous annealing at a temperature of not less than the recrystallization temperature and not more than 900 ° C. for 10 to 120 s, 55
Cool to 0 ° C or lower at an average cooling rate of 10 to 300 ° C / s. The annealing step is preferably performed in a continuous hot-dip galvanizing line. The continuous annealing temperature was higher than the recrystallization temperature. If the continuous annealing temperature is lower than the recrystallization temperature, recrystallization is not completed and the strength satisfies the target, but the ductility is low.
In order to further improve the formability, the continuous annealing temperature is preferably set to 700 ° C. or higher. On the other hand, if the continuous annealing temperature exceeds 900 ° C., nitrides such as AlN are precipitated, and the amount of solute N in the product steel plate becomes insufficient. For this reason, it is preferable that the continuous annealing temperature be equal to or higher than the recrystallization temperature and equal to or lower than 900 ° C.

The holding time at the continuous annealing temperature is preferably as short as possible from the viewpoint of refining the structure and securing a desired amount of solute N more than desired.
It is desirable to set it to s or more. If the holding time exceeds 120 s, it becomes difficult to refine the structure and secure the amount of dissolved N. For this reason, the holding time at the continuous annealing temperature is preferably in the range of 10 to 120 s. The atmosphere of the annealing treatment was H ₂
And a mixed gas atmosphere of N ₂ . The mixed gas of H ₂ and N ₂ is 3
It is preferable to use nitrogen gas containing 9% of H ₂ gas.

Cooling after soaking in continuous annealing is important from the viewpoint of refining the structure and securing the amount of solute N. In the present invention, the temperature range up to at least 550 ° C. after annealing is reduced by an average cooling rate of 10%. ~ 300 ° C / s. If the cooling rate is less than 10 ° C./s, it is difficult to secure a uniform and fine structure and a desired amount of solid solution N or more. On the other hand, if the cooling rate exceeds 300 ° C / s,
The uniformity of the material in the width direction of the steel plate is insufficient.

Then, a galvanizing process is performed on the cold-rolled sheet having undergone the annealing step to form a galvanized layer on the surface of the steel sheet. In the plating treatment step in the present invention, the hot-dip galvanizing is performed in the temperature range of 450 to 550 ° C. to form a hot-dip galvanized layer on the surface of the steel sheet, similarly to the conditions performed in the hot-dip galvanizing line. preferable. The zinc bath is preferably a Zn bath containing 0.10 to 0.15% Al. Needless to say, after the plating process, wiping for adjusting the basis weight may be performed as necessary.

After the plating process, the steel sheet is cooled.
In the temperature range up to 300 ° C. after the plating treatment, the cooling rate (average cooling rate) is preferably 5 to 25 ° C./s. Further, in the present invention, after the plating step, an alloying step of alloying the plating layer may be performed. The heating temperature in the alloying treatment is preferably 470 ° C. to (Ac ₁ transformation point). If the heating temperature is lower than 470 ° C., the progress of alloying is slow and the productivity is reduced. On the other hand, when the heating temperature is A
Beyond c ₁ transformation point, the plating layer becomes brittle alloying of the plating layer is too advanced. Therefore, in the present invention, the heating temperature of the alloying treatment is preferably set to a temperature of 470 ° C. to (Ac ₁ transformation point).

In the present invention, when performing the alloying treatment, the cooling rate (average cooling rate) is preferably 5 to 25 ° C./s in a temperature range up to 300 ° C. after the alloying treatment step. Thereby, high ductility and material uniformity can be ensured after cooling. The steel sheet after the plating process or the alloying process may be subjected to temper rolling for shape correction, adjustment of surface roughness, and the like.

[0064]

EXAMPLES Steel having the composition shown in Table 1 was melted in a converter and cast into pieces by continuous casting. The slab heating temperature SRT under the conditions shown in Table 2, the rough rolling and the finish rolling were performed, and after the rolling was completed, the slab was cooled under the conditions shown in Table 2 and wound at the winding temperature shown in Table 2.
A hot rolled sheet was obtained by the hot rolling step A or B step.
Next, these hot-rolled sheets were subjected to an internal oxide layer forming treatment step under the conditions shown in Table 2 or, without such treatment, pickled, and then subjected to a cold rolling step under the conditions shown in Table 2. It was a cold rolled sheet (sheet thickness 0.8 to 1.6 mm). When the hot-rolled sheet was subjected to the cold rolling step without performing the internal oxide layer forming step, the internal oxide layer forming step under the conditions shown in Table 2 was performed after the cold rolling step. The atmosphere for the heat treatment in the internal oxide layer forming treatment was a nitrogen atmosphere for batch annealing, and an atmosphere showing an oxidation potential of P ₀₂ : 1 × 10 ⁻¹¹ Pa for continuous annealing. Pickling was performed on the cold-rolled sheet after the internal oxide layer forming step. In some steel plates, the internal oxide layer forming process was not performed.

Next, these cold-rolled sheets were subjected to an annealing step, a plating step, or a further alloying step in a continuous galvanizing line under the conditions shown in Table 2. In addition,
The annealing temperatures were all above the recrystallization temperature. After the alloying treatment, cooling was performed at a cooling rate shown in Table 2. The plating process is
The steel sheet was immersed in a hot-dip galvanizing bath. The immersed steel sheet was pulled up, and the basis weight was adjusted by gas wiping. The conditions of the plating treatment were as follows: plating bath: 0.13% Al-Zn bath temperature: 485 ° C, basis weight: 45 g / m ² (per side).

The alloying treatment was performed under the conditions shown in Table 2, and the alloying treatment temperatures were all below the Ac ₁ transformation point. About the obtained plated steel sheet, the structure, the amount of solute N, the tensile properties, the plating properties, and the strain age hardening properties were investigated. The microstructure of the section of the steel sheet in the rolling direction (C section) was imaged using an optical microscope or a scanning electron microscope, and the average crystal grain size of ferrite was determined using an image analyzer.

The amount of solute N was determined by subtracting the amount of precipitated N from the total amount of N in steel determined by chemical analysis. The amount of precipitated N is
It was determined by an analytical method using electrolytic extraction. Here, this analysis method is an analysis method for determining the amount of N as a precipitate by chemical analysis of a residue extracted by dissolving ground iron by constant potential electrolysis using acetyl-acetone as a solvent.

The tensile properties were sampled from the steel sheet in the direction perpendicular to the rolling direction.
Using the JIS No. 5 test piece specified in JIS Z 2204
Degree: 3 × 10^-3/ S tensile test, yield strength YS, pull
Tensile strength TS and elongation El were measured. Also, how to roll steel plate
Direction (L direction), 45 ° to rolling direction (D direction), pressure
JIS No. 5 test piece from 90 ° direction (C direction) to the elongation direction
Was collected. 15% uniaxial tensile prestrain is applied to these specimens
The width distortion and thickness distortion of each test piece at the time of
R = ln (w / w₀) / Ln (t / t₀(Where w₀, T₀Is the width and thickness of the specimen before the test
And w and t are the width and thickness of the test piece after the test.
You. ) Is determined in each direction from the following equation._mean= (R_L+2 r_D+ R_c) / 4 gives the average r value r_meanI asked. Where r_LRolled
R value in the direction (L direction), and r_DIs the rolling direction (L direction
Direction) is the r value in the 45 ° direction (D direction) with respect to _cIs
The r value in the 90 ° direction (C direction) with respect to the rolling direction (L direction)
is there. However, if the uniform elongation is less than 15%,
The amount of strain is the smallest uniform elongation among the three directions of L, D and C.
The value should be adjusted to less than 15%.

The plating property was determined by visually observing the surface of the steel sheet, determining the presence or absence of non-plating defects, and calculating the non-plating area ratio. When the area ratio of non-plating was 0.3% or more, it was judged that non-plating occurred and evaluated as x. Further, the Fe content in the plating layer was investigated as an index of the powdering resistance and the degree of alloying of the plating layer.

The Fe content in the plating layer was determined by dissolving the plating layer with sulfuric acid, quantifying Fe in the dissolved solution by an atomic absorption method, and determining the Fe content in the plating layer. The powdering resistance was determined by applying a cellophane adhesive tape to the plated surface after peeling off the plated steel sheet by 90 °, peeling it off, and measuring the amount of zinc powder adhered to the tape by fluorescent X-rays. In addition, the amount of zinc powder was shown by the count (cps) of the counter tube. When the amount of zinc powder was 2000 cps or more, the powdering resistance was judged to be poor, and evaluated as x.

The strain aging hardening characteristic is determined by the steel plate (product plate)
S No. 5 test piece was sampled in the rolling direction, 5% tensile strain was given as pre-deformation, and pre-deformation stress σ _5% was measured.
After applying a heat treatment (aging treatment) equivalent to a paint baking treatment for × 20 minutes, a tensile test was performed at a strain rate of 3 × 10 ⁻³ / s, and the tensile properties after pre-deformation-heat treatment (yield stress YS _BH , Tensile strength TS) is determined, and BH amount = YS _BH −σ _5% , ΔTS = TS
_BH- TS was calculated. YS _BH and TS _BH are pre-deformed
TS is the yield stress and tensile strength after heat treatment, and TS is the tensile strength of the product sheet.

Table 3 shows the results.

[0073]

[Table 1]

[0074]

[Table 2]

[0075]

[Table 3]

In each of the examples of the present invention, high ductility and r _mean
Workability with a value of 1.5 or more, BH amount: 80 MPa or more, ΔT
S: High strain aging hardening property of 40 MPa or more, excellent workability and strain aging hardening property, no non-plating defect is observed, and excellent plating property. In particular, when the plating layer is alloyed, the Fe content in the plating layer is also 8 to 12%.
%, Which is an alloyed hot-dip galvanized steel sheet having excellent powdering resistance.

On the other hand, the comparative examples deviating from the present invention show that the ductility is reduced, the r value is low, or the strain age hardening property is reduced.
The target properties are not satisfied due to a decrease in plating property or a decrease in powdering resistance.

[0078]

According to the present invention, a hot-dip galvanized steel sheet or an alloyed hot-dip galvanized steel sheet having excellent ductility, workability, strain age hardening characteristics, no plating, and excellent plating properties can be stably manufactured. It has a significant industrial effect. Incidentally, the plated steel sheet of the present invention is soft at the time of forming, the tensile strength as well as the yield strength is improved by strain aging hardening after forming, the impact resistance of the product after forming is significantly improved, and for automotive parts This also has the effect of expanding the applications.

Continued on the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (reference) C23C 2/28 C23C 2/28 2/40 2/40 F term (reference) 4K027 AA02 AA23 AB02 AB28 AB42 AC02 AC12 AC18 AC73 AE12 AE18 4K037 EA01 EA02 EA04 EA13 EA15 EA17 EA18 EA19 EA20 EA23 EA25 EA27 EA31 EA32 EB02 EB03 FA02 FA03 FC03 FC04 FC05 FD04 FE01 FE02 FH01 FJ04 FJ05 FJ06 FK03 FK08 GA05

Claims (7)

[Claims] 1. A hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface of a steel sheet, wherein a ground at an interface between the steel sheet and the hot-dip galvanized layer or the alloyed hot-dip galvanized layer is provided. It has an internal oxide on the iron side, and the steel sheet is, in mass%, C: 0.008% or less, Si: 1.0% or less, Mn: more than 1.0% 2.0% or less, P: 0.1% or less, S: 0.01 % Or less, Al: 0.005 to 0.020
%, N: 0.0050 to 0.0250%, Nb: 0.005 to 0.045%, Ti: 0.005 to 0.
070%, V: 0.005 to 0.10%, containing one or more selected from the group and satisfying the following formula (1):
Strain age hardening at a tensile strength of 440MPa or more, characterized by having a composition containing 0.0015% or more of N in the solid solution state, a composition comprising the balance of Fe and unavoidable impurities, and a ferrite having an average crystal grain size of 20μm or less. High tensile galvanized steel sheet with excellent properties and workability. N / (Al + Nb + Ti + V) ≧ 0.3 (1) where N, Al, Nb, Ti, and V: content of each element (% by mass) 2. In addition to the above composition, the composition further contains one or two of the following groups A and B in mass%, and the following formula (2) is replaced by the above formula (1). The high tensile galvanized steel sheet according to claim 1, wherein the steel sheet is satisfied. Group A: one or more of Mo: 1.0% or less, Ni: 1.5% or less, Cu: 1.5% or less Group B: B: 0.01% or less N / (Al + Nb + Ti + V + B) ≧ 0.3 (2) Here, N, Al, Nb, Ti, V, B: content of each element (% by mass) 3. In mass%, C: 0.008% or less, Si: 1.0% or less, Mn: more than 1.0% 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.005 to 0.020
%, N: 0.0050 to 0.0250%, Nb: 0.005 to 0.045%, Ti: 0.005 to 0.
070%, V: 0.005 to 0.10%, contains one or more kinds selected from the group consisting of:
A steel slab containing one or two of the groups and having a composition satisfying the following formula (2) is heated to a slab temperature of 1000 ° C.
As described above, the sheet bar is roughly rolled to form a sheet bar. The sheet bar is subjected to finish rolling at a finish-rolling exit temperature of 800 ° C. or more to form a hot-rolled sheet.After the finish rolling, cooling is started within 0.5 s. A hot-rolling step of rapidly cooling at an average cooling rate of 40 ° C./s or higher and winding at a winding temperature of 650 ° C. or lower; and an internal oxide layer forming step of forming an internal oxide layer of the hot-rolled sheet; After pickling the hot-rolled sheet after the internal oxide layer forming step, the cold-rolling step is performed by cold rolling to form a cold-rolled sheet, and the cold-rolled sheet is subjected to 10 to 120 at a temperature of not less than a recrystallization temperature and not more than 900 ° C. After performing continuous annealing for holding for s, an annealing step of cooling to 550 ° C at an average cooling rate of 10 to 300 ° C / s, and a hot-dip galvanized steel sheet subjected to the annealing step is subjected to hot-dip galvanizing to melt the steel sheet surface. And a plating step of forming a galvanized layer, wherein the tensile strength is 440 MPa. The method for producing a high-strength hot-dip galvanized steel sheet having excellent strain aging characteristics and workability as described above. Note Group A: Mo: 1.0% or less, Ni: 1.5% or less, Cu: 1.5% or less Group B: B: 0.01% or less N / (Al + Nb + Ti + V + B) ≧ 0.3 (2) where N, Al, Nb, Ti, V, B: content of each element (% by mass) 4. In mass%, C: 0.008 or less, Si: 1.0% or less, Mn: more than 1.0% 2.0% or less, P: 0.1% or less, S: 0.01% or less, Al: 0.005 to 0.020
%, N: 0.0050 to 0.0250%, and further Nb: 0.005 to 0.045%, Ti: 0.4%.
005 to 0.070%, V: 0.005 to 0.10%, contains one or more of them, or further contains one or two of the following groups A and B, and has the following formula (2) A steel slab having a composition satisfying the following conditions is subjected to rough rolling at a slab heating temperature of 1000 ° C. or higher to form a sheet bar. The sheet bar is subjected to finish rolling at a finish rolling exit temperature of 800 ° C. or higher to obtain a hot rolled sheet. After the finish rolling, cooling is started within 0.5 s, quenched at an average cooling rate of 40 ° C./s or more, and wound at a winding temperature of 650 ° C. or less. Cold rolling after cold pickling and cold rolling to a cold rolled sheet, an internal oxide layer forming step for forming an internal oxide layer on the cold rolled sheet, and a cold rolled sheet passing through the internal oxide layer forming step Is pickled, and then at a temperature not lower than the recrystallization temperature and not higher than 900 ° C.
After performing continuous annealing for 10 to 120 seconds, the steel sheet is subjected to an annealing step of cooling to 550 ° C. at an average cooling rate of 10 to 300 ° C./s, and a hot-dip galvanized steel sheet subjected to the annealing step. A method for producing a high-strength hot-dip galvanized steel sheet having a tensile strength of 440 MPa or more and excellent strain aging characteristics and workability, characterized by sequentially performing a plating process of forming a hot-dip galvanized layer on the surface. Note Group A: Mo: 1.0% or less, Ni: 1.5% or less, Cu: 1.5% or less Group B: B: 0.01% or less N / (Al + Nb + Ti + V + B) ≧ 0.3 (2) where N, Al, Nb, Ti, V, B: content of each element (% by mass) 5. The production of a high-strength hot-dip galvanized steel sheet according to claim 3, wherein an alloying step of alloying the hot-dip galvanized layer is further performed subsequent to the plating step. Method. 6. The method for producing a hot-dip galvanized steel sheet according to claim 5, wherein an average cooling rate to 300 ° C. after the alloying step is 5 to 25 ° C./s. 7. The slab heating temperature: 1000 in the hot rolling step.
C. or higher, rough rolling is completed in the temperature range of 1000 ° C. to the Ar ₃ transformation point to form a sheet bar, and while the sheet bar is lubricated in a temperature range of 600 ° C. or more lower than the Ar ₃ transformation point, the rolling reduction: 80 %
The method for producing a high-strength hot-dip galvanized steel sheet according to any one of claims 3 to 6, wherein the hot-rolled sheet is subjected to the finish rolling described above to form a hot-rolled sheet, and then to a hot-rolling step.