JP2003247043A - High tensile strength galvanized, cold rolled steel sheet having excellent balance in strength-ductility and production method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high tensile strength galvanized, cold rolled steel sheet which has high strength, has an excellent balance in strength-ductility as well, and further has satisfactory hole expansibility and impact resistance. <P>SOLUTION: The steel sheet has a composition containing, by mass, 0.02 to 0.20% C, 0.2 to 1.0% Si, 1.6 to 3.0% Mn, ≤0.1% P, ≤0.02% S, 0.01 to 0.1% Al, ≤0.005% N, ≤0.1% Ti and ≤0.1% Nb, and in which the C, Si, Mn, Ti and Nb elements are contained in the ranges where temperatures Ac<SB>1</SB>, Ac<SB>3</SB>and Tre obtained by substituting the C, Si, Mn, Ti and Nb contents for the following formulae (1), (2) and (3) satisfy Ac<SB>1</SB>≤Tre≤Ac<SB>3</SB>, and the balance Fe with inevitable impurities, and has a steel structure essentially consisting of ferrite, and in which the mean crystal grain size of the above ferrite is controlled to ≤4.0 μm: Ac<SB>3</SB>=915-325.9(%C)-35.9(%Mn)+31.4(%Si) (1), Ac<SB>1</SB>=761.3+212(%C)-45.8(%Mn)+16.7(%Si) (2), and Tre=777.6+85.3(Ti*/ C*)+113.8(Nb*/C*) (3). <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a high-strength hot-dip galvanized cold-rolled steel sheet having a tensile strength of 780 MPa or more and an excellent strength-ductility balance, and a method for producing the same, which is suitable for use in applications such as steel sheets for automobiles. It is about. In the present invention, the hot-dip galvanized cold-rolled steel sheet also includes a so-called alloyed hot-dip galvanized cold-rolled steel sheet which is alloyed after the hot-dip galvanizing treatment.

[0002]

2. Description of the Related Art Since steel sheets for automobiles are generally required to have corrosion resistance and workability, various surface-treated steel sheets have been used. Among them, the hot-dip galvanized steel sheet not only has a high degree of corrosion resistance, but also has the advantage that it can be manufactured extremely inexpensively by a continuous hot-dip galvanizing line that can perform recrystallization annealing and galvanizing on the same line. . Further, the alloyed hot-dip galvanized steel sheet which is immediately heated and alloyed after the above galvanizing treatment is particularly excellent in corrosion resistance, weldability and press formability.

By the way, in recent years, there has been an increasing demand for weight reduction of automobiles in order to improve fuel economy for the purpose of improving the global environment, and there has been a demand for strengthening safety regulations at the time of collision in order to improve safety. Therefore, the hot-dip galvanized steel sheet also needs to have high strength (also referred to as high tensile strength).

In addition, recently, at the request of the user, the strength of a member such as a member or a pillar, which is considered to be difficult to be strengthened due to a collision-resistant member, which is difficult to form, has been improved. Is required. In order to meet the above requirements, a high degree of strength-ductility balance and further good hole expansibility are required, but as a means for achieving this, for example, in JP-A-11-131145, there is a work-induced transformation. A method has been proposed in which hot dip galvanizing is performed by utilizing retained austenite that contributes to the ductility of a steel sheet. In this method, a steel sheet adjusted to the specified composition is used in a continuous hot-dip galvanizing line (Ac ₁ point + 30 ° C).
Above, Ac ₃ point or less of a temperature range 30s or more annealing from its temperature range (Ac ₁ point + 20 ℃) ~Ac ₁ point to cooling below the cooling rate of 5 ° C. / s, continue 6 ° C. to 520 ° C. or less 520 ~ 40 in a series of manufacturing processes such as hot dip plating, plating amount adjustment, alloying treatment, etc.
3% volume ratio is obtained by allowing the sample to stay in the temperature range of 0 ℃ for 90 seconds or more and 300 seconds or less and then cooling it to 200 ℃ or less.
It is intended to contain the above retained austenite.

However, in this method, in order to further stabilize by concentrating the carbon austenite present during annealing, the steel sheet very narrow temperature range that during annealing (Ac ₁ point + 20 ℃) ~Ac ₁ point Since it has to be cooled, there is a problem that it is extremely difficult to control the temperature of the steel sheet. Also,
From the temperature range of (Ac ₁ point + 30 ° C) or more and Ac ₃ points or less (Ac ₁
(Point + 20 ° C) to the temperature range of Ac ₁ point, the aim is to grow ferrite crystal grains to promote the enrichment in order to concentrate the carbon in austenite, but from the viewpoint of obtaining a high-strength steel sheet. Therefore, it is not desirable to grow the crystal grain size. Furthermore, in order to improve the coating adhesion and alloying processability, the upper limit of the Al concentration in the plating bath should be 0.13
Since the mass% has to be suppressed to a low level, operating conditions are extremely strict, and as a result, there is a problem that it is difficult to achieve both stable film adhesion and alloying processability.

Further, in Japanese Patent Application Laid-Open No. 6-009292, a second structure in which the final structure is composed of ferrite and at least one phase of martensite, bainite, pearlite, retained austenite and low temperature transformation ferrite
The volumetric ratio of the second phase from the surface of the steel plate to the plate thickness 1/4 is 1.3 times the volume ratio of the second phase from the plate thickness 1/4 depth to the plate thickness center. As described above, a high-strength thin steel plate for working having excellent hole expanding characteristics and a method for manufacturing the same have been proposed. However, the greatest feature of this steel sheet is that the fraction of the second phase is intentionally made non-uniform, and for that purpose carburizing at high temperature is indispensable, so the complexity of the manufacturing process is avoided. Not only that, it is necessary to strictly control the atmosphere control and carburization rate during carburization. Also,
According to the examples, the obtained strength is only about 650 MPa, and there is also a problem that the strength at the 780 MPa level, which is increasing the needs of users, cannot be obtained.

Further, from the viewpoint of regulations regarding collision safety, which are rapidly increasing in Japan and abroad, for example, Japanese Patent Laid-Open No. 11-61327 discloses that in a steel sheet microstructure, ferrite is the main phase and martensite is the main phase. The steel sheet for automobiles, which has excellent collision resistance and formability and is characterized by having an occupancy rate of 3 to 30% and a yield ratio of 0.75 or less, has been proposed. However, it was insufficient to achieve the absorbed energy required to meet the impact resistance required at present.

In addition, Japanese Patent Laid-Open No. 10-273754 discloses a high-strength and high-yield ratio hot-dip galvanized steel sheet having a non-composite structure having a tensile strength of 45 kg / mm ² or more and a yield ratio of 80% or more. Although a manufacturing method has been proposed, in this technique, in order to obtain a non-composite structure, the cooling rate after the annealing step is made extremely slow, or the second annealing step is performed at 515 to 600 ° C for 15 minutes.
Since it is necessary to introduce a holding process for more than a second, it is inevitable that the manufacturing process is complicated.

[0009]

As described above, in order to provide a member which is difficult to form but requires high strength, a good strength-ductility balance, a good hole expansibility, and further Requires excellent impact resistance, but current steel sheets have not been sufficient to meet the needs of users. The present invention solves the above problems that conventional steel sheets have, has a tensile strength of 780 MPa or more, and is excellent in strength-ductility balance, and further has good hole expandability and excellent impact resistance. It is an object of the present invention to propose a new high-strength hot-dip galvanized cold-rolled steel sheet having the above-mentioned characteristics and having a good hot-dip galvanizing property together with its advantageous manufacturing method.

[0010]

[Means for Solving the Problems] As a result of intensive research aimed at solving the above-mentioned problems of achieving both high strength and ductility, the inventors have optimized alloy elements and have recrystallized temperatures of steel sheets. Is adjusted so as to be not less than the Ac ₁ transformation temperature and not more than the Ac ₃ transformation temperature, and the steel whose composition has been adjusted in this way is subjected to recrystallization annealing and then a temperature not less than the Ac ₁ transformation temperature and not more than the Ac ₃ transformation temperature. It was found that the heat treatment in the region is extremely effective in achieving the intended purpose. The present invention is based on the above findings.

[0011]

That is, the gist of the present invention is as follows. 1. % By mass, C: 0.02 to 0.20%, 0.2% ≤ Si ≤ 1.0
%, Mn: 1.6 to 3.0%, P ≦ 0.1%, S ≦ 0.02%, 0.01
% ≦ Al ≦ 0.1%, N ≦ 0.005%, Ti ≦ 0.1% and Nb ≦
0.1%, and the amounts of C, Si, Mn, Ti and Nb are as follows (1),
Temperatures Ac ₁ , Ac ₃ and Tre calculated by substituting in equations (2) and (3)
In a range satisfying Ac ₁ ≤ Tre ≤ Ac ₃ , the balance being a composition of Fe and inevitable impurities, having a steel structure with ferrite as the main phase, and having an average crystal grain size of 4.0 A high-strength hot-dip galvanized cold-rolled steel sheet having an excellent balance of strength and ductility, which is characterized by having a hot-dip galvanized layer on the surface and having a thickness of μm or less. Note Ac ₃ = 915 −325.9 (% C) −35.9 (% Mn) +31.4 (% Si) --- (1) Ac ₁ = 761.3 +212 (% C) −45.8 (% Mn) +16.7 (% Si) --- (2) Tre = 777.6 + 85.3 (Ti ^* / C ^* ) + 113.8 (Nb ^* / C ^* ) --- (3) However, Ti ^* / C ^* = {(% Ti) / 47.86-(% N) /14.00-(% S) /32.0
6} / {(% C) /12.01} Nb ^* / C ^* = {(% Nb) /92.90} / {(% C) /12.01} (% M) is the content of M element (mass%) Represents Moreover, all units of temperature are Celsius degrees (° C.).

2. In the above 1, the steel structure contains ferrite as a main phase and a volume fraction of 3 to 40% bainite and 2 to 20% retained austenite, and a high tensile strength excellent in balance between strength and ductility. Hot-dip galvanized cold rolled steel sheet.

3. In the above 1, the strength is characterized in that the steel structure contains ferrite as a main phase and contains martensite in a volume fraction of 5 to 12%, and the total of both phases is 95% or more in volume fraction. -High-strength hot-dip galvanized cold-rolled steel sheet with excellent ductility balance and hole expandability.

4. In the above 1, the steel structure contains ferrite: 70% or more and less than 88% and martensite: more than 12% and 30% or less in volume fraction, and the total of both phases is 96% or more in volume fraction. A high-strength hot-dip galvanized cold-rolled steel sheet having excellent strength-ductility balance and impact resistance.

5. In any one of 1 to 4 above, the steel sheet is further mass%, Mo ≦ 1.0%, Cr ≦ 1.0% and Ni.
A high-strength hot-dip galvanized cold-rolled steel sheet having an excellent balance of strength and ductility, which has a composition containing one or more selected from ≤1.0%.

6. % By mass, C: 0.02 to 0.20%, 0.2%
≤Si≤1.0%, Mn: 1.6-3.0%, P≤0.1%, S≤0.
02%, 0.01% ≤ Al ≤ 0.1%, N ≤ 0.005%, Ti ≤ 0.1%
And Nb ≦ 0.1%, and the temperatures Ac ₁ , Ac ₃ and Tre obtained by substituting the amounts of C, Si, Mn, Ti and Nb into the formulas (1), (2) and (3) below are Ac ₁ ≦ A steel material containing Tre <Ac ₃ in a range satisfying the following, with the balance being Fe and inevitable impurities, is heated to 1200 ° C or higher, and the finish rolling finish temperature is Ar ₃
After hot rolling at a temperature above the transformation point, 500 to 750
It is wound around a coil at a temperature of ℃, pickled and cold-rolled, and then at a temperature of Tre or higher and Ac ₃ or lower for 20 to 20 ° C.
Recrystallization annealing is applied for 60 seconds, then pickled, and then Ac ₁ ~
A high tensile strength excellent in strength-ductility balance, characterized by performing heat treatment in the above Ac ₃ temperature range, cooling and then performing hot dip galvanizing, or performing hot dip galvanizing and then alloying treatment. Manufacturing method of hot-dip galvanized cold-rolled steel sheet. Note Ac ₃ = 915 −325.9 (% C) −35.9 (% Mn) +31.4 (% Si) --- (1) Ac ₁ = 761.3 +212 (% C) −45.8 (% Mn) +16.7 (% Si) --- (2) Tre = 777.6 + 85.3 (Ti ^* / C ^* ) + 113.8 (Nb ^* / C ^* ) --- (3) However, Ti ^* / C ^* = {(% Ti) / 47.86-(% N) /14.00-(% S) /32.0
6} / {(% C) /12.01} Nb ^* / C ^* = {(% Nb) /92.90} / {(% C) /12.01} (% M) is the content of M element (mass%) Represents Moreover, all units of temperature are Celsius degrees (° C.).

7. In the above 6, the heat treatment after recrystallization annealing-pickling is performed for 5 to 30 seconds in the temperature range of Ac ₁ or more and (Ac ₁ + 70 ° C.) or less, and then 5 to 15 up to the hot dip galvanizing start temperature. Cool at a rate of ° C / s and continue to 5
After hot dip galvanizing at a temperature of 25 to 460 ℃, 300
Cool down to below ℃ at a rate of 10 ℃ / s or after the above hot dip galvanizing, perform alloying treatment at 500 to 560 ℃, and then cool down to 300 ℃ or below at a speed of 10 ℃ / s or above. And a method for producing a high-strength hot-dip galvanized cold-rolled steel sheet excellent in strength-ductility balance.

8. In the above 6, the heat treatment after recrystallization annealing-pickling is performed for 5 seconds or more in the temperature range of Ac ₁ or more and (Ac ₁ + 70 ° C) or less, and then 15 ° C / s until the hot dip galvanizing start temperature. Cool at super speed and continue to 525
After hot dip galvanizing at a temperature of ~ 460 ℃, 300 ℃
Cool down to 10 ° C / s or more, or after performing hot dip galvanizing and alloying at 500 to 560 ° C, cool down to 300 ° C or less at 10 ° C / s or more. A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent strength-ductility balance and hole expandability.

9. In the above 6, the heat treatment after recrystallization annealing-pickling is performed for 5 seconds or more in the temperature range of (Ac ₁ + 70 ° C.) or more and Ac ₃ or less, and then 5 ° C./s until the hot dip galvanizing start temperature. Cool at above speed and continue
After hot dip galvanizing at a temperature of 25 to 460 ℃, 300
Cool down to below ℃ at a rate of 10 ℃ / s or after the above hot dip galvanizing, perform alloying treatment at 500 to 560 ℃, and then cool down to 300 ℃ or below at a speed of 10 ℃ / s or above. And a method for producing a high-strength hot-dip galvanized cold-rolled steel sheet excellent in strength-ductility balance and impact resistance.

10. In any one of 6 to 9 above, the steel material further comprises, by mass%, Mo ≦ 1.0%, Cr ≦ 1.0% and
A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having an excellent balance of strength and ductility, which has a composition containing one or more selected from Ni ≤ 1.0%.

[0021]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be specifically described below. First, the reason why the composition of the steel sheet is limited to the above range in the present invention will be described. In addition, all% indications regarding the component compositions shown below are “mass%”. C: 0.02 to 0.20% C is an inexpensive strengthening element and is an element useful not only for securing the strength of steel but also for stabilizing the retained austenite that contributes to ductility, but the content is 0.02%. If less than 0.20%, the effect of addition is poor, while if added in excess of 0.20%, the weldability is impaired.
It is limited to the range of 0.20%. More preferably 0.02-0.12%
Is.

0.2% ≦ Si ≦ 1.0% Si has a large solid solution strengthening ability and acts as a solid solution in ferrite to increase the tensile strength, but if the content exceeds 1.0%, the plating property is impaired. The upper limit is 1.0%. The lower limit is 0.2% to promote the stabilization of the ferrite phase and the concentration of carbon in the austenite phase.

Mn: 1.6 to 3.0% Mn is an element useful for stabilizing the austenite phase and ensuring strength, but if the content is less than 1.6%, stable austenite phase formation and desired strength are achieved. It is difficult to obtain, and if the content exceeds 3.0%, the workability decreases and the strength-ductility balance deteriorates.
The range is to 3.0%.

P ≦ 0.1% P is an element which is not only limited to the level of unavoidable impurities but also attains high strength at low cost as is well known in the art, and is also useful for improving elongation. It may be added appropriately for adjustment, or may be added for other purposes. However, in any case, the content is
If it exceeds 0.1%, it segregates at the grain boundaries and causes embrittlement, which not only lowers workability but also deteriorates toughness. Therefore, the upper limit of the P content is limited to 0.1%.

S ≦ 0.02% S is a harmful element that causes hot cracking during hot rolling and is present as an inclusion in the steel sheet to deteriorate ductility, so it is desirable to reduce it as much as possible. Since 0.02% or less is acceptable, the content is limited to 0.02% or less.

0.01% ≦ Al ≦ 0.1% Al is an element that acts as a deoxidizer and is effective in improving the cleanliness of steel, but if its content is less than 0.01%, its addition effect is poor, while 0.1% ≦ 0.1% %, The manufacturing cost rises, so Al is limited to the range of 0.01 to 0.1%.

N ≦ 0.005% N not only causes aging deterioration but also yield elongation, so it is desirable to reduce it as much as possible.
Since it is acceptable if it is 5% or less, its content is 0.005
% Or less.

Ti ≤ 0.1%, Nb ≤ 0.1% All of Ti and Nb have the effect of increasing the recrystallization temperature of the steel sheet by precipitating TiC, NbC, etc. when added. Further, in order to make the crystal grain size fine, as described later,
It is necessary to satisfy c ₁ ≤ Tre ≤ Ac ₃ . More preferably T
re is to satisfy the range of 0.5 × (Ac ₁ + Ac ₃ ) ≦ Tre ≦ Ac ₃ , and when Ac ₁ ≦ Tre <0.5 × (Ac ₁ + Ac ₃ ), either or both of Ti and Nb Is added as appropriate to raise the recrystallization temperature, and Tre is 0.5 × (Ac ₁ + Ac ₃ ).
It is desirable to adjust so as to satisfy the range of ≦ Tre ≦ Ac ₃ . However, the above Ac ₃ and Ac ₁ are the A of steel, respectively.
Predicted values of c ₃ transformation point and Ac ₁ transformation point, and Tre are predicted values of recrystallization temperature during continuous annealing. Even if Ti and Nb are not added, they are not necessarily added if Tre satisfies the range of 0.5 × (Ac ₁ + Ac ₃ ) ≦ Tre ≦ Ac ₃ . However, Ti and Nb have the effect of increasing the strength of ferrite by precipitating fine carbides in ferrite, improving local ductility such as stretch flangeability, and increasing yield strength to improve impact resistance. From this point of view, it is preferable to add Ti and Nb even if Tre satisfies 0.5 × (Ac ₁ + Ac ₃ ) ≦ Tre ≦ Ac ₃ without adding Ti and Nb.

The above effect is obtained in both Ti and Nb.
Since it becomes remarkable at 0.01% or more, when they are added, it is preferable to add 0.01% or more, respectively, and these may be added individually or in combination. However, if Ti is added in excess of 0.1%, not only the effect will be saturated, but also the amount of precipitates will increase and ductility will be reduced, so the upper limit is made 0.1%. Also, Nb, similar to Ti, will saturate the effect even if added in excess of 0.1%.
The upper limit is set to 0.1% because the amount of precipitates increases and ductility decreases.

Although the basic components of the present invention have been described above, other elements described below can be appropriately contained in the present invention. Mo ≦ 1.0% Mo, like Mn, not only is useful for enhancing the hardenability and obtaining the desired second phase, but also effectively contributes to the improvement of strength and the stabilization of austenite. However, if the content exceeds 1.0%, it causes a delay in alloying, deteriorates the weldability and raises the cost, so the content is set to 1.0% or less. In addition, in order to obtain the above effects remarkably, it is preferable to contain 0.1% or more.

Cr ≦ 1.0% Cr, like Mn, is an element which is useful not only for improving the hardenability and obtaining the desired second phase, but also for improving the strength and stabilizing the austenite. . However, 1.0%
If the content of Ni exceeds 1.0%, the wettability with respect to zinc is impaired during hot dip galvanizing, so the content was made 1.0% or less. In addition, in order to obtain the above effects remarkably, the content of 0.1% or more is preferable.

Similar to Mn and Cr, Ni ≦ 1.0% Ni is useful for enhancing the hardenability and obtaining the desired second phase, and also effectively contributes to the improvement of strength and the stabilization of austenite. However, if the content exceeds 1.0%, the steel sheet is excessively hardened, the workability such as elongation decreases, and the cost increases, so the content is set to 1.0% or less. In addition, in order to obtain the above effects remarkably, the content of 0.1% or more is preferable.

Although the proper composition range of each component has been described above, in the present invention, it is not enough that each component simply satisfies the above composition range, and in order to refine the crystal grains, steel is required. Regarding the contents of C, Si, Mn, N, S, Ti and Nb among the components, the temperatures Ac ₁ , Ac ₃ and Tre obtained by substituting the formulas (1), (2) and (3) below are , Ac ₁ ≤ Tre ≤
It is important to adjust the components so as to satisfy the range of Ac ₃ . The above Ac ₁ and Ac ₃ are the predicted values of the Ac ₁ transformation point and the Ac ₃ transformation point of each steel found at the end of various studies, and Tre is the predicted value of the recrystallization temperature during continuous annealing. Is. These predicted values can be suitably applied when heating at a heating rate of 2 ° C./s or more and 20 ° C./s or less. Note Ac ₃ = 915 −325.9 (% C) −35.9 (% Mn) +31.4 (% Si) --- (1) Ac ₁ = 761.3 +212 (% C) −45.8 (% Mn) +16.7 (% Si) --- (2) Tre = 777.6 + 85.3 (Ti ^* / C ^* ) + 113.8 (Nb ^* / C ^* ) --- (3) However, Ti ^* / C ^* = {(% Ti) / 47.86-(% N) /14.00-(% S) /32.0
6} / {(% C) /12.01} Nb ^* / C ^* = {(% Nb) /92.90} / {(% C) /12.01} (% M) is the content of M element (mass%) Represents Moreover, all units of temperature are Celsius degrees (° C.).

As will be described later, ferrite is the main phase,
Furthermore the mean crystal grain size of the ferrite and the steel sheet is less than 4.0μm is, Ac ₁ or more predicted value Tre recrystallization temperature of the steel sheet is a predicted value of the Ac ₁ transformation point, the predicted value of the Ac ₃ transformation point It is important to adjust the components so as to satisfy Ac ₃ or less. This is because when the recrystallization temperature of the steel sheet becomes less than the Ac ₁ transformation point, the recrystallization ends in the ferrite single phase region, and the ferrite crystal grains grow very quickly, resulting in insufficient strength. In addition to the above, there is a problem that the diffusion distance of the additive element to austenite becomes longer during the heat treatment before plating performed in a continuous hot-dip galvanizing line, etc. Also, when the recrystallization temperature of the steel sheet exceeds the Ac ₃ transformation point, the same applies. In addition, since recrystallization is completed in the austenite single phase region, crystal grains of ferrite grow very quickly, resulting in a problem of insufficient strength, so that the desired steel structure as described above may be obtained. Because you can't.

Preferably, 0.5 × (Ac ₁ + Ac ₃ ) ≦
The components are adjusted so that Tre ≦ Ac ₃ . I mean,
This temperature range has a large effect of suppressing the growth of crystal grains.
Among the phase regions, it is a temperature region that has a particularly large grain growth suppressing effect, and a fine crystal grain size can be achieved by suppressing the growth of recrystallized crystal grains, and in order to increase the strength-ductility balance of the steel sheet. Therefore, it is extremely advantageous. For this purpose, it is effective to add Ti and Nb as described above.

Next, the reasons for limiting the steel structure will be described. Ferrite is soft and has a structure useful for improving ductility and workability. Therefore, in order to maintain an appropriate strength-ductility balance, it is necessary to make the steel structure a structure of ferrite main phase. From this viewpoint, it is preferable that the volume fraction of ferrite is 50% or more of the entire structure. Also,
The average crystal grain size of the main phase ferrite needs to be 4.0 μm or less in order to obtain a high strength-ductility balance with a high strength of 780 MPa or more.

Further, in the present invention, the matrix is strengthened,
Moreover, in order to increase the yield strength while maintaining this strength, and further to refine the crystal grains, it is important to finely disperse and precipitate TiC, NbC, etc. in the ferrite, as described above. Has a carbide particle size of 30 nm
It is preferable to suppress the thickness to 20 nm or less. Thus, as described above, not only the effects of strengthening the matrix phase and increasing the yield ratio and suppressing the growth of crystal grains are obtained,
With respect to impact resistance, the deformation energy per unit volume at a strain rate of 2000 / s is 270 MJ / m ³ or more, which is an extremely high absorbed energy.

In the present invention, in order to improve the strength-ductility balance, it is necessary not only to make the structure of the ferrite main phase as described above, but also to suppress the average crystal grain size to 4.0 μm or less. By controlling the microstructure other than ferrite, it is possible to obtain a steel sheet that is more excellent in strength-ductility balance, a steel sheet that is excellent in strength-ductility balance and hole expandability, and a steel sheet that is excellent in strength-ductility balance and impact resistance. it can.

Now, in order to obtain a steel sheet having a more excellent strength-ductility balance, the steel structure has ferrite as the main phase, and
It is important to have a structure containing 3-40% by volume of bainite and 2-20% by volume of retained austenite. That is, ferrite has a soft structure, and ductility and workability are improved. Further, since it is difficult for carbon to form a solid solution, it contributes effectively to the concentration of carbon in austenite during the heat treatment before plating. Here, in order to maintain the balance between ductility and workability and strength, it is preferable that the main phase ferrite is 50% or more of the entire structure in terms of volume fraction. It is more preferably 60 to 95%, further preferably 70 to 95%.

Bainite has a hard structure and is important for obtaining the strength of the steel sheet. Further, in the process of precipitation of bainite mainly during passage through the hot-dip galvanizing bath or subsequent alloying treatment, it has a function of concentrating carbon in the retained austenite and stabilizing the retained austenite. In order to obtain these effects, bainite needs to have a volume fraction of 3% or more with respect to the entire structure, but when the volume fraction exceeds 40%, the steel plate excessively increases in strength and the strength-ductility balance. Is deteriorated, so 40% or less. It is preferably 30% or less. Further, the retained austenite is a structure that contributes to the ductility of the steel sheet, and in order to obtain the effect, it is necessary that the volume fraction is 2% or more of the entire structure. On the other hand, in the present component system, when considering the manufacturing conditions, the upper limit is substantially 20% in terms of volume fraction.

As described above, the steel structure in this case is basically of three kinds, that is, ferrite, bainite and retained austenite, but martensite may be present in addition to these. However, since this martensite is a structure that increases the strength of the steel sheet, if it is present in a large amount, it excessively strengthens the steel sheet and deteriorates the ductility. Therefore, when martensite is present, it is desirable that the proportion of martensite is 12% or less in terms of volume fraction with respect to the entire structure.

Next, in order to obtain a steel sheet excellent in strength-ductility balance and hole expandability, the steel structure contains ferrite as a main phase and contains martensite in a volume fraction of 5 to 12%.
And it is important that the total volume of both phases is 95% or more. As described above, ferrite has a soft structure and ductility and workability are improved. Further, since it is difficult for carbon to form a solid solution, it is possible to enhance the hardenability by concentrating carbon in austenite during annealing in the two-phase region before plating, and effectively contribute to obtaining the desired second phase. Here, in order to maintain good hole expandability and strength-ductility balance, it is necessary to use ferrite as the main phase. The ferrite, which is the main phase, preferably accounts for 50% or more of the entire structure in terms of volume fraction. It is more preferably 83 to 95%.

Martensite is a useful structure for obtaining strength, and its effect is effective when the volume fraction is 5% or more, but if it is excessively present, the steel sheet is hardened and the parent phase The difference in hardness from ferrite is too large, which rather deteriorates the hole expandability.
Must be 12% or less. Further, in order to obtain high strength and good hole expandability, it is necessary to set the volume fraction of the above-mentioned two structures of ferrite and martensite to 95% or more. In addition to the above-mentioned ferrite and martensite, bainite: 3
%, Residual austenite: less than 2%, cementite: 5% or less and pearlite: 5% or less are acceptable.

Next, in order to obtain a steel sheet excellent in balance between strength and ductility and impact resistance, the steel structure has a volume fraction of ferrite: 70% or more and less than 88% and martensite: 12
% And less than 30%, and the sum of both phases is the volume fraction.
It is important that the organization is 96% or more. As described above, ferrite is soft and has a structure useful for improving ductility and workability. Further, since it is difficult for carbon to form a solid solution, it is possible to enhance the hardenability by concentrating carbon in austenite during annealing in the two-phase region before plating, and effectively contribute to obtaining the desired second phase. Here, in order to have an appropriate strength-ductility balance and to improve impact resistance, the main phase, ferrite, must be in the range of 70% or more and less than 88% of the entire structure in volume fraction. is there.

Further, martensite is a structure useful for obtaining strength, and particularly with respect to impact resistance, its effect is remarkable when the volume fraction is more than 12%. However, if it is excessively present, the steel sheet is hardened more than necessary, so the content ratio is set to 30% or less.
In order to obtain strength-ductility balance and impact resistance together, the above two structures are combined and the volume fraction is 96%.
It must be at least%. In addition to the above-mentioned ferrite and martensite, bainite, pearlite,
Although cementite and retained austenite and the like may be present, since all of these structures contain carbides or carbon itself, the carbon concentration in martensite is reduced, and it becomes difficult to secure strength by martensite. In each case, it is necessary to suppress the volume fraction to 4% or less.

Next, the manufacturing method of the present invention will be described. The steel adjusted to the above preferable composition is melted in a converter or the like, and then made into a slab by a continuous casting method or the like. This steel material is placed in a heating furnace after being heated to a high temperature or once cooled and then heated to 1200 ° C. or higher, and then the finish rolling end temperature, that is, the finish hot rolling side temperature is set to a temperature not lower than the Ar ₃ transformation point. After hot rolling to make a hot-rolled sheet, it is coiled at a coiling temperature of 500 to 750 ℃.

In the above manufacturing process, if the heating temperature of the slab is less than 1200 ° C., TiC or the like does not sufficiently form a solid solution and becomes coarse, and the effect of suppressing the grain growth in the subsequent recrystallization annealing process is insufficient. Therefore, the heating temperature of the slab needs to be 1200 ° C or higher. On the other hand, if the finish hot rolling temperature is lower than the Ar ₃ transformation point, ferrite and austenite are generated during rolling, a band-like structure is generated in the steel sheet, and the crystal grain size is apt to be coarsened. This band-like structure tends to remain even after cold rolling or annealing, which causes anisotropy in material properties. Therefore, it is necessary that the finish rolling finish temperature is set to the Ar ₃ transformation point or higher. Furthermore, the winding temperature after hot rolling is 500
If the temperature is lower than ℃ or higher than 750 ℃, precipitation of AlN for suppressing aging deterioration due to nitrogen is insufficient, resulting in deterioration of material properties. Further, the coiling temperature is preferably 500 ° C. or higher and 750 ° C. or lower in order to make the structure of the steel sheet uniform and make the crystal grain size as fine and uniform as possible. Therefore, the coil winding temperature was limited to the range of 500 to 750 ℃.

Then, the oxide scale on the surface of the hot rolled steel sheet is removed by pickling, and then cold rolling is performed to obtain a cold rolled steel sheet having a predetermined sheet thickness. The pickling condition and the cold rolling condition are not particularly limited, and may be a conventional method. The rolling reduction during cold rolling is preferably 40% or more from the viewpoint of increasing the nucleation sites during recrystallization annealing and promoting the refinement of crystal grains. However, if the reduction ratio is too large, the work becomes difficult due to work hardening of the steel sheet,
The upper limit of the rolling reduction is preferably about 80%.

Then, the cold rolled steel sheet thus obtained is subjected to recrystallization annealing. That is, the cold-rolled steel sheet obtained in the above process is heated in the temperature range of Tre or higher and Ac ₃ or lower to perform recrystallization annealing. In the present invention steel material subjected to component adjustment as described above, is the predicted value of the recrystallization temperature between Ac ₃ is the predicted value of the Ac ₁ and Ac ₃ transformation point is the predicted value of the Ac ₁ transformation point Since Tre is secured, recrystallization annealing can be performed at a temperature below the Ac ₃ transformation point. That is, the annealing temperature Tre above, by the Ac ₃ or less, it is possible to perform recrystallization annealing at a 2-phase region temperature. The purpose of this recrystallization annealing is to suppress the grain growth of ferrite by setting the recrystallization temperature in the two-phase temperature range, to secure the fine ferrite grains after the recrystallization annealing, and to make the austenite phase in the subsequent heat treatment before plating. To make it easier to concentrate the carbon to obtain the desired second phase, and to make the average crystal grain size of ferrite in the finally obtained hot-dip galvanized cold-rolled steel sheet finer to 4.0 μm or less, and The purpose is to improve the plating property when performing a plating treatment in a continuous hot dip galvanizing line or the like by promoting the concentration of components on the surface and removing the concentrated components by subsequent pickling.

Here, recrystallization occurs during the recrystallization annealing time.
Good between 20 and 60 seconds. If the time is shorter than 20 seconds, recrystallization is insufficient and the structure that has been stretched in the rolling direction remains, so that ductility cannot be secured. Further, since the crystal grains are coarsened for a time longer than 60 seconds, the desired strength and yield ratio cannot be obtained, and it is disadvantageous in the concentration of the additive element to austenite in the subsequent heat treatment. However, it becomes difficult to secure the desired second phase. The rate of temperature rise during recrystallization annealing is usually 2 to
It may be about 20 ° C / s, and cooling after recrystallization annealing is
In order to suppress the growth of crystal grains as much as possible,
It is preferable to cool at an average cooling rate of up to 200 ° C of 10 ° C / s or more.

Then, after the above-mentioned recrystallization annealing, pickling is carried out to remove the surface oxide which adversely affects the plating property.
That is, P, Si, Mn, Cr precipitated on the steel plate surface during annealing.
The surface concentrated layer that has been concentrated as an oxide is removed. Since such a surface concentrated layer to be removed can be removed by light pickling, the conventional light pickling before continuous hot dip galvanizing is sufficient.

After the above pickling, heat treatment is performed in the temperature range of Ac ₁ to Ac ₃ and then cooled to the hot dip galvanizing start temperature, and hot dip galvanizing treatment (including the case of alloying treatment) is performed. To do. In order to make ferrite the main phase and suppress the grain growth of ferrite so that the average crystal grain size of the finally obtained hot-dip galvanized cold-rolled steel sheet is 4.0 μm or less, recrystallization annealing-after pickling The heat treatment is the above Ac ₁ to A
Must be done in the temperature range of c ₃ . The above heat treatment is preferably performed in a continuous hot-dip galvanizing line.

As the heat treatment conditions of the present invention, the above-mentioned heat treatment conditions are basically sufficient. However, in the present invention, by controlling the microstructures other than ferrite, a steel sheet having a more excellent strength-ductility balance, A steel sheet excellent in strength-ductility balance and hole expandability, and a steel sheet excellent in strength-ductility balance and impact resistance can be obtained. For that purpose, it is necessary to change the heat treatment conditions after the above-mentioned recrystallization annealing-pickling and before the hot dip galvanizing treatment according to the desired characteristics and structure.

In order to obtain a steel sheet having a more excellent strength-ductility balance, the following heat treatment is necessary. That is, after heating in the temperature range of Ac ₁ to (Ac ₁ + 70 ° C.) for 5 to 30 seconds, it is cooled to the hot dip galvanizing start temperature at a rate of 5 to 15 ° C./s. By performing heat treatment under such conditions, substitutional alloying elements such as C, Mn, Mo, and Ni are concentrated and stabilized in the austenite phase generated by transformation, and as a result, residual austenite obtained by cooling is obtained. The phase can be stabilized even at room temperature, which is suitable for obtaining good ductility.

In the manufacturing method of the present invention, as described above,
Since the crystal grains of the steel are made fine by the recrystallization annealing performed prior to the above heat treatment, the distance that carbon and other alloy elements move to the γ phase is short, and the carbon and Mn, Mo to austenite are reduced. , Substitutional alloying elements such as Ni are easily concentrated, and retained austenite can be stably obtained. If the heating time in this heat treatment is less than 5 seconds, the transformation time is short and it is disadvantageous for the generation of austenite. On the other hand, if the heating time is longer than 30 seconds, the amount of carbon in austenite approaches the equilibrium amount at the heat treatment temperature. It is reduced, which is disadvantageous in producing stable retained austenite. Therefore, the heating time in the heat treatment in this case is 5 to
Limited to 30 seconds range. Further, the cooling after the heat treatment requires a cooling rate of 5 ° C./s or more in order to suppress the crystal grain growth, but conversely, if the cooling rate is too fast, martensite is formed and ductility rather deteriorates. Therefore, the cooling rate up to the hot dip galvanizing start temperature needs to be 5 to 15 ° C / s. The above heat treatment is preferably performed in a continuous hot dip galvanizing line.

Next, in order to obtain a steel sheet excellent in strength-ductility balance and hole expandability, the following heat treatment is necessary. That is, after heating in the temperature range of Ac ₁ to (Ac ₁ + 70 ° C.) for 5 seconds or more, cooling to the starting temperature of hot dip galvanizing at a rate of more than 15 ° C./s. By heat treatment under such conditions, substitutional alloying elements such as C, Mn, Mo, and Ni are concentrated and stabilized in the austenite phase generated by the phase transformation during annealing, and the subsequent cooling rate is increased. By doing so, as a result of obtaining fine martensite after cooling, good strength-ductility balance and hole expandability can be obtained.

In this manufacturing method, as described above, since the crystal grains of the steel are made fine by the recrystallization annealing performed prior to the above heat treatment, the distance over which carbon and other alloy elements move to the γ phase Shorter, carbon and M to austenite
Substitutional alloying elements such as n, Mo, and Ni are easily concentrated, and the subsequent cooling rate is higher than 15 ° C / s,
Martensite can be stably obtained. If the heating time in this heat treatment is less than 5 seconds, the transformation time is too short, which is disadvantageous in the formation of austenite. Therefore, the heating time in the heat treatment in this case is set to 5 seconds or more. In addition,
Considering the efficiency in actual operation, the upper limit of the heating time is preferably about 50 seconds. Further, cooling after the heat treatment needs to generate martensite, so cooling to the hot-dip galvanizing start temperature was performed at a rate of more than 15 ° C / s. The above heat treatment is preferably performed in a continuous hot dip galvanizing line.

Next, in order to obtain a steel sheet excellent in strength-ductility balance and impact resistance, the following heat treatment is required. That is, (Ac ₁ + 70 ° C.) to Ac ₃
After heating in the temperature range of 5 seconds or more for 5 seconds or more, it is cooled to the hot-dip galvanizing start temperature at a rate of 5 ° C./s or more.

In this manufacturing method, as described above, since the crystal grains of the steel are made fine by the recrystallization annealing performed prior to the above heat treatment, the distance that carbon and other alloy elements move to the γ phase Is short, C, Mn, Mo to austenite,
Since the substitutional alloying elements such as Ni are easily concentrated, and the heat treatment is performed in the temperature range of (Ac ₁ + 70 ° C) to Ac ₃ having a high austenite fraction, the hardenability is improved and martensite is formed. It can be stably obtained. If the heating time in this heat treatment is less than 5 seconds, the transformation time is short and it is disadvantageous for the generation of austenite. Therefore, the heating time in the heat treatment in this case is set to 5 seconds or more. Considering the efficiency of actual operation, the upper limit of the heating time is 50.
It is preferable to set it to about a second. Further, the cooling after the heat treatment needs to be performed at a rate of 5 ° C./s or more up to the hot-dip galvanizing start temperature from the viewpoint of suppressing crystal grain growth. The above heat treatment is preferably performed in a continuous hot dip galvanizing line.

Further, it is preferable that the above heat treatment is carried out in a reducing atmosphere regardless of the desired steel structure. The reducing atmosphere as used herein means a reducing atmosphere having a dew point of -20 ° C or lower, and nitrogen gas, argon gas, hydrogen gas, or a mixture of two or more of these gases is advantageously suitable. . In an atmosphere with a dew point higher than -20 ° C, thick Fe oxide is generated on the surface of the steel sheet, which is not preferable.

After cooling after the above heat treatment, hot dip galvanization is performed or alloying treatment is further performed to have a steel structure having ferrite as a main phase, and the average grain size of ferrite is 4.0 μm or less. A high-strength hot-dip galvanized cold-rolled steel sheet is obtained. The hot dip galvanizing treatment or the subsequent alloying treatment may be performed under normal conditions.

Next, in the case of obtaining a steel sheet having a more excellent strength-ductility balance, if the plating temperature becomes unnecessarily high, it becomes disadvantageous in securing the amount of retained austenite, and if the temperature becomes too low. Since the reaction of zinc and iron in the zinc bath slows down and productivity tends to decrease,
The plate temperature during hot dip galvanizing shall be 460-525 ℃. Also,
When performing the alloying treatment after the hot dip galvanizing, if the temperature becomes too high, it is also disadvantageous in securing the retained austenite, and if the alloying temperature is too low, the alloying progresses slowly and the productivity decreases. Therefore, the plate temperature during the alloying process should be 500-560 ° C. Furthermore, in order to secure the necessary retained austenite after plating and to suppress the crystal grain growth even a little, the cooling rate up to 300 ° C after plating, or when further alloying treatment is performed, the cooling rate up to 300 ° C after alloying treatment 10 ℃ / s or more.

Next, in the case of obtaining a steel sheet excellent in strength-ductility balance and hole expandability, if the hot dip galvanizing temperature becomes unnecessarily high, the isothermal transformation of bainite proceeds and the temperature becomes low. If too much, the reaction between zinc and iron in the zinc bath slows down, and productivity tends to decrease.
The plate temperature during hot dip galvanizing shall be 460-525 ℃. Also,
When performing an alloying treatment after hot dip galvanizing, if the temperature becomes unnecessarily high, the isothermal transformation of bainite also proceeds, and if the alloying temperature is too low, the progress of alloying slows down and the productivity decreases. Therefore, the plate temperature during alloying treatment should be 500-560 ° C. Furthermore, in order to make the second phase a desired martensite after plating and to suppress the growth of crystal grains as much as possible, up to 300 ° C. after plating, or if further alloying treatment is performed, after alloying treatment 300 ℃
It is necessary to set the cooling rate up to 10 ° C / s or more.

Next, in the case of obtaining a steel sheet excellent in strength-ductility balance and impact resistance, if the hot dip galvanizing temperature becomes unnecessarily high, bainite is easily produced, while the temperature is low. If the temperature is too high, the reaction between zinc and iron in the zinc bath tends to be slow and the productivity tends to decline, so the plate temperature during hot dip galvanizing is 460 to 525 ° C. Further, when performing an alloying treatment after hot dip galvanizing, if the temperature is unnecessarily high, carbides are generated, while if the alloying temperature is too low, the progress of alloying slows down and the productivity is reduced. Therefore, the plate temperature during alloying treatment should be 500-560 ° C. Furthermore, in order to suppress crystal grain growth after plating as much as possible, the cooling rate up to 300 ° C after plating, or when further alloying treatment is performed up to 300 ° C after alloying treatment, should be 10 ° C / s or more. There is a need to.

[0065]

EXAMPLE A steel slab having the composition shown in Table 1 is heated and then hot-rolled at various finish rolling finish temperatures. The hot-rolled sheet is pickled, and then cold-rolled. :
After finishing to a cold rolled steel sheet of 1.6 mm, recrystallization annealing was performed in a continuous annealing line at an average cooling rate from the annealing temperature to 200 ° C of 10 to 30 ° C / s, followed by heat treatment in a continuous hot dip galvanizing line. Subsequent to the above, a galvanizing treatment and an alloying treatment were performed to manufacture a hot-dip galvanized steel sheet. In the above manufacturing process, slab heating temperature, finish rolling end temperature and winding temperature and recrystallization annealing temperature and annealing time in a continuous annealing line, heat treatment temperature and heat treatment time before plating in a continuous hot dip galvanizing line, and further The processing conditions such as plating temperature, alloying temperature, and subsequent cooling rate are shown in Tables 2-3, 6-7, and 10-11, respectively. Also,
Tables 4-5, 8-9, and 12-13 show the results of investigations on the material properties and steel structures of the obtained hot-dip galvanized steel sheets, respectively.

In Table 1, steel symbols A to P, T, V and W are steels of the present invention, Q to S and U are comparative steels, and these comparative steels Q to
In both S and U, the recrystallization temperature of the steel sheet is the Ac ₃ transformation point or higher. In addition, Tables 4-5, 8-9, 12-13
Inside, F is ferrite, M is martensite, P is pearlite, B is bainite, θ is cementite, and γ is retained austenite.

The material properties were cut in the rolling direction.
A JIS 5 test piece was used to conduct a tensile test for investigation. The structure was investigated by observing the cross section in the rolling direction of the steel sheet with an optical microscope or a scanning electron microscope. The volume fraction of ferrite is determined by image analysis to determine the occupied area ratio of the relevant phase within a square of 1 mm square by taking multiple field-of-view photographs of a cross-section structure of 4000 times. Regarded The average crystal grain size of ferrite was measured by JIS G 05 after taking a photograph with a scanning electron microscope as described above.
It was determined in accordance with the cutting method in the ferrite grain size test method for steel specified in 52. The amount of retained austenite was determined by polishing the steel plate in the plate thickness direction to a position of 1/4 of the plate thickness and measuring the diffraction X-ray intensity on this 1/4 surface. MoK for incident X-rays
Using alpha rays, {110}, {20 of ferrite phase
0}, {211} {200}, {220}, {311} of the retained austenite phase with respect to the diffracted X-ray intensity of each surface
The diffracted X-ray intensity ratio of each surface was obtained, and the average value thereof was taken as the volume ratio of retained austenite. The volume fraction of bainite, martensite, etc. is obtained by synthesizing the image analysis using a cross-sectional micrograph of 4000 times and observing the texture with a transmission electron microscope to determine the occupied area ratio of each phase. Was regarded as the volume fraction.

Further, the hole expandability is determined by the Japan Iron and Steel Federation standard JFST.
After punching a ₁₀ mmφ (D ₀ ) punched hole on a test piece taken in accordance with 1001, it was expanded by a conical punch with an apex angle of 60 °, and the hole diameter D ( m
m) was obtained, and the evaluation was made with the λ value obtained from λ = {(D−D ₀ ) / D ₀ } × 100%. Impact resistance is described in "Journal of t
he Society of Materials Science Japan.47, 10 (199
8) Based on the stress-strain curve measured at a strain rate of 2000 / s in accordance with the high-speed tensile test method described in 1058 ”, the absorption energy E obtained by integrating the stress in the range of 0 to 30% is evaluated. did. Further, in order to evaluate the superiority or inferiority of the impact resistance with respect to the strength level of the steel sheet, the ratio of the absorbed energy E (MJ / m ³ ) to the tensile strength TS (MPa), E / TS (MJ / m ³ · MPa) was obtained. . The particle size of the carbide was measured by observing the structure using a transmission electron microscope.

[0069]

[Table 1]

[0070]

[Table 2]

[0071]

[Table 3]

[0072]

[Table 4]

[0073]

[Table 5]

[0074]

[Table 6]

[0075]

[Table 7]

[0076]

[Table 8]

[0077]

[Table 9]

[0078]

[Table 10]

[0079]

[Table 11]

[0080]

[Table 12]

[0081]

[Table 13]

The survey results obtained will be described below. The invention examples shown in Tables 2 to 13 all have a high tensile strength of 780 (MPa) or more and an excellent strength-ductility balance of TS × El of 17500 (MPa ·%) or more. First, the steel sheets having excellent strength-ductility balance shown in Tables 2 to 3 and 4 to 5 will be described in detail. As shown in Tables 4 to 5, all of the products obtained according to the present invention have high tensile strength of 780 MPa or more and TS × El of 17500 (MPa ·%).
Excellent strength-ductility balance, or more
It has a very good strength-ductility balance of over 20000 (MPa ·%).

On the other hand, in Comparative Example No. 3, since the slab heating temperature was low, TiC was coarsened, the crystal grain suppressing effect of the steel sheet was small, and the crystal grains were inevitably coarsened. In Comparative Example No. 5, since the finish rolling end temperature after heating was lower than the Ar ₃ transformation point, the grain size was coarse in the state of the hot rolled sheet, and the grain size was also coarse after cold rolling and annealing. . Comparative example No.7
For, the coil winding temperature was high and the crystal grains grew, and the tensile strength was less than 780 MPa. Comparative Example N
In o.8, the coil winding temperature was low, so bainite and martensite were the main structures in the hot-rolled sheet state, and even after cold rolling and annealing, grains easily grew as colonies,
The particle size did not become fine. Comparative Examples Nos. 9 to 11 were coarse grains because recrystallization annealing was performed at a temperature above the Ac ₃ transformation point.
Sufficient strength was not obtained. Comparative Example No. 12 was annealed at a temperature higher than the Ac ₃ transformation point, so the grain size became coarse,
The material properties have deteriorated. In Comparative Example No. 14, since the holding time during recrystallization annealing was too short, the expanded grains remained after rolling,
In addition to being excessively hardened, the ductility deteriorated. Comparative Example No. 16 is
Since the heating temperature before plating was less than the Ac ₁ transformation point, the structure was just tempered, and the hard second phase could not be formed, and the material properties deteriorated. In Comparative Examples Nos. 40, 41, and 44, since recrystallization annealing was performed at a temperature of Ac ₃ transformation point or higher, coarse grains were formed and sufficient strength was not obtained. Although it is an example of the invention, No. 18 had a short heating time before plating, so the concentration of carbon into austenite was insufficient, and residual austenite contributing to ductility did not remain, and TS ×
El ≧ 17500 (MPa ・%) was secured, but TS × El ≧ 20
000 (MPa ·%) could not be achieved. In No. 19, too, since the heating time before plating was too long, the carbon concentration in austenite decreased, and residual austenite that contributes to ductility did not remain, and TS × El ≧ 17500 (MPa ·%) was secured, but TS
× El ≧ 20000 (MPa ·%) could not be achieved. No.25
, The cooling rate after alloying was slow, so bainite transformation proceeded, the amount of retained austenite was small, and TS ×
El ≧ 17500 (MPa ・%) was secured, but TS × El ≧ 20
000 (MPa ·%) could not be achieved. Nos. 26 and 27 had high plating bath temperature and alloying temperature, so that bainite transformation proceeded and residual austenite that contributed to ductility did not remain and TS × El ≧ 17500 (MPa ·%) was secured. , TS × El ≧ 20000 (MPa ·%) could not be achieved.
In Nos. 32, 35, and 38, the cooling rate before plating was slow, so bainite transformation proceeded, the amount of retained austenite was small, and TS × El ≧ 17500 (MPa ・%) was secured, but TS × El ≧ 20000 (MPa ·%) could not be achieved.

From Tables 4 to 5, focusing on the TS × EL balance (strength-ductility balance), the comparative example had a strength-ductility balance of less than 17500 (MPa ×%), whereas the recrystallization temperature was In each of the invention examples in which the components were designed to be contained in the two-phase region and the annealing temperature during recrystallization annealing was adjusted to the two-phase region temperature, the crystal grain size was as fine as 2.0 to 3.6 μm and the strength-ductility was high. Balance is at least 17500 (MPa%) or more,
When adjusted so that the strength-ductility balance is further excellent, an excellent value of 20000 (MPa ×%) or more is obtained.

Next, steel sheets having excellent strength-ductility balance and hole expandability shown in Tables 6 to 7 and 8 to 9 will be described. As shown in Tables 8 to 9, all of the products obtained according to the present invention had TS of 780 (MPa) or more and TS × El of 1750.
0 (MPa ·%) or more, and when adjusted to have excellent hole expandability, λ value is 47% or more and TS × λ value is 38000 MPa ·% or more, excellent strength-hole expansion In addition to having a good balance of strength and high tensile strength of 810 MPa or more, it has an excellent strength-ductility balance of 18,000 (MPa x%) or more.

On the other hand, in Comparative Example No. 3, since the slab heating temperature was low, the TiC coarsely precipitated during solidification and cooling of the slab was not sufficiently re-dissolved, and the fine particles that contributed to the increase in the recrystallization temperature were obtained. Since TiC did not precipitate, the effect of suppressing the crystal grain growth of the steel sheet during cold rolling and annealing was small, and the crystal grains were coarsened, and the desired strength could not be obtained. In Comparative Example No. 5, since the finish rolling end temperature after heating was lower than the Ar ₃ transformation point, the grain size became coarse in the state of the hot rolled sheet, and the grain size became coarse after cold rolling and annealing. The material properties have deteriorated. In Comparative Example No. 7, since the coil winding temperature was high, the crystal grains grew excessively and the tensile strength was less than 780 MPa. Comparative Example No. 8, because the coil winding temperature was low, bainite and martensite are the main structures in the state of the hot rolled sheet, cold rolling, grain growth easily as colonies even after annealing, grain size Was not fine, and the desired strength could not be obtained. In each of Comparative Examples Nos. 9 to 11, recrystallization annealing was performed at a temperature above the Ac ₃ transformation point, so coarse grains were formed and sufficient strength was not obtained. Comparative example No. 12 is A
Since annealing was performed at a temperature higher than the c ₃ transformation point, the grain size became coarse and the material properties deteriorated. In Comparative Example No. 14, the holding time at the time of recrystallization annealing was too short, so the expanded grains remained after rolling, and the material properties deteriorated. In Comparative Example No. 16, the heating temperature before plating was less than the Ac ₁ transformation point, so that the structure was just tempered, and the hard second phase could not be formed, and the material properties were deteriorated. In Comparative Examples Nos. 39, 42, and 43, the recrystallization annealing temperature exceeded the Ac ₃ transformation point, so that the crystal grains became coarse and sufficient strength could not be obtained. Although it is an example of the invention, No. 18 is because the heating time before plating was too short,
The structure is such that carbides are formed only by being tempered, the second phase formed during heating is small, and it transforms to bainite during cooling, TS × El ≧ 17500 (MPa ・
%) Was secured, but the hole expandability deteriorated. No. 24
Since the cooling rate after alloying was slow, pearlite transformation, bainite transformation, etc. proceeded, and the hole expandability deteriorated. In Nos. 25 and 26, the plating bath temperature and the alloying temperature were high, so that the pearlite transformation and the bainite transformation proceeded, and the hole expandability deteriorated. In Nos. 31, 34, and 37, since the cooling rate before plating was slow, pearlite, bainite, etc. were generated during cooling, and the hole expandability deteriorated.

Next, steel sheets having excellent strength-ductility balance and impact resistance shown in Tables 10 to 11 and 12 to 13 will be described. As shown in Tables 12 to 13, all of the products obtained according to the present invention had TS × El of 17500 (MPa ·%) or more and strength −
It has a high tensile strength (TS) of 780 MPa or more, which is adjusted to have excellent ductility balance and impact resistance.
Needless to say, it is possible to obtain a high yield ratio (YR) of over 0.75 and shock absorption energy of 270 M.
Excellent impact resistance of J / m ³ or more could be obtained.

On the other hand, in Comparative Example No. 3, since the slab heating temperature was low, TiC coarsely precipitated during solidification and cooling of the slab did not sufficiently re-dissolve, and the fine particles that contribute to the increase of the recrystallization temperature were obtained. Since TiC did not precipitate, the effect of suppressing the crystal grain growth of the steel sheet during cold rolling and annealing was small, and the crystal grains were coarsened, and the desired strength could not be obtained. In Comparative Example No. 5, since the finish rolling end temperature after heating was lower than the Ar ₃ transformation point, the grain size became coarse in the state of the hot rolled sheet, and the grain size became coarse after cold rolling and annealing. It was not possible to obtain the desired strength. In Comparative Example No. 7, the coil winding temperature was high, so that the crystal grains grew too much, and as a result, the ferrite grain size became large and the tensile strength was less than 780 MPa. Comparative Example No. 8, because the coil winding temperature was low, bainite and martensite are the main structures in the state of the hot rolled sheet, cold rolling, grain growth easily as colonies even after annealing, grain size Was not fine, and the desired strength could not be obtained. Comparative Examples Nos. 9-11 are all
Since recrystallization annealing was performed at a temperature above the Ac ₃ transformation point, coarse grains were formed and desired strength could not be obtained. Comparative Example N
In the case of o.12, since the annealing was performed at a temperature higher than the Ac ₃ transformation point, the grain size became coarse and the desired strength could not be obtained. In Comparative Example No. 14, the holding time at the time of recrystallization annealing was too short, so that the expanded grains remained after the rolling and the material properties were deteriorated. In Comparative Example No. 16, the heating temperature before plating was less than the Ac ₁ transformation point, so that the structure was just tempered, and the hard second phase could not be formed, and the material properties were deteriorated. In Comparative Examples Nos. 39, 42, and 43, the recrystallization annealing temperature exceeded the Ac ₃ transformation point, so that the crystal grains became coarse and sufficient strength could not be obtained. Although it is an example of the invention, No. 18
Since the heating time before plating was too short, it had a structure in which carbide was formed almost only by being tempered, the second phase generated during heating was small, and it transformed into bainite during cooling. Since the site did not generate,
Although TS × El ≧ 17500 (MPa ·%) was secured, shock absorption energy ≧ 270 MJ / m ³ was not satisfied. No. 24
Since the cooling rate after alloying was slow, bainite transformation proceeded and the impact resistance decreased. In Nos. 25 and 26, since the plating bath temperature and the alloying temperature were high, bainite transformation proceeded and the impact resistance decreased. No.31, 3
As for 4 and 37, since the cooling rate before plating was slow, pearlite, bainite, etc. were generated during cooling, and the impact resistance decreased.

[0089]

As described above, according to the present invention, even when galvanizing is performed by using equipment such as a continuous hot dip galvanizing line, the characteristics such as strength-ductility balance, hole expansibility and impact resistance can be obtained. Excellent and tensile strength of 780 MPa
It is possible to provide a high-strength hot-dip galvanized cold-rolled steel sheet having the above-mentioned high strength, which in turn reduces the weight of the automobile body and improves the safety at the time of a collision, and not only measures for the global environment but also the safety of the driver. Contribute significantly to improvement.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ Identification code FI theme code (reference) C23C 2/40 C23C 2/40 (72) Inventor Kazuhiro Seto 1 Kawasaki-cho, Chuo-ku, Chiba-shi Kawasaki Inside the Steel Research Laboratory (72) Inventor Takashi Sakata 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Kawasaki Steel Research Institute Inside (72) Inventor Kaneharu Okuda, Kawasaki-cho, Chuo-ku, Chiba Kawasaki Steel Co., Ltd. Technical Research Institute F-term (reference) 4K027 AA05 AA23 AB02 AB13 AB28 AB42 AC73 AE12 AE22 4K037 EA01 EA04 EA05 EA15 EA16 EA18 EA19 EA23 EA25 EA27 EA31 FA03 FC07 FE02 FE03 FJ05 FJ06 GA05

Claims (10)

[Claims] 1. In mass%, C: 0.02 to 0.20%, 0.2% ≦
Si≤1.0%, Mn: 1.6-3.0%, P≤0.1%, S≤0.02
%, 0.01% ≤ Al ≤ 0.1%, N ≤ 0.005%, Ti ≤ 0.1% and Nb ≤ 0.1%, and the amounts of C, Si, Mn, Ti and Nb are as follows (1), (2), (3). Steels containing ferrite in the main phase, with the contents Ac ₁ , Ac ₃ and Tre determined by substituting in the formula in the range satisfying Ac ₁ ≤Tre ≤Ac ₃ , the balance being Fe and inevitable impurities. A high-strength hot-dip galvanized cold-rolled steel sheet having a structure, having an average crystal grain size of 4.0 μm or less, and having a hot-dip galvanized layer on the surface thereof and having an excellent strength-ductility balance. Note Ac ₃ = 915 −325.9 (% C) −35.9 (% Mn) +31.4 (% Si) --- (1) Ac ₁ = 761.3 +212 (% C) −45.8 (% Mn) +16.7 (% Si) --- (2) Tre = 777.6 + 85.3 (Ti ^* / C ^* ) + 113.8 (Nb ^* / C ^* ) --- (3) However, Ti ^* / C ^* = {(% Ti) / 47.86-(% N) /14.00-(% S) /32.0
6} / {(% C) /12.01} Nb ^* / C ^* = {(% Nb) /92.90} / {(% C) /12.01} (% M) is the content of M element (mass%) Represents Moreover, all units of temperature are Celsius degrees (° C.). 2. The strength-ductility according to claim 1, characterized in that the steel structure contains ferrite as a main phase in a volume fraction of 3 to 40% bainite and 2 to 20% residual austenite. High-strength hot-dip galvanized cold-rolled steel sheet with excellent balance. 3. The steel structure according to claim 1, wherein the main structure of the steel is ferrite and contains 5 to 12% by volume fraction of martensite, and the total of both phases is 95% or more by volume fraction. A high-strength hot-dip galvanized cold-rolled steel sheet excellent in strength-ductility balance and hole expandability, which is characterized in that 4. The steel structure according to claim 1, wherein the volume fraction of the steel structure includes ferrite: 70% or more and less than 88%, and martensite: more than 12% and 30% or less, and the total amount of both phases is volume. A high-strength hot-dip galvanized cold-rolled steel sheet excellent in strength-ductility balance and impact resistance, characterized by a fraction of 96% or more. 5. The steel sheet according to any one of claims 1 to 4, wherein the steel sheet further comprises, by mass%, Mo ≦ 1.0%, Cr ≦ 1.0% and Ni.
A high-strength hot-dip galvanized cold-rolled steel sheet having an excellent balance of strength and ductility, which has a composition containing one or more selected from ≤1.0%. 6. In mass%, C: 0.02 to 0.20%, 0.2% ≦
Si≤1.0%, Mn: 1.6-3.0%, P≤0.1%, S≤0.02
%, 0.01% ≤ Al ≤ 0.1%, N ≤ 0.005%, Ti ≤ 0.1% and Nb ≤ 0.1%, and the amounts of C, Si, Mn, Ti and Nb are as follows (1), (2), (3). A steel material containing Ac in the range where Ac ₁ , Ac ₃ and Tre obtained by substituting into the formula satisfy Ac ₁ ≤ Tre ≤ Ac ₃ and the balance is Fe and inevitable impurities is set to 1200 ° C or more. After heating, hot rolling is performed under the conditions that the finish rolling finish temperature is at or above the Ar ₃ transformation point, and then 500 to 750 ° C.
After being wound on a coil at a temperature of, then pickled and cold-rolled, at a temperature of Tre or higher and Ac ₃ or lower, the temperature is 20 to 60.
Seconds subjected to recrystallization annealing and then pickling, after conducting a heat treatment in the temperature range of the Ac ₁ ~ above Ac _3, cooled, or continue to galvanizing after the galvanizing,
A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet excellent in strength-ductility balance, which is characterized by further performing an alloying treatment. Note Ac ₃ = 915 −325.9 (% C) −35.9 (% Mn) +31.4 (% Si) --- (1) Ac ₁ = 761.3 +212 (% C) −45.8 (% Mn) +16.7 (% Si) --- (2) Tre = 777.6 + 85.3 (Ti ^* / C ^* ) + 113.8 (Nb ^* / C ^* ) --- (3) However, Ti ^* / C ^* = {(% Ti) / 47.86-(% N) /14.00-(% S) /32.0
6} / {(% C) /12.01} Nb ^* / C ^* = {(% Nb) /92.90} / {(% C) /12.01} (% M) is the content of M element (mass%) Represents Moreover, all units of temperature are Celsius degrees (° C.). 7. The heat treatment after recrystallization annealing-pickling according to claim 6, wherein the heat treatment is performed for 5 to 30 seconds in the temperature range of Ac ₁ or higher and (Ac ₁ + 70 ° C. or lower), and then hot dip galvanizing. Cool to the starting temperature at a rate of 5 to 15 ° C / s and continue to 5
After hot dip galvanizing at a temperature of 25 to 460 ℃, 300
Cool down to a temperature below 10 ° C / s at a rate of 10 ° C / s or after the hot dip galvanizing, perform an alloying treatment at 500 to 560 ° C, and then cool down to a temperature below 300 ° C at a rate of 10 ° C / s or more. And a method for producing a high-strength hot-dip galvanized cold-rolled steel sheet excellent in strength-ductility balance. 8. The heat treatment after recrystallization annealing-pickling according to claim 6, wherein the heat treatment is performed for 5 seconds or more within a temperature range of Ac ₁ or more and (Ac ₁ + 70 ° C.) or less, and then hot dip galvanization is started. Cool to temperature at rates above 15 ° C / s and continue to 525
After hot dip galvanizing at a temperature of ~ 460 ℃, 300 ℃
Cool down to 10 ° C / s or more, or after performing hot dip galvanizing and alloying at 500 to 560 ° C, cool down to 300 ° C or less at 10 ° C / s or more. A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent strength-ductility balance and hole expandability. 9. The heat treatment after recrystallization annealing-pickling according to claim 6, is a heat treatment for 5 seconds or more in a temperature range of (Ac ₁ + 70 ° C.) or higher and Ac ₃ or lower, and then starts hot dip galvanization. Cool to temperature at a rate of 5 ° C / s or more, and continue 5
After hot dip galvanizing at a temperature of 25 to 460 ℃, 300
Cool down to a temperature below 10 ° C / s at a rate of 10 ° C / s or after the hot dip galvanizing, perform an alloying treatment at 500 to 560 ° C, and then cool down to a temperature below 300 ° C at a rate of 10 ° C / s or more. And a method for producing a high-strength hot-dip galvanized cold-rolled steel sheet excellent in strength-ductility balance and impact resistance. 10. The steel material according to any one of claims 6 to 9, further comprising, in mass%, one or more selected from Mo ≦ 1.0%, Cr ≦ 1.0% and Ni ≦ 1.0%. A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet excellent in strength-ductility balance, which is characterized in that the composition is contained.