JP4172268B2 - Manufacturing method of high-strength hot-dip galvanized steel sheet with excellent stretch flangeability, strength-ductility balance, and strain age hardening characteristics - Google Patents

Description

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本発明例は、いずれも、440MPa以上の引張強さ（ＴＳ）を有し、高い穴拡げ率（λ）、高い強度−延性バランス（ＴＳ×Ｅｌ）、高いＢＨ量および高い△ＴＳを示し、伸びフランジ性、強度−延性バランス、歪時効硬化特性に優れた高張力溶融亜鉛めっき鋼板となっている。これに対し、本発明の範囲を外れる比較例では、伸びフランジ性、強度−延性バランス、歪時効硬化特性のいずれかが低い値となっている。また、表面のめっき性を観察した結果は、本発明例はいずれもめっき性は良好であることが確認された。
【００８０】
【発明の効果】
以上のように本発明によれば、440MPa以上の引張強さを有し、伸びフランジ性、強度−延性バランス、歪時効硬化特性が同時に優れ、自動車車体用として好適な高張力溶融亜鉛めっき鋼板を安価にしかも安定して製造でき、産業上格段の効果を奏する。
【図面の簡単な説明】
【図１】強度−延性バランスＴＳ×Ｅｌと｛12（Ｃ＋Ｎ）＋MnーSi｝との関係を示すグラフである。
【図２】穴拡げ率λと｛12（Ｃ＋Ｎ）＋MnーSi｝との関係を示すグラフである。
【図３】ＢＨ量、ΔＴＳと｛12（Ｃ＋Ｎ）＋MnーSi｝との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-tensile hot-dip galvanized steel sheet having a tensile strength of 440 MPa or more, which is mainly suitable for use in automobile body parts, etc., and particularly excellent in stretch flangeability, strength-ductility balance, and strain age hardening characteristics. The present invention relates to a composite structure type high-tensile hot dip galvanized steel sheet and a method for producing the same.
[0002]
In the present invention, “excellent stretch flangeability” means that the hole expansion ratio λ is 80% or more, and “excellent strength-ductility balance” means tensile strength TS and total elongation. The product of El, that is, the strength-ductility balance TS x El is 17000 MPa% or more. “Excellent strain age hardening” means that after pre-deformation of 5% tensile strain, hold at a temperature of 170 ° C. for 20 min. When the aging treatment is performed under the above conditions, the deformation stress increase amount before and after the aging treatment (BH amount: BH amount = (yield stress after aging treatment) − (pre-deformation stress before aging treatment)) is 80 MPa or more. And the tensile strength increase (before ΔTS: ΔTS = (tensile strength after aging treatment) − (tensile strength before predeformation)) before and after the strain aging treatment (predeformation + the aging treatment) is 50 MPa. That is all.
[0003]
[Prior art]
In recent years, improvement in fuel efficiency of automobiles is required from the viewpoint of protecting the global environment, and improvement in automobile safety is also required from the viewpoint of protecting passengers in the event of a vehicle collision. In order to meet these demands, studies are being actively conducted to reduce the weight and strengthen the automobile body. In order to satisfy the weight reduction and strengthening of the car body at the same time, it is said that it is effective to increase the strength of the parts material. in use.
[0004]
Since many automobile body parts made of steel sheets are formed by press working, high-tensile steel sheets used for body parts are required to have excellent press formability. Therefore, as the mechanical properties of the steel sheet, high stretch flangeability (high hole expansion ratio λ), high strength-ductility balance (TS × El), and high strain aging characteristics (high BH content, high ΔTS) are required. Yes.
[0005]
A typical example of a high-tensile steel sheet having good press formability is a composite-structure high-tensile steel sheet having a structure in which soft ferrite and hard martensite are combined. In particular, a composite structure type high-tensile steel sheet produced by subjecting a cold-rolled steel sheet to continuous annealing and then gas jet cooling is a steel sheet having low yield stress and high ductility, and also has bake hardenability. However, this type of high-strength steel sheet of a composite structure type is generally good in workability under normal conditions, but has a problem in forming under severe conditions because of poor stretch flange formability. There is also a problem that the bake hardenability is not so high.
[0006]
In recent years, as a steel sheet that can satisfy both good press formability and high strength after forming, it is soft and easy to press form before press forming, and can be hardened by paint baking after press forming to increase the strength of parts. BH steel plate (Bake-Hardenable Steel) has been developed.
As an example of such a BH steel sheet, for example, in Patent Document 1, Nb is added according to the contents of C, N, and Al, and further, the cooling rate after annealing is controlled, thereby solidifying the steel sheet. A method for producing a cold-rolled steel sheet that adjusts dissolved C and solute N, and Patent Document 2 describe a cold-rolled steel sheet that has improved bake hardenability by a combined addition of Ti and Nb. Patent Document 3 proposes a method for producing a cold-rolled steel sheet having improved bake hardenability by adding W, Cr, or Mo alone or in combination.
[0007]
In the techniques described in Patent Literature 1, Patent Literature 2, and Patent Literature 3, the strength is increased by post-forming coating baking treatment using a small amount of solid solution C and solid solution N in the steel sheet. As is well known, BH steel sheets can increase the yield strength of the material, but not the tensile strength. Therefore, in the steel plates manufactured by these conventional techniques, the deformation start stress of the part can be increased, but it can be said that the effect of increasing the stress required for the deformation throughout the entire deformation from the start of the deformation to the end of the deformation is sufficient. Absent. Moreover, it cannot be said that the effect of increasing the tensile strength after molding is sufficient.
[0008]
As a cold-rolled steel sheet whose tensile strength increases after forming, for example, in Patent Document 4, after forming, the tensile strength increases by 60 MPa or more before and after forming by performing heat treatment in a temperature range of 200 ° C. to 450 ° C. An alloyed hot-dip galvanized steel sheet is disclosed. The plated steel sheet produced by the technique described in Patent Document 4 contains C: 0.01 to 0.08%, Mn: 0.01 to 3.0%, and includes one or more of W, Cr, and Mo in a total amount: A composition containing 0.05 to 3.0%, and optionally containing one or more of Ti: 0.005 to 0.1%, Nb: 0.005 to 0.1%, V: 0.005 to 0.1%, and ferrite or ferrite mainly It is a steel plate which has the microstructure of these and has an alloying hot-dip galvanization layer in the surface layer. This plated steel sheet is heat-treated in a temperature range of 220 to 370 ° C. after processing, and fine carbides are formed in the steel. The amount is increased, and the strength (tensile strength) after processing is remarkably increased. However, the technique described in Patent Document 4 has a drawback that the heat treatment after processing needs to be performed in a temperature range of 220 to 370 ° C., which is higher than a general baking coating temperature.
[0009]
In addition, although it is a hot-rolled steel sheet, Patent Document 5 describes the production of a hot-rolled steel sheet that is soft at the time of processing and greatly increases in tensile strength to 100 MPa or more along with the yield stress by baking treatment at about 170 ° C after processing. A method is described. In the technique described in Patent Document 5, a steel containing a large amount of C (0.02 to 0.13%) and N (0.0080 to 0.0250%) is reheated to 1100 ° C or higher, and finish rolling is finished at 850 to 900 ° C. Rolled, then cooled to a temperature of less than 150 ° C. at a cooling rate of 15 ° C./s or more, wound up, leaving a large amount of solute N remaining in the steel, and having a metal structure mainly composed of ferrite and martensite It is a complex organization.
[0010]
However, using the hot-rolled steel sheet obtained by the technique described in Patent Document 5 as a starting material, cold-rolling and recrystallization annealing are also used as the cold-rolled steel sheet. It is hard to say that the strength is increased and a high BH of 80 MPa or more can be obtained. This is because the cold rolling and recrystallization annealing result in a microstructure different from that during hot rolling, and large strain accumulation occurs during cold rolling, so that carbides, nitrides or carbonitrides are likely to be formed. This is because the state of dissolved N changes.
[0011]
For such problems, Patent Document 6 includes C: 0.005 to 0.15%, Mn: 0.3 to 3.0%, Mo: 0.06 to 1.0%, Al: 0.005 to 0.02%, N: 0.050 to 0.0200%, In addition, a composition containing N / Al of 0.3 or more, a solid solution N of 0.0010% or more, and a structure containing ferrite as a main phase and martensite as a second phase at a volume ratio of 3% or more. Excellent high-tensile hot-dip galvanized steel sheets have been proposed. In the technology described in Patent Document 6, the BH amount is 80 MPa or more and before and after the strain aging treatment even under the aging treatment condition of holding 20 minutes at 170 ° C after pre-deformation of 10% tensile strain by effectively utilizing solute N. It is said that high strain age hardening characteristics can be obtained with an increase in tensile strength of 50 MPa or more.
[0012]
[Patent Document 1]
Japanese Unexamined Patent Publication No. 55-141526
[Patent Document 2]
Japanese Patent Publication No.61-45689
[Patent Document 3]
JP-A-5-25549
[Patent Document 4]
JP-A-10-310847
[Patent Document 5]
Japanese Patent Publication No. 8-23048
[Patent Document 6]
JP 2001-247946
[0013]
[Problems to be solved by the invention]
However, recently, the forming conditions have become more severe, and even high-tensile hot-dip galvanized steel sheets manufactured by the technique described in Patent Document 6 do not necessarily have characteristics that can sufficiently meet these forming conditions. However, further improvement in moldability is desired.
[0014]
In general, when the strength of a steel plate is increased, the hole expansion ratio λ and the total elongation El are decreased and press workability is decreased, and when the strength is increased, El is decreased. In addition, the compatibility with the strain aging characteristics, which has recently been attracting attention from the viewpoint of improving the formability of high-strength steel sheets, has not been realized with many technical problems. Thus, no high-tensile hot-dip galvanized steel sheet having a high λ and a high TS × El, and further having a high strain aging characteristic advantageous for improving the formability was not found.
[0015]
The present invention advantageously solves the above-mentioned problems of the prior art, has a tensile strength of 440 MPa or more, and has a composite structure type high-tensile molten zinc excellent in stretch flangeability, strength-ductility balance, and strain age hardening characteristics. It aims at proposing the manufacturing method of a plated steel plate.
[0016]
[Means for Solving the Invention]
Conventionally, it has been difficult to obtain a high-tensile hot-dip galvanized steel sheet that is excellent in stretch flangeability, strength-ductility balance, and strain age hardening characteristics at the same time. In order to achieve the above-described problems, the present inventors first considered as follows.
(1) In the case of a composite structure consisting of a ferrite phase and a martensite phase, stress is concentrated around the martensite phase, which is softer than the ferrite phase. In order to reduce the hardness difference between the ferrite phase and the martensite phase, the quenching after annealing and subsequent tempering soften the martensite phase (generate an appropriate amount of tempered martensite phase), as well as component content and annealing. It is effective from the viewpoint of improving the stretch flangeability to distribute an appropriate amount of intrinsic strengthening elements such as C, N, Si, Mn, etc. to the ferrite phase and to make the ferrite phase hard by optimizing the thermal history at the time.
(2) In order to improve the strength-ductility balance, it is generally effective to make the structure a composite structure of ferrite and martensite and increase the ferrite phase or refine the martensite phase. When strengthening, when the phase fraction of the martensite phase is excessively increased, stretch flangeability is deteriorated. In order to improve the stretch flangeability and strength-ductility balance at the same time, the phase fraction of the martensite phase is adjusted to a range where no significant deterioration of the strength-ductility balance and stretch flangeability occurs, and the martensite It is effective to achieve softening.
(3) In order to improve the strain aging characteristics, it is only necessary to increase the amount of solid solution N in the ferrite phase, which is expected to have a large contribution to the strain aging hardening characteristics by introducing more dislocations. Conceivable. In order to increase the amount of solute N, it is desirable to cool at a high speed as much as possible after annealing in order to suppress the precipitation of N as precipitates during cooling after annealing. However, containing excessive solid solution N in the ferrite phase may significantly deteriorate the ductility of the ferrite and cause a significant decrease in the strength-ductility balance. Further, increasing the cooling rate after annealing may significantly increase the martensite phase and lower the ductility, and may cause deterioration in aging resistance at room temperature. For this reason, in order to simultaneously improve stretch flangeability, strength-ductility balance, and strain age hardening characteristics, it is necessary to optimize the steel composition, annealing temperature, and heat pattern during tempering. further,
(4) In order to obtain a hot dip galvanized steel sheet having excellent stretch flangeability, strength-ductility balance, and strain age hardening characteristics, the concentration of components formed on the steel sheet surface layer in the steel sheet annealing process before the hot dip galvanizing process. After removing the layer, it is effective to perform a hot dip galvanizing step including a pre-plating annealing treatment and a hot dip galvanizing treatment in a continuous hot dip galvanizing line.
[0017]
Specifically, based on this idea,
(1) Optimization of C, Mn, Si content, which greatly changes the phase fraction and hardness of ferrite phase and martensite phase and greatly affects stretch flangeability and strength-ductility balance,
(2) Optimization of the amount of N that effectively works to improve strain age hardening characteristics and has a great effect on the hardness of the ferrite phase and martensite phase, and
(3) Optimization of pre-plating annealing conditions affecting the refinement and softening of martensite,
Various studies were conducted.
[0018]
Next, basic experimental results performed by the present inventors will be described.
The basic composition is C: 0.042%, Mo: 0.15%, P: 0.009%, S: 0.002%, Al: 0.008%, N: 0.0164%, and C, Si, and Mn are added to C: 0.015- A sheet bar having a composition changed in the range of 0.031%, Si: 0.02 to 0.5%, and Mn: 0.2 to 2.0% was heated to 1250 ° C and soaked, so that the finish rolling finish temperature would be 900 ° C. Pass rolling was performed to obtain a hot rolled sheet having a thickness of 4.0 mm. After finishing rolling, the hot-rolled sheet was subjected to heat retention (600 ° C. × 1 h) corresponding to coil winding processing.
[0019]
Subsequently, these hot-rolled sheets were cold-rolled with a reduction ratio of 70% to obtain cold-rolled sheets having a thickness of 1.2 mm. The obtained cold-rolled sheet was annealed at 850 ° C for 40 s, then gas-cooled to 700 ° C so that the average cooling rate was 30 ° C / s, and then between 700 ° C and 100 ° C or less. Water cooling was performed so that the average cooling rate of the water became about 600 ° C./s, and then pickling.
[0020]
After that, these cold-rolled sheets were annealed before plating at 680 ° C for 40 s, and then gas-cooled so that the average cooling rate up to 475 ° C was 15 ° C / s, and then a 475 ° C zinc bath (0.13 % Al-Zn) for 3 s to form a hot-dip galvanized layer on the surface of the cold-rolled sheet, and then the basis weight is 45 g / m by gas wiping.²Adjusted. Then, after heating again to 520 ° C and alloying the hot-dip galvanized layer, gas cooling was performed so that the average cooling rate up to 100 ° C was 15 ° C / s, and an alloyed hot-dip galvanized steel sheet was obtained. . The obtained galvannealed steel sheet is a composite structure type hot dip galvanized steel sheet having a structure composed of ferrite, martensite, and tempered martensite.
[0021]
The obtained galvannealed steel sheet was subjected to a tensile test, a hole expansion test, and a strain age hardening test.
The tensile test is performed in accordance with the provisions of JIS Z 2241 using a JIS No. 5 tensile test piece whose major axis is perpendicular to the rolling direction, and tensile properties (yield strength YS, tensile strength TS, elongation El) are measured. Asked.
[0022]
The hole expansion test conforms to the provisions of the Japan Iron and Steel Federation Standard JFS T 1001, using a test piece of size 80 mm x 80 mm x plate thickness 1.2 mm, initial hole diameter 10 mm, die inner diameter 10.3 mm, and clearance plate The hole expansion rate λ was determined under the condition of 12.5% of the thickness. Hole expansion rate λ is calculated using λ (%) = (Dr-Do) / (Do) x 100 (where Dr: hole diameter after fracture (mm), Do: initial hole diameter (mm)) did.
[0023]
In the strain age hardening test, pre-deformation with a tensile pre-strain of 5% was applied using a JIS No. 5 tensile test piece with the long axis perpendicular to the rolling direction, and a heat treatment of 170 ° C x 20 min was performed as a coating baking equivalent process. After applying tensile test, pre-deformation-yield stress YS after heat treatment_BH, Tensile strength TS_BH, BH amount = YS_BH-S_Five%, ΔTS = TS_BH-TS was calculated. S_Five%Is the deformation stress when pre-deformed 5%, and TS is the tensile strength.
[0024]
The obtained results are shown in FIGS.
1 shows the relationship between the strength-ductility balance TS × El and {12 (C + N) + Mn−Si}, FIG. 2 shows the relationship between the hole expansion ratio λ and {12 (C + N) + Mn−Si}, and FIG. 3 shows the BH amount. , ΔTS and {12 (C + N) + Mn−Si}, respectively. C, N, Mn, and Si are the contents (mass%) of each element.
[0025]
1, 2, and 3, by adjusting {12 (C + N) + Mn−Si} to a range of 0.5 to 2.0, the hole expansion ratio λ, the strength-ductility balance TS × El, the BH amount, and ΔTS are simultaneously adjusted. It can be seen that a high value can be obtained, and a hot-dip galvanized steel sheet excellent in stretch flangeability, strength-ductility balance, and strain age hardening characteristics can be produced. It was also found that it is important to adjust {12 (C + N) + Mn-Si} to a range of 0.5 to 2.0 and appropriately control the annealing temperature and the heat pattern during subsequent cooling.
[0026]
  The present invention has been completed by further study based on the above-described findings, and the gist of the present invention is as follows..
(1) By mass%, C: 0.01 to 0.10%, Si: 0.01 to 0.5%, Mn: 0.1 to 2.0%, P: 0.08% or less, S: 0.005% or less, Al: 0.02% or less, N: 0.0050 to 0.0250% , Mo: 0.05 to 1.0%, N / Al is 0.3 or more, and C, N, Mn, and Si are represented by the following formula (1)
                  0.5 ≦ 12 (C + N) + Mn−Si ≦ 2.0 (1)
(Here, C, N, Mn, Si: content of each element (mass%))
Contain to satisfyAnd the rest Fe And inevitable impuritiesA steel slab having a composition is heated to a heating temperature of 1000 ° C. or higher, and then roughly rolled to form a sheet bar. The sheet bar is subjected to finish rolling to a finish rolling exit temperature of 800 ° C. or higher, and the winding temperature is set to:200 ℃ or moreA hot-rolling process for winding a hot-rolled sheet at 750 ° C. or lower; a cold-rolling process for pickling and cold-rolling the hot-rolled sheet to form a cold-rolled sheet;_ThreeTransformation point -50 ° C) to (Ac_ThreeAfter performing an annealing treatment to heat to an annealing temperature in the temperature range of the transformation point + 50 ° C., from the annealing temperature (Ac₁Transformation point + 20 ° C) to (Ac₁The average cooling rate to the predetermined temperature in the temperature range of the transformation point −100 ° C. is 5 to 50 ° C./s, and further the average cooling rate from 300 to 100 ° C. is at least 300 ° C. or less. After performing a steel plate annealing step for performing cooling at a temperature of at least ° C./s and a component concentrated layer removing step for removing the component concentrated layer formed on the surface layer by pickling in the steel plate annealing step, (Ac₁Transformation point) ~ (Ac₁After the pre-plating annealing treatment is applied to the temperature range of the transformation point + 50 ° C., it is cooled to a predetermined hot dip galvanizing temperature at a cooling rate of 10 ° C./s or more, and melted on the surface at the hot dip galvanizing temperature. A hot dip galvanizing process for forming a galvanized layer, and then cooling at a cooling rate of 10 ° C./s or more from the hot dip galvanizing temperature to 100 ° C. or lower, or 100 ° C. Without cooling at a cooling rate of 10 ° C./s or more until the following, after heating to a temperature range of 470 to 600 ° C., alloying the hot-dip galvanized layer, and then to 100 ° C. or less And an alloying process step of cooling at a cooling rate of 10 ° C./s or more, and having excellent stretch flangeability, strength-ductility balance, and strain age hardening characteristics, and a tensile strength of 440 MPa or more. Composite structure type high tension Manufacturing method of hot dip galvanized steel sheet.
(2) (1In addition to the said composition, in addition to the said composition, 1 type or 2 types of Cu and Ni are further contained in total 2.0% or less, The manufacturing method of the composite structure type | mold high-tensile-dip galvanized steel sheet characterized by the above-mentioned .
(3) (1) Or (2In addition to the above composition, in addition to the above composition, one or more of Nb, Ti, V, and B are represented by the following formula (2):
                  N / (Al + Nb + Ti + V + B) ≧ 0.3 (2)
(N, Al, Nb, Ti, V, B: content of each element (mass%))
Is included so as to satisfy the above-described requirements, a method for producing a composite structure type high-tensile galvanized steel sheet.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
  Of the present inventionObtained by manufacturing methodHigh-tensile hot-dip galvanized steel sheet has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the surface layer, and has a high tensile strength (TS) of 440 MPa or more, stretch flangeability, strength-ductility balance, and strain. It is a composite structure type high-tensile hot-dip galvanized steel sheet with excellent age hardening characteristics.
[0028]
  First, the present inventionObtained by manufacturing methodThe reason for limiting the composition of the hot-dip galvanized steel sheet will be described. The mass% is simply written as%.
  C: 0.01-0.10%
  In the present invention, C is contained by 0.01% or more from the viewpoint of securing a tensile strength of 440 MPa or more and making the structure a composite structure. On the other hand, if the content exceeds 0.10%, the ductility and further formability of the steel sheet are significantly deteriorated due to an increase in the fraction of carbides in the steel. Further, as an important problem, when C is contained in excess of 0.10%, weldability such as spot weldability and arc weldability is significantly lowered. For this reason, in this invention, C was limited to 0.01 to 0.10% of range. From the viewpoint of improving moldability, C is preferably 0.08% or less. For applications in which good ductility is particularly important, C is more preferably 0.05% or less.
[0029]
Si: 0.01-0.5%
Si is a useful strengthening element that can increase the strength of a steel sheet without significantly reducing the ductility of the steel. Such an effect is recognized when the content is 0.01% or more. However, when Si is contained in excess of 0.5%, adverse effects such as inhibiting wettability among plating properties are exhibited. For this reason, Si was limited to the range of 0.01 to 0.5%. In addition, Preferably it is 0.1% or more. Moreover, when manufacturing a steel plate having a high strength and a high ductility such that the tensile strength exceeds 500 MPa, Si is more preferably 0.2% or more from the viewpoint of a strength-ductility balance.
[0030]
Mn: 0.1 to 2.0%
Mn has the effect of strengthening steel, further reducing the critical cooling rate to obtain a composite structure of ferrite and martensite, promoting the formation of a composite structure of ferrite and martensite, and refining crystal grains Has the effect of In order to obtain such an effect, in the present invention, Mn is 0.1% or more, and it is contained in an amount corresponding to the cooling rate after recrystallization annealing. Mn is an effective element for preventing hot cracking due to S, and is preferably added according to the amount of S contained. Further, when high strength exceeding TS: 500 MPa is required, Mn is preferably contained in an amount of 0.5% or more. More preferably, it is 1.0% or more. By increasing the Mn content to this level, there is a great advantage that the variation in the mechanical properties of the steel sheet, particularly the variation in the strain age hardening characteristics, is significantly reduced with respect to fluctuations in hot rolling conditions. However, if Mn is contained excessively exceeding 2.0%, the detailed mechanism is unknown, but it tends to increase the hot deformation resistance of the steel sheet, and further tends to deteriorate the weldability and weldability. Further, since the formation of ferrite is suppressed, the ductility tends to be remarkably lowered. From the above, Mn was made 2.0% as the upper limit. In applications where better corrosion resistance and formability are required, Mn is desirably 0.80% or less.
[0031]
P: 0.08% or less
P has an effect of strengthening steel and is preferably contained in an amount of 0.001% or more depending on the desired strength. However, if it exceeds 0.08%, the press formability deteriorates. For this reason, P was limited to 0.08% or less. In addition, when more excellent press formability is required, P is preferably 0.05% or less. Furthermore, when C, Mn, etc. are contained in a large amount in applications requiring high strength of TS: 590 MPa or more, P is preferably 0.05% or less from the viewpoint of weldability.
[0032]
S: 0.005% or less
S is present as an inclusion in the steel sheet, and is an element that causes deterioration of the ductility and formability of the steel sheet, particularly the stretch flange formability, and therefore it is preferably reduced as much as possible in the present invention. If S is reduced to 0.005% or less, the adverse effect on stretch flangeability can be ignored. Therefore, in the present invention, S is made 0.005% as the upper limit. In addition, when more excellent stretch flange formability is required, or when a large amount of C, Mn, etc. is contained to ensure TS: 590 MPa or more and excellent weldability is required, S is It is preferable to make it 0.003% or less.
[0033]
Al: 0.02% or less
Al acts as a deoxidizer for steel, is an element useful for improving the cleanliness of steel, and is also an element useful for refining the structure of steel, and it must contain 0.001% or more. preferable. More preferably, it is 0.005% or more. In the present invention, solid solution N is also used as a strengthening element, but the aluminum killed steel to which Al is added in an appropriate range is superior in mechanical properties to the conventional rimmed steel to which no aluminum is added. Yes. On the other hand, when the Al content exceeds 0.02%, the surface properties are deteriorated and the solid solution N is remarkably lowered, and it is difficult to secure extremely large age-hardening characteristics, which is the object of the present invention. Therefore, in the present invention, the upper limit of Al is 0.02%, which is lower than that of the conventional steel. From the viewpoint of the stability of the material, Al is preferably in the range of 0.001 to 0.015%. Further, although there is a concern that the reduction of the Al content leads to the coarsening of the crystal grains, in the present invention, it is prevented by limiting the other alloy elements to the optimum amount and setting the annealing conditions in the optimum range.
[0034]
N: 0.0050-0.0250%
N is an important element for developing excellent strain age hardening characteristics. N also has the effect of lowering the transformation point of steel, and is effective in stabilizing the operation in the situation where it is not desired to perform rolling with a thin material that greatly cuts the transformation point. In the present invention, by containing N in an appropriate range and controlling the production conditions, the necessary and sufficient solid solution N is ensured in the state of the hot dip galvanized steel sheet (product). The effect of increasing the strength (yield stress YS and tensile strength TS) by strain aging hardening is sufficiently obtained, and the target tensile strength of 440 MPa or more and deformation stress increase of 80 MPa or more (BH amount before and after strain aging treatment) ) And an increase in tensile strength (ΔTS) of 50 MPa or more can be stably obtained.
[0035]
The above-described effects can be stably obtained when the N content is approximately 0.0050% or more. However, if the content exceeds 0.0250%, the rate of occurrence of internal defects in the steel sheet increases, and the occurrence of slab cracking during continuous casting becomes significant. For this reason, N was limited to the range of 0.0050 to 0.0250%. Note that N is more preferably in the range of 0.0070 to 0.0170% from the viewpoint of improving the stability and yield of the material in consideration of the entire manufacturing process. In addition, if it is N content in the range of this invention, there will be no bad influence on weldability etc. at all.
[0036]
Mo: 0.05-1.0% or less
Mo is an element that has the effect of improving the hardenability of steel and promoting the formation of martensite phase, and is an element that has little effect on hot dip galvanizing properties. In the present invention, a composite structure can be obtained stably. Therefore, it contains 0.05% or more positively. On the other hand, if the content exceeds 1.0%, the alloying of the plating layer is delayed. For this reason, Mo was limited to the range of 0.05 to 1.0%.
[0037]
Solid solution N: 0.0030% or more
In order for sufficient strength to be ensured with a steel sheet and for strain-age hardening by N to be effectively exhibited, N in a solid solution state (hereinafter also referred to as solid solution N) needs to be approximately 0.0030% or more.
The solute N amount is a value obtained by subtracting the precipitated N amount from the total N amount in the steel. In the present invention, the amount of precipitated N is determined by an analysis method to which a dissolution method by electrolytic extraction is applied. Various methods have been examined as methods for analyzing the amount of precipitated N, but the method of applying a dissolution method by electrolytic extraction using the constant potential electrolysis method adopted in the present invention method is the best and responds well to actual material changes. Based on. In addition, it is preferable to use an acetylacetone system as the electrolytic solution. The residue extracted by the dissolution method by electrolytic extraction using a constant potential electrolysis method was chemically analyzed to determine the amount of N in the residue, and this was defined as the amount of precipitated N.
[0038]
Further, when it is necessary to increase the yield stress and increase the tensile strength TS due to larger strain age hardening, it is effective to make the amount of solute N approximately 0.0050% or more.
N / Al ratio: 0.3 or more
In order to make solid solution N stably remain at 0.0030% or more, it is desirable to limit the content of Al having an action of strongly fixing N. For hot-dip galvanized steel sheets with various combinations of components, N / which is the ratio of N remaining in a solid solution state in the hot-dip galvanized steel sheet and N content (mass%) and Al content (mass%) As a result of investigating the relationship with the Al ratio, in the steel composition range of the steel of the present invention, by setting the N / Al value to 0.3 or more, the solid solution N amount can be stably increased to 0.0030% or more, and the target strain aging is achieved. It was confirmed that the effect of curing was exhibited. For this reason, the ratio of N / Al is 0.3 or more.
[0039]
In the present invention, C, N, Mn, and Si are represented by the following formula (1):
0.5 ≦ 12 (C + N) + Mn−Si ≦ 2.0 (1)
(Here, C, N, Mn, Si: content of each element (mass%))
Is contained so as to satisfy. Thereby, as shown in FIGS. 1, 2, and 3, all of stretch flangeability, strength-ductility balance, and strain age hardening characteristics can be improved at the same time. For this reason, it is important to strictly control the contents of C, N, Mn, and Si, that is, {12 (C + N) + Mn-Si}. Note that {12 (C + N) + Mn-Si} is preferably 0.5 to 1.8.
[0040]
Although there are many unclear points at present regarding this detailed mechanism, the present inventors consider as follows.
Si is considered to decrease the solid solution C and solid solution N in the ferrite and soften the ferrite phase, thereby improving the ductility. However, excessive content of Si sharply lowers strain aging characteristics. This is thought to be due to the decrease in solute N due to precipitation of a compound containing Si and N, but the decrease in solute N varies depending on the balance between the amount of Mn and the amount of Si, particularly {12 (C + N ) + Mn-Si} becomes less than 0.5. Contrary to Si, when the total amount of C, N, and Mn exceeds a certain range, the ferrite phase is excessively hardened, so that the strength-ductility balance is rapidly lowered. In the present invention, as a preferable production method, high-speed cooling is performed after annealing as will be described later. However, such high-speed cooling makes it easy to obtain high stretch flangeability and high strain aging characteristics, but ductility tends to deteriorate. In order to prevent this deterioration of ductility, the inclusion of Si as a ferrite stabilizing element is effective.
[0041]
In the present invention, in addition to the basic composition described above, one or two of Cu and Ni can be contained in a total of 2.0% or less.
Less than 2.0% in total of one or two of Cu and Ni
Cu and Ni have the effect | action which strengthens steel, and can select and contain 1 type or 2 types according to desired intensity | strength. However, when Cu or Ni is contained alone or in combination and exceeds 2.0% in total, the strength-ductility balance TS × El tends to decrease. For this reason, it is preferable to limit Cu or Ni to one or two in total to 2.0% or less. In addition, in order to acquire the above-mentioned effect, it is preferable to contain Cu: 0.05% or more and Ni: 0.05% or more, respectively, and you may contain individually or in combination.
[0042]
In the present invention, in addition to the basic composition described above, one or more of Nb, Ti, V, and B are represented by the following formula (2):
N / (Al + Nb + Ti + V + B) ≧ 0.3 (2)
(N, Al, Nb, Ti, V, B: content of each element (mass%))
It is preferable to contain so that it may satisfy.
[0043]
B, Nb, Ti and V have the effect of forming nitrides and increasing the strength of the steel by precipitation hardening, and can be selected and contained in one or more types depending on the desired strength. When the formula (2) is not satisfied, that is, when N / (Al + Nb + Ti + V + B) is less than 0.3, the strain age hardening characteristics tend to deteriorate. In order to obtain the effects described above, B: 0.0001% or more, Nb: 0.001% or more, Ti: 0.001% or more, and V: 0.001% or more are more preferable. Good.
[0044]
In the present invention, in addition to the above components, there is no problem even if Ca, Zr, REM, etc. are contained within the range of the normal steel composition.
The balance other than the above components is Fe and inevitable impurities. Inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. These inevitable impurity elements include Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less. acceptable.
[0045]
  Next, the present inventionObtained by manufacturing methodThe microstructure of the hot dip galvanized steel sheet will be described.
  Of the present inventionObtained by manufacturing methodThe hot-dip galvanized steel sheet is composed of a ferrite phase, which is the main phase, in an area ratio of 50 to 96% with respect to the entire structure, and as a second phase, 1 to 30% martensite phase and 3% or more tempered martensite phase. It has a complex tissue containing.
  If the ferrite phase, which is the main phase, is less than 50% in terms of the area ratio relative to the entire structure, it becomes difficult to ensure high ductility and the press formability tends to decrease. In applications where further good ductility is required, the ferrite phase is preferably 70% or more in terms of the area ratio relative to the entire structure. In order to take advantage of the composite structure, the ferrite phase needs to be 96% or less.
[0046]
Further, if the martensite phase, which is the second phase, is less than 1% in terms of the area ratio with respect to the entire structure, a high strength-ductility balance TS × El cannot be ensured. On the other hand, if the martensite phase exceeds 30% in terms of the area ratio with respect to the entire structure, stretch flangeability and strength-ductility balance deteriorate. For this reason, a martensite phase shall be 1-30% with the area ratio with respect to the whole structure | tissue. From the viewpoint of obtaining a higher strength-ductility balance, the martensite phase is preferably 3% or more in terms of the area ratio relative to the entire structure. The presence of this martensite phase is important for obtaining excellent aging resistance at room temperature.
[0047]
In the present invention, from the viewpoint of improving stretch flangeability, it is necessary to further include a tempered martensite phase of 3% or more in terms of the area ratio with respect to the entire structure as the second phase. Thereby, the hardness difference between the ferrite phase which is the main phase and the second phase is reduced, and the starting point of the crack at the time of hole expansion can be reduced. The term “tempered martensite phase” as used herein refers to a structure in which carbon and alloy elements that are supersaturated in martensite are precipitated as carbides by tempering. The martensite phase and the tempered martensite phase can be easily distinguished by structural observation with a scanning electron microscope or the like.
[0048]
In addition to the main phase and the second phase described above, any one of a pearlite phase, a bainite phase, and a retained austenite phase may be mixed as a subphase. However, these pearlite phase, bainite phase, and retained austenite phase exhibit the effect of the martensite phase more effectively. Therefore, the sum of these subphases is a structure other than the ferrite phase (second phase + subphase). It is preferable to limit the area ratio to 50% or less with respect to the total of the phases.
[0049]
  Next, the manufacturing method of the hot dip galvanized steel sheet of this invention is demonstrated.
  The composition of the steel slab is the same set as the steel sheet composition described above except for the solid solution N.WithTo do.
  AboveGroupIn order to prevent macro segregation of the components, it is desirable that the molten steel is melted by a known melting method such as a converter, electric furnace, etc., but it is desirable to form a steel slab by a continuous casting method. The steel slab may be obtained by a known casting method such as a slab casting method or a thin slab continuous casting method.
[0050]
The obtained steel slab is then formed into a hot-rolled sheet by a hot rolling process. In the hot rolling process, the steel slab is cooled to room temperature and then reheated, and is not cooled to room temperature. Energy saving processes such as direct feed rolling and direct rolling, which are rolled immediately after the application, can be applied without any problem. In particular, direct rolling is one of the useful techniques for effectively securing N in a solid solution state.
[0051]
Next, the reasons for limiting the hot rolling conditions will be described.
Slab heating temperature: 1000 ℃ or more
Slab heating temperature shall be 1000 degreeC or more from a viewpoint of ensuring N of a solid solution state as an initial state. The upper limit of the slab heating temperature is not particularly limited, but is preferably 1280 ° C. or less because of an increase in loss accompanying an increase in oxidized weight. The heated slab is made into a sheet bar having a predetermined thickness by rough rolling, and then the sheet bar is subjected to finish rolling at a finish rolling exit temperature of 800 ° C. or more to obtain a hot rolled sheet.
[0052]
  Finishing rolling delivery temperature: 800 ℃ or more
  By setting the finish rolling exit temperature to 800 ° C. or higher, a uniform and fine hot-rolled mother board structure can be obtained, and can be used without any problem in use. However, when the finish rolling exit temperature is less than 800 ° C, the hot rolled sheet structure becomes non-uniform, and the non-uniformity of the structure remains after the subsequent cold rolling, annealing, and plating treatments, and press forming. Sometimes the risk of developing various problems increases. In addition, when the finish rolling exit temperature is low, even if a high coiling temperature is employed to avoid the remaining of the processed structure, the same problem occurs with the generation of coarse grains, and the solid solution N is significantly reduced. Therefore, it becomes difficult to secure the target tensile strength of 440 MPa or more. For this reason, the finish rolling exit temperature is set to 800 ° C or higher.TheIn order to further improve the mechanical properties, it is desirable that the finish rolling outlet temperature is 820 ° C. or higher. The upper limit of the finish rolling exit temperature does not need to be specifically limited. However, when rolling at an excessively high finish rolling exit temperature, there is a risk of increasing scale wrinkling, so it may be approximately up to about 1000 ° C. preferable.
[0053]
  Winding temperature: 750 ℃ or less
  By decreasing the winding temperature after finishing rolling, the strength tends to increase. In order to obtain the target tensile strength of 440 MPa or more, the coiling temperature should be 750 ° C. or less.TheThe lower limit of the coiling temperature is not strictly limited in terms of the material, but if the coiling temperature falls below 200 ° C., the steel plate shape is significantly disturbed, increasing the risk of causing problems in actual use. In addition, the uniformity of the material tends to decrease. For this reason, the coiling temperature of the hot-rolled sheet should be 750 ° C or lower and 200 ° C or higher.TheWhen higher material uniformity is required, the coiling temperature is desirably 300 ° C. or higher.
[0054]
The hot rolled sheet is then subjected to a cold rolling process to form a cold rolled sheet. The cold rolling process usually consists of pickling and cold rolling, but if it is in a very thin scale, it can be directly cold rolled without pickling, and in the cold rolling process, At least cold rolling may be performed. In addition, when pickling, it is preferable to carry out according to a normal method. The cold rolling only needs to be a cold-rolled sheet having a desired size and shape, and in the present invention, it is not necessary to particularly limit the rolling reduction and the like, but it is 40% or more from the viewpoint of surface flatness and structure uniformity. It is desirable to set the reduction ratio.
[0055]
Next, the cold-rolled sheet is subjected to a steel sheet annealing process comprising annealing treatment and subsequent cooling. Cold-rolled sheet annealing treatment is (Ac_ThreeTransformation point -50 ° C) to (Ac_Three(Transformation point + 50 ° C) In general, when the annealing temperature is in the two-phase region of ferrite + austenite, N is preferentially distributed to the austenite phase thermodynamically, and it becomes difficult to uniformly disperse the solid solution N in the ferrite phase and martensite phase. It is considered that age hardening cannot be expected. However, in the present invention, as will be described later, since rapid cooling is performed after the annealing treatment, the concentration of C and N in the austenite is suppressed during cooling, and the precipitation amount of N remains small. To (Ac_ThreeIn the temperature range above the transformation point (-50 ° C.), high strain aging characteristics can be obtained even if annealing is performed in the two-phase region.
[0056]
  However, (Ac_ThreeWhen annealing is performed in a two-phase region in a temperature range lower than the transformation point (-50 ° C), it is expected that the annealing after the pre-plating annealing will be performed at a high speed in the annealing process before the hot dip galvanizing process described later. Therefore, the concentration of C and N in the austenite is promoted, and a hot dip galvanized steel sheet having high strain aging characteristics cannot be obtained. On the other hand, the annealing temperature in the annealing process is (Ac_ThreeWhen the temperature exceeds the transformation point + 50 ° C., the crystal grains become coarse, the stretch flangeability, the strength-ductility balance, and the strain age hardening characteristics deteriorate, and the aging resistance at room temperature decreases. For this reason, the annealing temperature of the annealing treatment is (Ac_ThreeTransformation point -50 ° C) to (Ac_ThreeTemperature range of transformation point + 50 ° C)TheThe heating rate up to the annealing temperature in the annealing treatment is preferably 1 ° C./s or more from the viewpoint of preventing the concentration of C and N in the austenite when passing through the two-phase region of ferrite and austenite. From the viewpoint of ensuring ductility, it is more preferably set to 50 ° C./s or less.
[0057]
In addition, the annealing treatment is desirably as short as possible from the viewpoint of refining the structure and securing solid solution N, but soaking of about 10 s or more is generally desirable from the viewpoint of operational stability. Further, from the viewpoint of uniform and refinement of the structure, it is generally preferable to be 120 s or less. In such a uniform and fine crystal structure, the area of the grain boundary, which is considered to be a stable site of solute N, greatly increases, and it is considered effective for improving the aging resistance at room temperature.
[0058]
The cold-rolled sheet is cooled after being subjected to an annealing process for heating and soaking under the above-described conditions. Cooling after the annealing treatment is important from the viewpoints of controlling the phase fraction of the ferrite phase and martensite phase, refining the structure, and securing solid solution N. In the present invention, cooling from the annealing temperature is two-stage cooling. In the first stage of cooling, from the annealing temperature (Ac₁Transformation point + 20 ° C) to (Ac₁The average cooling rate to a predetermined temperature in the temperature range of transformation point −100 ° C. is 5 to 50 ° C./s.
[0059]
  The second stage cooling following the first stage cooling is further (Ac₁Transformation point + 20 ° C) to (Ac₁From a predetermined temperature in the temperature range of the transformation point −100 ° C. to below 100 ° C.₁Transformation point + 20 ° C) to (Ac₁Cooling in which the average cooling rate between a predetermined temperature in the temperature range of the transformation point −100 ° C. and 100 ° C. is 300 ° C./s or more.
  In the first stage cooling, from the annealing temperature (Ac)₁Transformation point + 20 ° C) to (Ac₁The average cooling rate up to the specified temperature in the temperature range of transformation point -100 ° C is limited to 5-50 ° C / s.TheIf the average cooling rate is less than 5 ° C / s, the second phase becomes coarse and the strength-ductility balance deteriorates. On the other hand, when it exceeds 50 ° C./s, it becomes difficult to secure an appropriate amount of martensite phase. Further, the predetermined temperature which is the cooling end temperature of the first stage cooling is set to (Ac₁If the temperature is higher than the transformation point + 20 ° C., an appropriate composite structure cannot be obtained, and it becomes difficult to obtain a high strength-ductility balance. Therefore, the upper limit of the cooling end temperature of the first stage cooling (Ac₁Transformation point + 20 ℃)TheFurther, the predetermined temperature which is the cooling end temperature of the first stage cooling is (Ac₁If it is less than the transformation point (−100 ° C.), the strain age hardening characteristics are lowered. For this reason, the lower limit of the cooling end temperature of the first stage cooling (Ac₁Transformation point –100 ° C)TheNote that the cooling end temperature of the first stage cooling may be set as appropriate within the above temperature range.
[0060]
  In the second stage of cooling, in order to ensure solid solution N and the form and amount of the martensite phase to be appropriate, immediately after the first stage of cooling, at least the first stage cooling end temperature to 100 ° C or less. Rapid cooling at 300 ° C / s or higher with an average cooling rate from 1 to 100 ° CTheWhen the average cooling rate is less than 300 ° C./s, it is impossible to ensure a solid solution N of a predetermined amount or more, and it becomes impossible to ensure excellent stretch flangeability after the hot dip galvanizing process described later.
[0061]
The cold-rolled sheet that has undergone the steel plate annealing step is then subjected to a component concentrated layer removal step.
By the annealing treatment in the steel sheet annealing step, P is precipitated on the surface layer of the cold rolled sheet, and Mn, Si, Cr, etc. are concentrated as oxides. In the present invention, a component concentrated layer removing step for removing such a component concentrated layer by pickling is performed. In addition, pickling does not have any problem as usual pickling. The acid to be used is preferably an aqueous hydrochloric acid solution.
[0062]
Note that a micro oxide layer such as Mn or Si is formed at the grain boundary near the surface of the cold-rolled sheet by the annealing treatment in the steel sheet annealing step. These micro oxide layers cannot be completely removed by pickling in the component concentrated layer removal process, remain in the vicinity of the cold-rolled sheet surface, and during heating in the subsequent hot dip galvanizing process, , Prevents diffusion of alloy elements such as Mn to the surface and suppresses the concentration of Si and Mn on the surface of the cold rolled sheet. Thereby, the plating property is improved.
[0063]
  Following the component concentrated layer removal step, the hot dip galvanizing step is performed on the cold-rolled plate in a continuous hot dip galvanizing line.
  The hot dip galvanizing process includes an annealing process (also referred to as a pre-plating annealing process) and a hot dip galvanizing process before the hot dip galvanizing process.
  Pre-plating annealing treatment (Ac₁Transformation point) ~ (Ac₁Heating to a temperature range of (transformation point + 50 ° C.) and holding for 10 to 120 seconds. The annealing temperature of pre-plating annealing is (Ac₁When the temperature exceeds the transformation point + 50 ° C, an appropriate amount of tempered martensite phase cannot be secured, and it becomes difficult to obtain high stretch flangeability, and excessive partitioning of N to the austenite phase occurs, resulting in strain age hardening. The characteristics are significantly deteriorated. On the other hand, the annealing temperature of the pre-plating annealing treatment is (Ac₁If the transformation point is less than 0 ° C., a composite structure cannot be obtained, and a high strength-elongation balance cannot be obtained. For this reason, the pre-plating annealing treatment is (Ac₁Transformation point) ~ (Ac₁(Transformation point + 50 ℃)Yeah.
[0064]
In addition, the annealing treatment before plating is preferably as short as possible from the viewpoint of refining the structure, securing solid solution N, suppressing the formation of excessive martensite phase, and securing tempered martensite. From the viewpoint of stability, it is desirable that the soaking be about 10 s or more. Further, from the viewpoint of producing appropriate amounts of martensite phase and tempered martensite phase, it is generally preferable to be 120 s or less.
[0065]
  In addition, it is preferable that the heating rate to the annealing temperature of the annealing process before plating is 1 ° C./s or more from the viewpoint of preventing the concentration of C and N in the austenite when passing through the two-phase region of ferrite and austenite. . The upper limit of the heating rate is not particularly limited.
  The cold-rolled sheet that has been subjected to pre-plating annealing is then cooled to a predetermined hot-dip galvanizing temperature at a cooling rate of 10 ° C / s or higher.TheIf the average cooling rate is less than 10 ° C./s, a composite structure cannot be obtained stably, and it is difficult to obtain a high strength-ductility balance. Furthermore, it becomes difficult to ensure a solid solution N of a predetermined amount or more, and strain aging Curing characteristics are also significantly degraded.
[0066]
The cold-rolled sheet cooled to a predetermined hot-dip galvanizing temperature is then immersed in a hot-dip galvanizing bath and subjected to hot-dip galvanizing treatment that forms a hot-dip galvanized layer on the surface. In the hot dip galvanizing treatment, hot dip galvanizing is performed in a temperature range of about 450 to 550 ° C. to form a hot dip galvanized layer on the surface of the steel sheet in the same manner as in a normal hot dip galvanizing line. The zinc bath is preferably a Zn bath containing 0.10 to 0.15% Al. Further, after the hot dip galvanizing treatment, wiping for adjusting the basis weight may be performed as necessary.
[0067]
  After the hot dip galvanized layer is formed on the steel sheet surface, the temperature from the hot dip galvanizing temperature to 100 ° C or lower is cooled at a cooling rate of 10 ° C / s or higher.TheIf the average cooling rate is less than 10 ° C./s, the composite structure cannot be stably obtained, and it is difficult to obtain a high strength-ductility balance.
  Further, in the present invention, after forming the hot dip galvanized layer, alloying is performed in which the hot dip galvanized layer is alloyed without being cooled at a cooling rate of 10 ° C./s or higher up to 100 ° C. or less. A processing step may be performed. The heating temperature in the alloying treatment is preferably in the temperature range of 470 to 600 ° C. When the heating temperature is less than 470 ° C., the alloying progresses slowly and the productivity decreases. On the other hand, if the heating temperature exceeds 600 ° C., alloying of the plating layer proceeds too much and the galvannealed alloy layer becomes brittle. For this reason, in the present invention, the heating temperature for the alloying treatment is preferably set to a temperature range of 470 to 600 ° C. The holding time at the alloying treatment temperature is preferably 10 s or longer from the viewpoint of operational stability and 120 s or shorter from the viewpoint of structure uniformity and refinement.
[0068]
  After the alloying treatment, the cold-rolled sheet is cooled, but the average cooling rate from the alloying treatment temperature to 100 ° C is set to 10 ° C / s or more.TheIf the average cooling rate is less than 10 ° C./s, the composite structure cannot be stably obtained, and it is difficult to obtain a high strength-ductility balance.
  In the present invention, temper rolling with an elongation of 2% or less may be added after the above-described steps in order to adjust shape correction, surface roughness, and the like.
[0069]
Needless to say, the hot-dip galvanized steel sheet of the present invention may be subjected to special treatment as an upper layer of the plating layer in order to improve chemical conversion treatment properties, weldability, press formability, corrosion resistance, and the like.
[0070]
【Example】
Molten steel having the composition shown in Table 1 was melted in a converter and made into a steel slab by a continuous casting method. Subsequently, these steel slabs were formed into hot-rolled steel strips (hot-rolled sheets) having a thickness of 4.0 mm under the hot rolling conditions shown in Table 2. Subsequently, these hot-rolled steel strips (hot-rolled sheets) were pickled and subjected to a cold rolling process in which cold rolling was performed at a reduction ratio of 70% to obtain a cold-rolled steel strip (cold-rolled sheets) having a thickness of 1.2 mm. . Then, a steel plate annealing process was performed on these cold-rolled steel strips (cold-rolled sheets) by recrystallization annealing under the annealing conditions shown in Table 2 in a continuous annealing line. Subsequently, the obtained steel strip (cold rolled sheet) was subjected to a component concentrated layer removing step of pickling with a 5% HCl aqueous solution.
[0071]
Subsequently, these steel strips (cold rolled plates) were subjected to pre-plating annealing treatment and hot dip galvanizing treatment in the continuous plating line under the conditions shown in Table 2 to form a hot dip galvanized layer on the surface of the steel strip. The hot dip galvanizing treatment was performed by immersing a steel plate in a hot dip galvanizing bath, and after lifting the immersed steel plate, the basis weight was adjusted by gas wiping. Hot dip galvanizing treatment is performed at a plate temperature of 475 ° C, a plating bath of 0.13% Al-Zn, a plating bath temperature of 475 ° C, an immersion time of 3 s, and a basis weight of 45 g / m per side.²It was. Further, some steel strips (cold rolled sheets) were subjected to alloying treatment, and the hot dip galvanized layer was used as an alloyed hot dip galvanized layer. The alloying treatment was 520 ° C. × 20 s. In addition, it cooled on the conditions shown in Table 2 after the hot dip galvanization process or alloying process. After the hot dip galvanizing process or alloying process, the obtained steel strip (hot dip galvanized plate) was further subjected to temper rolling with an elongation of 0.5%.
[0072]
Test pieces were collected from the obtained steel strip (hot dip galvanized plate) and subjected to structure observation, tensile test, hole expansion test, and strain age hardening test to evaluate tensile properties, stretch flange properties, and strain age hardening properties. . The test method was as follows.
(1) Microscopic observation
A specimen is taken from the obtained steel strip, and the cross section (C cross section) perpendicular to the rolling direction is imaged with a microscopic structure using an optical microscope or a scanning electron microscope, and as a main phase using an image analyzer. The ferrite phase and the martensite phase as the second phase, the tempered martensite phase, the subphase, and the like were identified, and the structure fractions were determined. In addition, the determination of the martensite phase and the tempered martensite phase was performed by observing the presence or absence of carbide precipitation using a scanning electron microscope.
(2) Tensile test
A JIS No. 5 tensile test piece with the major axis perpendicular to the rolling direction was taken from the obtained steel strip and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to obtain tensile properties (yield stress (YS), Tensile strength (TS), elongation (El), yield ratio (YR)) were determined. The yield ratio YR was determined as YR = (YS / TS) × 100 (%).
(3) Hole expansion test
In accordance with JFS T 1001, the hole expansion test uses a test piece of size 80mm x 80mm x plate thickness 1.2mm, initial hole diameter is 10mm, die inner diameter is 10.3mm, and clearance is 12.5% of the plate thickness. The hole expansion rate λ was obtained.
(4) Strain age hardening test
In the strain age hardening test, pre-deformation with a tensile pre-strain of 5% was applied using a JIS No. 5 tensile test piece with the major axis orthogonal to the rolling direction, and the coating baking equivalent process was 170 ° C x 20 min. Pre-deformation-Deformation stress YS after heat treatment after tensile test after heat treatment_BH, Tensile strength TS_BH, BH amount = YS_BH-S_Five%, △ TS = TS_BH-TS was calculated. S_Five%Is the deformation stress after 5% pre-deformation, and TS is the tensile strength of the steel strip (hot dip galvanized plate).
[0073]
In addition, the amount of solid solution N used the value which deducted the amount of precipitation N measured by the constant-potential electrolysis method from the total N amount obtained by the chemical analysis. In addition, the plating property was confirmed by visually observing the surface of the hot dip galvanized steel sheet to check for the presence of non-plating defects.
The obtained results are shown in Table 3.
[0074]
[Table 1]

[0075]
[Table 2]

[0076]
[Table 3]

[0077]
[Table 4]

[0078]
[Table 5]

[0079]
Examples of the present invention all have a tensile strength (TS) of 440 MPa or more, exhibit a high hole expansion ratio (λ), a high strength-ductility balance (TS × El), a high BH amount, and a high ΔTS. It is a high-tensile hot-dip galvanized steel sheet with excellent stretch flangeability, strength-ductility balance, and strain age hardening characteristics. On the other hand, in the comparative example outside the scope of the present invention, any one of stretch flangeability, strength-ductility balance, and strain age hardening characteristics is a low value. Moreover, as a result of observing the plating property of the surface, it was confirmed that all of the examples of the present invention have good plating property.
[0080]
【The invention's effect】
As described above, according to the present invention, a high-tensile hot-dip galvanized steel sheet having a tensile strength of 440 MPa or more, excellent in stretch flangeability, strength-ductility balance, and strain age hardening properties is suitable for use in automobile bodies. It can be manufactured at a low cost and stably, and has a remarkable industrial effect.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between a strength-ductility balance TS × E1 and {12 (C + N) + Mn−Si}.
FIG. 2 is a graph showing a relationship between a hole expansion rate λ and {12 (C + N) + Mn−Si}.
FIG. 3 is a graph showing the relationship between the BH amount, ΔTS and {12 (C + N) + Mn−Si}.

Claims (3)

% By mass
C: 0.01 to 0.10%, Si: 0.01 to 0.5%,
Mn: 0.1 to 2.0%, P: 0.08% or less,
S: 0.005% or less, Al: 0.02% or less,
N: 0.0050-0.0250%, Mo: 0.05-1.0%
It includes, and N / Al has a 0.3 or higher, further C, N, Mn, and Si contained so as to satisfy the following formula (1), the composition balance of Fe and unavoidable impurities ing steel The slab is heated to a heating temperature of 1000 ° C. or higher and then roughly rolled into a sheet bar. The sheet bar is subjected to finish rolling with a finish rolling exit temperature of 800 ° C. or higher, and a winding temperature of 200 ° C. or higher to 750 ° C. ℃ and hot rolling step of the winding hot rolled sheet below, and a cold-rolling step of cold rolling the performs cold-rolled sheet in the hot-rolled sheet, the cold-rolled sheet (Ac ₃ transformation point -50 ° C. ) To (Ac ₃ transformation point + 50 ° C.), after annealing at an annealing temperature in the temperature range of (Ac ₁ transformation point + 20 ° C.) to (Ac ₁ transformation point −100 ° C.) Cooling is performed so that the average cooling rate to a predetermined temperature in the range is 5 to 50 ° C./s, and further to 100 ° C. or less, at least from the predetermined temperature to 100 ° C. The steel sheet annealing process which performs cooling with an average cooling rate of 300 ° C./s or more, and the component concentrated layer removal process for removing the component concentrated layer formed on the surface layer in the steel sheet annealing process by pickling Then, after performing the pre-plating annealing treatment to heat to the temperature range of (Ac ₁ transformation point) to (Ac ₁ transformation point + 50 ° C), the predetermined hot dip galvanizing treatment temperature at a cooling rate of 10 ° C / s or more Cooling to 10 ° C./s or more from the hot dip galvanizing temperature to 100 ° C. or lower after the hot dip galvanizing treatment to form a hot dip galvanized layer on the surface at the hot dip galvanizing temperature. Without chilling at a cooling rate of 10 ° C./s or more, between the hot dip galvanizing step of cooling at a rate and further up to 100 ° C. or less, in the temperature range of 470 to 600 ° C., After alloying, the temperature is kept below 100 ° C. It is excellent in stretch flangeability, strength-ductility balance and strain age-hardening characteristics, and tensile strength is 440 MPa or more. A method for producing a composite structure type high-tensile hot-dip galvanized steel sheet.
Record
0.5 ≦ 12 (C + N) + Mn−Si ≦ 2.0 (1)
Here, C, N, Mn, Si: Content of each element (mass%) The composite structure type high-tensile hot-dip galvanized steel sheet according to claim 1 , further comprising, in addition to the composition, one or two of Cu and Ni in a mass% of 2.0% or less in total. Manufacturing method. In addition to the above composition, by mass%, Nb, Ti, V, according to claim 1 or 2 one or more of B, characterized in that it contains so as to satisfy the following formula (2) The manufacturing method of the composite structure type | mold high-tensile-strength hot-dip galvanized steel plate of description.
Record
N / (Al + Nb + Ti + V + B) ≧ 0.3 (2)
Here, N, Al, Nb, Ti, V, B: Content of each element (mass%)

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