JP3925064B2 - Hot-dip galvanized steel sheet excellent in press formability and strain age hardening characteristics and method for producing the same - Google Patents

Description

【００７０】
【表２】

【００７１】
【表３】

【００７２】
本発明例は、いずれも、低い降伏強さＹＳと高い伸びＥｌと、低い降伏比ＹＲを有し、さらに大きな穴拡げ率λを示して、伸びフランジ成形性を含むプレス成形性に優れるとともに、大きなΔＹＳと極めて大きなΔＴＳを示し、歪時効硬化特性に優れた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、降伏強さＹＳが高いか、伸びＥｌが低いか、あるいは穴拡げ率λが小さいか、ΔＴＳが小さく、プレス成形性、歪時効硬化特性が低下しためっき鋼板となっている。
【００７３】
（実施例２）
表４に示す組成の溶鋼を転炉で溶製し、連続鋳造法でスラブとした。これらスラブを、表５に示す条件の熱間圧延により板厚1.6mm 、4.0mm の熱延鋼帯（熱延板）にした。これら1.6mm 厚の熱延鋼帯（熱延板）を酸洗した後、連続溶融亜鉛めっきライン（ＣＧＬ）で、表５に示す条件で焼鈍を行い、ついで溶融亜鉛めっき処理を施し、鋼板表面に溶融亜鉛めっき層を形成した。ついで、表５に示す条件で溶融亜鉛めっき層の合金化処理を行った。なお、一部の鋼板は溶融亜鉛めっき処理のままとした。
【００７４】
また、4.0mm 厚の熱延鋼帯（熱延板）を、さらに酸洗したのち、表５に示す条件で冷延工程により冷延鋼帯（冷延板）とした。これら冷延鋼帯（冷延板）を、連続溶融亜鉛めっきライン（ＣＧＬ）で、表５に示す条件で焼鈍を行い、ついで溶融亜鉛めっき処理を施し、鋼板表面に溶融亜鉛めっき層を形成した。ついで、溶融亜鉛めっき層の合金化処理を行った。なお、一部の鋼板は溶融亜鉛めっき処理のままとした。
【００７５】
なお、一部の鋼板には、連続溶融亜鉛めっきライン（ＣＧＬ）での焼鈍に先立ち、連続焼鈍ライン（ＣＡＬ）で、表５に示す条件の前加熱処理と、ついで酸洗処理を行う前処理工程を施した。前処理工程での酸洗は、ＣＧＬ入側の酸洗槽にて行った。
なお、亜鉛めっき浴温は460 〜480 ℃の範囲とし、浸漬する鋼板の温度は、めっき浴温以上、（浴温＋10℃）以下とした。また、合金化処理は、合金化処理温度に再加熱し、その温度に15〜28ｓ間保持した。これらめっき鋼板には、さらに伸び率1.0 ％の調質圧延を施した。
【００７６】
上記した工程により得られた溶融亜鉛めっき鋼板（鋼帯）について、実施例１と同様に、微視組織、引張特性、歪時効硬化特性、穴拡げ率を求めた。なお、プレス成形性は、伸びＥｌ（延性）、降伏強さおよび穴拡げ率から評価した。
これらの結果を表６に示す。
【００７７】
【表４】

【００７８】
【表５】

【００７９】
【表６】

【００８０】
本発明例は、いずれも、低い降伏強さＹＳと高い伸びＥｌと、低い降伏比ＹＲを有し、さらに大きな穴拡げ率λを示して、伸びフランジ成形性を含むプレス成形性に優れるとともに、大きなΔＹＳと極めて大きなΔＴＳを示し、歪時効硬化特性に優れた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、降伏強さＹＳが高いか、伸びＥｌが低いか、あるいは穴拡げ率λが小さいか、ΔＴＳが小さく、プレス成形性、歪時効硬化特性が低下しためっき鋼板となっている。
【００８１】
【発明の効果】
本発明によれば、優れたプレス成形性を維持しつつ、プレス成形後の熱処理により引張強さが顕著に上昇する溶融亜鉛めっき鋼板を、安定して製造することが可能となり、産業上格段の効果を奏する。本発明の溶融亜鉛めっき鋼板を自動車部品用に適用した場合、プレス成形が容易で、かつ完成後の部品特性を安定して高くでき、自動車車体の軽量化に十分に寄与できるという効果もある。
【図面の簡単な説明】
【図１】予変形−熱処理後のΔＴＳと再結晶焼鈍温度の関係におよぼすCu含有量の影響を示すグラフである。
【図２】予変形−熱処理後のΔＴＳと熱処理温度の関係におよぼすCu含有量の影響を示すグラフである。
【図３】λとＹＲとの関係におよぼすCu含有量の影響を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention mainly relates to cold-rolled steel sheets for automobiles, and in particular, press formability such as bending workability, stretch flange workability and drawability is good, and the tensile strength is remarkably increased by heat treatment after press forming. The present invention relates to a hot-dip galvanized steel sheet having extremely large strain age hardening characteristics and a method for producing the same. The extremely large strain age hardening characteristic referred to in the present invention, that is, “excellent strain age hardening characteristic” means that it has a strain age hardening characteristic of ΔTS: 80 MPa or more. In the present invention, ΔTS is the tensile strength before and after heat treatment when pre-deformation treatment with a plastic strain amount of 5% or more and heat treatment at a temperature in the range of 150 to 350 ° C. with a holding time of 30 seconds or more. Increase amount == (tensile strength after heat treatment) − (tensile strength before pre-deformation treatment)}. In the present invention, ΔYS refers to the amount of increase in deformation stress before and after heat treatment (denoted as ΔYS; ΔYS = (yield stress after heat treatment) − (yield stress before pre-deformation treatment)).
[0002]
[Prior art]
In recent years, in connection with exhaust gas regulations due to global environmental conservation issues, the reduction of vehicle body weight has become a very important issue. Recently, in order to reduce the weight of the vehicle body, it has been studied to increase the strength of a steel plate for automobiles and reduce the thickness of the steel plate.
Since many automotive body parts made of steel sheets are formed by press working, the steel sheets used are required to have excellent press formability. In order to obtain a steel sheet having excellent press formability, it is important to secure low yield strength and high ductility. In addition, stretch flange molding is often used, and it is necessary to have a high hole expansion rate. However, generally, when the strength of a steel plate is increased, the yield strength increases and the shape freezing property deteriorates, the ductility decreases, the hole expansion rate decreases, and the press formability tends to decrease. For this reason, conventionally, a high-strength steel sheet having high ductility and excellent press formability has been demanded.
[0003]
Recently, in order to protect an occupant in the event of a collision, importance is placed on the safety of the automobile body. For this reason, an improvement in impact resistance that is a measure of safety in the event of a collision is required. The higher the strength of the finished vehicle, the more advantageous for improving the impact resistance. Therefore, when molding automotive parts, steel sheets with high strength and excellent impact resistance were most strongly desired at the time when finished with a low strength, high ductility and excellent press formability. .
[0004]
In response to such a demand, a steel sheet having both press formability and high strength has been developed. This steel sheet is a paint bake hardening type steel sheet whose yield stress increases when it is subjected to a paint baking process including holding at a high temperature of usually 100 to 200 ° C. after press working. In this steel sheet, the amount of C remaining in the final solid solution state (solid solution C amount) is controlled within an appropriate range, soft at the time of press forming, ensuring shape freezing and ductility, and baking after press forming. At the time of processing, the remaining solid solution C adheres to the dislocations introduced during press forming, thereby preventing the dislocations from moving and increasing the yield stress. However, in this paint bake hardened automotive steel sheet, although the yield stress can be increased, the tensile strength cannot increase it.
[0005]
JP-B-5-24979 discloses a composition containing C: 0.08 to 0.20%, Mn: 1.5 to 3.5%, the balance being Fe and inevitable impurities, and having a uniform structure with a ferrite content of 5% or less. A bake hardenable high-tensile cold-rolled steel sheet made of bainite or bainite partially containing martensite is disclosed. The cold-rolled steel sheet described in Japanese Patent Publication No. 5-24979 is formed by rapidly cooling a temperature range of 400 to 200 ° C. in the cooling process after continuous annealing, and then gradually cooling the structure, so that the structure can be changed from a conventional ferrite-based structure. It is a bainite-based structure, and is intended to obtain a high bake hardening amount that has not existed before. However, the steel sheet manufactured by the technique described in Japanese Patent Publication No. 5-24979 increases the yield stress after baking, and can obtain a higher bake hardening amount than before, but still increases the tensile strength. There is a problem that the impact resistance cannot be improved.
[0006]
On the other hand, some steel sheets that are subjected to heat treatment after press forming to increase not only the yield stress but also the tensile strength are proposed.
For example, Japanese Patent Publication No. 8-23048 discloses steel containing C: 0.02 to 0.13%, Si: 2.0% or less, Mn: 0.6 to 2.5%, sol.Al: 0.10% or less, and N: 0.0080 to 0.0250%. , Reheat to 1100 ° C or higher, perform hot rolling to finish finish rolling at 850-950 ° C, then cool down to a temperature of less than 150 ° C at a cooling rate of 15 ° C / s or more, and take ferrite and martensite There has been proposed a method for producing a hot-rolled steel sheet having a composite structure mainly composed of. However, in the steel sheet manufactured by the technique described in Japanese Patent Publication No. 8-23048, although tensile strength increases with yield stress due to strain aging hardening, it is wound at an extremely low winding temperature of less than 150 ° C. There was a problem that the fluctuation of mechanical characteristics was large. In addition, there is a large variation in the amount of increase in yield stress after press molding-paint baking, and there is also a problem that the hole expansion rate (λ) is low, stretch flangeability is lowered, and press formability is insufficient.
[0007]
On the other hand, automobile parts are also required to have high corrosion resistance depending on the application site. A hot-dip galvanized steel sheet is suitable as a material to be applied to a part that requires high corrosion resistance, and there is a demand for a hot-dip galvanized steel sheet that is excellent in press formability at the time of forming and is significantly hardened by heat treatment after forming.
In response to such a demand, for example, Japanese Patent No. 2802513 proposes a method for producing a hot dip galvanized steel sheet using a hot-rolled sheet as a plating original sheet. In this method, a steel slab containing C: 0.05% or less, Mn: 0.05 to 0.5%, Al: 0.1% or less, and Cu: 0.8 to 2.0% is hot-rolled at a coiling temperature of 530 ° C. or less. Subsequently, after heating to a temperature of 530 ° C. or lower to reduce the surface of the steel sheet, hot-dip galvanization is performed, whereby significant hardening by heat treatment after forming is obtained. However, in the steel sheet produced by this method, in order to obtain remarkable hardening by the heat treatment after forming, the heat treatment temperature needs to be 500 ° C. or higher, and the heat treatment temperature is high, which leaves a problem in practical use.
[0008]
Japanese Patent Application Laid-Open No. 10-310824 proposes a method for producing an alloyed hot-dip galvanized steel sheet in which a hot rolled plate or a cold rolled plate is used as a plating original plate and an increase in strength can be expected by heat treatment after forming. This method includes C: 0.01 to 0.08%, and Si, Mn, P, S, Al, and N are used in appropriate amounts, and one or more of Cr, W, and Mo are added in a total amount of 0.05 to 3.0. % Hot-rolled steel, or further cold-rolled or temper-rolled and annealed, and then hot-dip galvanized and then heat-alloyed. This steel sheet is said to have an increased tensile strength by heating in a temperature range of 200 to 450 ° C. after forming. However, since the obtained steel sheet has a microstructure of ferrite single phase, ferrite + pearlite, or ferrite + bainite structure, high ductility and low yield strength cannot be obtained, and press formability is deteriorated. was there.
[0009]
[Problems to be solved by the invention]
As described above, the present invention has been made in view of the fact that there has never been a technique for industrially and stably producing a steel sheet that satisfies these characteristics, despite extremely strong demands. Strain aging that advantageously solves the above-mentioned problems, has excellent press formability, suitable as a steel sheet for automobiles, and has extremely high tensile strength due to heat treatment at a relatively low temperature after press forming. It is an object of the present invention to propose a high-tensile hot-dip galvanized steel sheet having excellent hardening characteristics and a production method capable of stably producing the hot-dip galvanized steel sheet.
[0010]
[Means for solving problems]
  In order to achieve the above-mentioned problems, the present inventors have conducted extensive studies on the influence of alloy elements on strain age hardening characteristics. As a result, the C content is set to a low carbon range, Cu within an appropriate range, orW or evenCr,Mo ofOne or two selected from our houseSeedIn addition, by adding a steel and steel structure to a composite structure of ferrite and martensite, after pre-deformation treatment with a pre-strain amount of 5% or more and heat treatment at a relatively low temperature of 150 to 350 ° C. In addition to increased yield stress and high strain age hardening that significantly increases tensile strength, it has excellent ductility, low yield strength, and high hole expansion ratio, and excellent press formability. I found out that it would be a steel plate.
[0011]
First, basic experimental results performed by the present inventors will be described.
In mass%, C: 0.04%, Si: 0.02%, Mn: 1.6%, P: 0.01%, S: 0.004%, Al: 0.04%, N: 0.002%, Cu 0.3% and 1.3% The sheet bar having the composition contained was heated and soaked to 1150 ° C., and then subjected to three-pass rolling so that the finish rolling finish temperature was 900 ° C. to obtain a plate thickness of 4.0 mm. In addition, after finishing rolling and winding up a coil, the heat | fever equivalent process of 600 degreeC x 1h was performed. Subsequently, cold rolling with a rolling reduction of 70% was performed to obtain a cold rolled sheet having a sheet thickness of 1.2 mm.
[0012]
These cold-rolled sheets are subjected to recrystallization annealing under various conditions, then rapidly cooled to a temperature range of 450-500 ° C, immersed in a hot dip galvanizing bath (0.13 mass% Al-Zn bath), and melted on the surface A galvanized layer was formed. Subsequently, it re-heated to the temperature range of 450-550 degreeC, and the alloying process (Fe content rate in a plating layer: about 10%) of the hot dip galvanization layer was implemented.
About the obtained hot-dip galvanized steel sheet, the tensile test was implemented and the tensile characteristic was investigated. Furthermore, the strain age hardening characteristics of these plated steel sheets were investigated.
[0013]
The specimens taken from these hot dip galvanized steel sheets were subjected to a pre-deformation treatment with a tensile pre-strain amount of 5%, followed by a heat treatment at 50 to 350 ° C. × 20 min, and then a tensile test was conducted to obtain tensile properties. The strain age hardening property was evaluated by the increase in tensile strength ΔTS before and after the heat treatment. ΔTS is the tensile strength TS after heat treatment_HTAnd the difference in tensile strength TS when heat treatment is not performed (= (tensile strength TS after heat treatment_HT)-(Tensile strength TS before pre-deformation treatment)). The tensile test was carried out using JIS No. 5 tensile test pieces.
[0014]
FIG. 1 shows the effect of Cu content on the relationship between ΔTS and the recrystallization annealing temperature. ΔTS is obtained by subjecting the specimens obtained from the obtained hot-dip galvanized steel sheet to a pre-deformation treatment with a tensile pre-strain amount of 5%, followed by a heat treatment at 250 ° C. for 20 minutes, and then conducting a tensile test. Asked.
As shown in FIG. 1, when the Cu content is 1.3% by mass, the recrystallization annealing temperature is set to 700 ° C. or higher, and the steel plate structure is made of a ferrite + martensite composite structure, whereby a high strain age hardening characteristic of ΔTS: 80 MPa or higher is obtained. It turns out that it is obtained. On the other hand, when the Cu content is 0.3% by mass, ΔTS: less than 80 MPa at any recrystallization annealing temperature, and high strain age hardening characteristics cannot be obtained. It can be seen from FIG. 1 that it is possible to produce a hot-dip galvanized steel sheet having high strain age hardening characteristics by optimizing the Cu content and making the structure a ferrite + martensite composite structure.
[0015]
FIG. 2 shows the influence of the Cu content on the relationship between ΔTS and the heat treatment temperature after the pre-deformation treatment. ΔTS is the heat treatment after pre-deformation treatment for hot-dip galvanized steel sheets manufactured by annealing cold-rolled sheets at 800 ° C, which is a two-phase region of ferrite and austenite, for 40 s as a recrystallization annealing condition. Obtained by changing the temperature. The microstructure after annealing was a composite structure of ferrite and martensite with a martensite area ratio of 7%.
[0016]
From FIG. 2, ΔTS increases as the heat treatment temperature increases, but the amount of increase greatly depends on the Cu content. It can be seen that when the Cu content is 1.3% by mass, a high strain age hardening property of ΔTS: 80 MPa or more can be obtained at a heat treatment temperature of 150 ° C. or higher. When the Cu content is 0.3% by mass, ΔTS: less than 80 MPa at any heat treatment temperature, and high strain age hardening characteristics cannot be obtained.
[0017]
In addition, for cold-rolled steel sheets with a Cu content of 0.3 mass% and 1.3 mass%, after performing recrystallization annealing by changing the recrystallization annealing conditions after cold rolling, up to a temperature range of 450 to 500 ° C It was quenched and then immersed in a hot dip galvanizing bath (0.13 mass% Al-Zn bath) to form a hot dip galvanized layer on the surface, and the structure was changed from ferrite + martensite to a ferrite single phase. Subsequently, it re-heated to the temperature range of 450-550 degreeC, and the alloying process (Fe content rate in a plating layer: about 10%) of the hot dip galvanization layer was implemented. As a result, a material (steel plate) having a yield ratio YR (= (yield strength YS / tensile strength TS) × 100%) of 50 to 90% was obtained.
[0018]
About the obtained material (steel plate), the hole expansion test was implemented and the hole expansion rate ((lambda)) was calculated | required. In the hole expansion test, a punch hole is formed in the specimen by punching with a 10mmφ punch, and then a conical punch with a vertex angle of 60 ° is used, and a crack that penetrates the plate thickness is generated with the beam facing outward. Hole expansion was performed until the hole expansion rate λ was obtained. The hole expansion ratio λ is λ (%) = {(d−d₀) / D₀} × 100. D₀: Initial hole diameter, d: Inner hole diameter when cracking occurs.
[0019]
These results are summarized in the relationship between the hole expansion ratio λ and the yield ratio YR, and the influence of the Cu content on the relationship between the hole expansion ratio λ and the yield ratio YR is shown in FIG.
From FIG. 3, in the steel sheet with Cu: 0.3% by mass, the ferrite + martensite composite structure is formed, and when YR is less than 70%, λ decreases with a decrease in YR. It can be seen that a high λ value is maintained even when the YR becomes low because of a composite structure of ferrite and martensite. On the other hand, in a steel sheet having a Cu content of 0.3% by mass, low YR and high λ cannot be obtained simultaneously.
[0020]
It can be seen from FIG. 3 that a steel sheet satisfying both a low yield ratio and a high hole expansion rate can be produced by setting the Cu content within an appropriate range and forming a composite structure of ferrite and martensite.
In the hot-dip galvanized steel sheet of the present invention, pre-deformation with a strain amount greater than 2%, which is a pre-strain amount when measuring the increase in deformation stress before and after normal heat treatment, and a relatively low temperature of 150 ° C. to 350 ° C. Due to the heat treatment in the region, ultrafine Cu is precipitated in the steel sheet. According to the study by the present inventors, it is considered that high strain age hardening characteristics in which the tensile strength is remarkably increased in addition to the increase in yield stress are obtained by the precipitation of this ultrafine Cu. Such precipitation of ultrafine Cu by heat treatment in a low temperature region was not observed at all in the extremely low carbon steel or low carbon steel reported so far. The reason why ultrafine Cu is precipitated by heat treatment in a low temperature region is not clear until now, but during annealing in the α + γ two-phase region, a large amount of Cu is distributed in the γ phase, It is thought that it was taken over after cooling and Cu was in a supersaturated solid solution in martensite, and was deposited very finely by adding 5% or more of pre-strain and low-temperature heat treatment.
[0021]
  In addition, the detailed mechanism by which the hole expansion rate of steel sheets with a Cu-added microstructure with a composite structure of ferrite and martensite is high has not been clarified so far. It is thought that this is because the difference in hardness between the two is small.
  Based on the above-described novel findings, the present inventors have conducted further intensive studies, and as a result, have found that the phenomenon described above also occurs in a steel sheet not containing Cu. Instead of Cu,W or evenMo,Cr of1 or 2 of themSeedContaining and making the structure a composite structure of ferrite + martensite, when pre-strain is added and heat treatment is performed at a low temperature, ultrafine carbides are strain-induced precipitated in the martensite and the tensile strength increases. I found. This strain-induced fine precipitation during low-temperature heating isW or evenMo,Cr of1 or 2 of themSeedsIn addition, it has also been found that the addition of one or more of Nb, V, and Ti becomes more prominent.
[0022]
  The present invention has been completed by further study based on the above findings.
  That is, the first aspect of the present invention is a hot dip galvanized steel sheet having a hot dip galvanized layer or an alloyed hot dip galvanized layer on the surface of the steel sheet, wherein the steel sheet is C: 0.15% or less, Si: 2.0 by mass%. %: Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less, N: 0.02% or less, Cu:1.28~ 3.0%Including or further, the following group A to group C
          Group A: Ni: 2.0% or less
          Group B: One or two of Cr and Mo in total 2.0% or less
          Group C: A total of 0.2% or less of one or more of Nb, Ti and V
1 or 2 or more of them, the balance is composed of Fe and inevitable impurities, and the structure is a composite structure composed of a ferrite phase and a martensite phase with an area ratio of 2% or more. It is a hot-dip galvanized steel sheet excellent in press formability and strain age-hardening characteristics of ΔTS: 80 MPa or more.
[0023]
  In the first aspect of the present invention, the steel sheet is, in mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1 % Or less, including N: 0.02% or less, and, W: 0.05-2.0%TheContains,Or even more Mo : 0.05 ~ 2.0 %, Cr: 0.05 ~ 2.0 %, One or two selected from Mo , Cr In total 2.0 % Or less,Alternatively, it is preferable that one or more of Nb, Ti, and V are contained in a total of 2.0% or less, with the balance being composed of Fe and inevitable impurities.
[0024]
  In the second aspect of the present invention, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al: 0.1% or less, N: 0.02 %, Cu:1.28~ 3.0%Including or further, the following group A to group C
          Group A: Ni: 2.0% or less
          Group B: One or two of Cr and Mo in total 2.0% or less
          Group C: A total of 0.2% or less of one or more of Nb, Ti and V
In a line for continuously hot-dip galvanizing a steel sheet containing one or more of these groups, with the balance being composed of Fe and inevitable impurities,_ThreeTransformation point ~ Ac₁After annealing to the two-phase region of ferrite + austenite in the temperature range of the transformation point, it is cooled to the temperature of the hot dip galvanizing treatment, and then hot dip galvanizing treatment is performed to form a hot dip galvanized layer on the steel sheet surface It is a method for producing a hot-dip galvanized steel sheet, which is excellent in press formability and strain age hardening characteristics of ΔTS: 80 MPa or more.
[0025]
Further, in the second aspect of the present invention, before the annealing, a pretreatment consisting of a preheating treatment that heats at a temperature of 700 ° C. or higher and a pickling treatment that follows the preheating treatment may be performed in a continuous annealing line. preferable. In the second aspect of the present invention, it is preferable that the hot dip galvanizing treatment is performed to form a hot dip galvanized layer on the surface of the steel sheet, and then the hot dip galvanized layer is alloyed. In the second aspect of the present invention, the steel plate may be either a hot rolled plate or a cold rolled plate.
[0026]
  In the second aspect of the present invention, instead of the steel sheet, in mass%, C: 0.15% or less, Si: 2.0% or less, Mn: 3.0% or less, P: 0.1% or less, S: 0.02% or less, Al : 0.1% or less, N: 0.02% or less, and, W: 0.05-2.0%TheContains,Or even more Mo : 0.05 ~ 2.0 %, Cr : 0.05 ~ 2.0 %, One or two selected from Mo , Cr In total 2.0 % Or less,Alternatively, it contains 2.0% or less of one or more of Nb, Ti and V in total.The restIt is preferable that the steel plate has a composition in which the part is composed of Fe and inevitable impurities.
[0027]
DETAILED DESCRIPTION OF THE INVENTION
The hot-dip galvanized steel sheet of the present invention is a plated steel sheet in which a hot-dip galvanized layer or an alloyed hot-dip galvanized layer is formed on the surface of the steel sheet. The hot-dip galvanized steel sheet of the present invention is a high-tensile hot-dip galvanized steel sheet having a tensile strength of TS: 440 MPa or more, has excellent press formability, and has a remarkable tensile strength by heat treatment at a relatively low temperature after press forming. It is a plated steel sheet with excellent strain age hardening characteristics that rises to ΔTS: 80 MPa or more. The steel plate may be either a hot rolled plate or a cold rolled plate.
[0028]
In the present invention, “excellent in strain age hardening characteristics” means, as described above, after a pre-deformation treatment with a tensile plastic strain amount of 5% or more, at a temperature in the range of 150 to 350 ° C. and a holding time of 30 seconds or more. This means that when heat treatment is performed, the amount of increase in tensile strength ΔTS {= (tensile strength after heat treatment) − (tensile strength before pre-deformation treatment)} before and after this heat treatment is 80 MPa or more. Desirably, ΔTS is 100 MPa or more. It goes without saying that the yield stress is increased by this heat treatment, and ΔYS: 80 MPa or more is obtained. Note that ΔYS {= (yield strength after heat treatment) − (yield strength before pre-deformation treatment)} means an increase in yield strength before and after heat treatment.
[0029]
The amount of pre-strain (pre-deformation) is an important factor when defining strain age hardening characteristics. The present inventors investigated the influence of the amount of pre-strain on the subsequent strain age hardening characteristics assuming a deformation mode to which the steel sheet for automobiles is applied. As a result, except for extremely deep drawing, it can be organized by the amount of uniaxial equivalent strain (tensile strain), and in actual parts, the amount of uniaxial equivalent strain exceeds about 5%, and the strength of the parts Was found to correspond well with the strength obtained after 5% pre-strain strain aging treatment. For these reasons, in the present invention, the pre-strain (deformation) of the strain aging treatment is a tensile plastic strain of 5% or more.
[0030]
Conventional baking treatment conditions of 170 ° C x 20 min have been adopted as standard. However, when using ultrafine Cu precipitation strengthening as in the present invention, a heat treatment temperature of 150 ° C or higher is required. . On the other hand, when the temperature exceeds 350 ° C., the effect is saturated, and conversely, it tends to soften somewhat. In addition, when heated to a temperature exceeding 350 ° C., the occurrence of thermal strain and temper color becomes remarkable. Therefore, in the present invention, the heat treatment temperature for strain age hardening is set to 150 to 350 ° C. The holding time at the heat treatment temperature is 30 s or longer. With respect to the heat treatment holding time, if it is kept at about 150 to 350 ° C. for about 30 seconds or more, almost sufficient strain age hardening is achieved. When it is desired to obtain a larger and stable strain age hardening, the holding time is desirably 60 seconds or more, and more preferably 300 seconds or more.
[0031]
The heating method in the heat treatment after the pre-deformation is not particularly limited, but, for example, induction heating, non-oxidizing flame, heating by laser, plasma, etc. are all suitable in addition to the atmosphere heating by a furnace as in the ordinary paint baking process. It is. In addition, so-called warm pressing in which the temperature of the steel sheet is increased and pressed is also an extremely effective method in the present invention.
[0032]
  Next, the structure of the hot dip galvanized steel sheet of the present invention will be described.
  The plated steel sheet of the present invention has a ferrite phase and a martensite phase with an area ratio of 2% or more.Consisting ofHas a complex tissue.
  In order to obtain a steel sheet having low yield strength YS and high ductility (El) and having excellent press formability, in the present invention, the structure of the steel sheet is composed of a ferrite phase as a main phase,The second phaseMartensiConsisting ofIt needs to be a complex organization. The ferrite as the main phase is preferably 50% or more in terms of area ratio. If the ferrite content is less than 50%, it is difficult to ensure high ductility, and press formability deteriorates. Further, when a better ductility is required, the area ratio of the ferrite phase is preferably 80% or more. In order to take advantage of the composite structure, the ferrite phase is preferably 98% or less.
[0033]
  As the second phase, in the present invention, it is necessary to contain martensite in an area ratio of 2% or more. If the martensite is less than 2%, low YS and high El cannot be satisfied simultaneously. The second phase is a martensite phase alone with an area ratio of 2% or more..
[0034]
The hot-dip galvanized steel sheet having the above-described structure is a steel sheet having low yield strength, high ductility, excellent press formability, and excellent strain age hardening characteristics.
Next, the reason for limiting the composition of the hot dip galvanized steel sheet of the present invention will be described. The mass% is simply written as%.
C: 0.15% or less
C is an element that increases the strength of the steel sheet and further promotes the formation of a composite structure of ferrite and martensite. In the present invention, C is preferably contained in an amount of 0.01% or more in order to obtain a composite structure of ferrite and martensite. . On the other hand, if the content exceeds 0.15%, the fraction of carbides in the steel increases, and ductility and further press formability decrease. Furthermore, as a more important problem, when the C content exceeds 0.15%, spot weldability, arc weldability, and the like are significantly reduced. For this reason, in the present invention, C is limited to 0.15% or less. From the viewpoint of moldability, the content is preferably 0.10% or less.
[0035]
Si: 2.0% or less
Si is a useful strengthening element that can increase the strength of the steel sheet without significantly reducing the ductility of the steel sheet, but when its content exceeds 2.0%, it causes deterioration of press formability, Reduces plating ability. For this reason, Si was limited to 2.0% or less.
[0036]
Mn: 3.0% or less
Mn has the effect of strengthening steel, further lowers the critical cooling rate at which a composite structure of ferrite and martensite is obtained, and has the action of promoting the formation of a composite structure of ferrite and martensite. It is preferably contained depending on the cooling rate after annealing. Mn is an effective element for preventing hot cracking due to S, and is preferably contained according to the amount of S contained. Such an effect becomes remarkable when the content is 0.5% or more. On the other hand, the content exceeding 3.0% deteriorates press formability and weldability. For this reason, Mn was limited to 3.0% or less. More preferably, it is 1.0% or more.
[0037]
P: 0.10% or less
P has an effect of strengthening steel and can be contained in a necessary amount according to a desired strength. However, when P is contained excessively, press formability deteriorates. For this reason, P was limited to 0.10% or less. When more excellent press formability is required, the content is preferably 0.08% or less.
[0038]
S: 0.02% or less
S is an element present in the steel sheet as an inclusion, and is an element that causes deterioration of the ductility and formability of the steel sheet, particularly stretch flangeability, and is preferably reduced as much as possible. In the present invention, the upper limit of S is 0.02%. When excellent stretch flange formability is required, S is preferably 0.010% or less.
[0039]
Al: 0.10% or less
Al is added as a deoxidizing element for steel and is an element useful for improving the cleanliness of steel. However, even if it exceeds 0.10%, a further deoxidizing effect cannot be obtained. Press formability deteriorates. For this reason, Al was limited to 0.10% or less. In the present invention, it does not exclude a melting method by a deoxidation method other than Al deoxidation. For example, Ti deoxidation or Si deoxidation may be performed. Included in the range. At this time, even if Ca, REM, etc. are added to the molten steel, the characteristics of the steel sheet of the present invention are not hindered at all.
[0040]
N: 0.02% or less
N is an element that increases the strength of the steel sheet by solid solution strengthening and strain age hardening. However, if it exceeds 0.02%, nitride increases in the steel sheet, thereby increasing the ductility of the steel sheet and further press formability. Is significantly deteriorated. For this reason, N was limited to 0.02% or less. In the case where further improvement in press formability is required, the content is preferably 0.01% or less, preferably 0.0005% or more.
[0041]
  Cu:1.28~ 3.0%
  Cu is an element that remarkably increases the strain age hardening (predeformation—increase in strength after heat treatment) of the steel sheet, and is one of the most important elements in the present invention. If the Cu content is less than 0.5%, even if the predeformation-heat treatment conditions are changed, an increase in tensile strength of ΔTS: 80 MPa or more cannot be obtained. For this reason, in this invention, Cu needs to contain 0.5% or more. On the other hand, if the content exceeds 3.0%, the effect is saturated, an effect commensurate with the content cannot be expected, and it is economically disadvantageous, which causes deterioration of press formability and further deteriorates the surface properties of the steel sheet.. YoTo achieve both large ΔTS and excellent press formabilityIn addition,Cu1.28~3.0%Limited to. More preferably 2.5 % Or less.
[0042]
In the present invention, in addition to the above-described composition containing Cu, the following groups A to C:
Group A: Ni: 2.0% or less
Group B: One or two of Cr and Mo in total 2.0% or less
Group C: A total of 0.2% or less of one or more of Nb, Ti and V
It is preferable to contain 1 group or 2 groups or more.
[0043]
Group A: Ni: 2.0% or less
Group A: Ni is an element effective for preventing surface defects generated on the steel sheet surface when Cu is added, and can be contained if necessary. When contained, the content depends on the Cu content, and is preferably about half of the Cu content. Even if the content exceeds 2.0%, the effect is saturated and an effect commensurate with the content cannot be expected, which is economically disadvantageous, and conversely, press formability deteriorates. For this reason, Ni is preferably limited to 2.0% or less.
[0044]
Group B: One or two of Cr and Mo in total 2.0% or less
Group B: Cr and Mo, like Mn, have the effect of lowering the critical cooling rate at which a composite structure of ferrite and martensite is obtained and promoting the formation of a composite structure of ferrite and martensite. , If necessary. If one or two of Cr and Mo are contained in total exceeding 2.0%, the press formability is lowered. For this reason, it is preferable to limit one or two of Group B: Cr and Mo to 2.0% or less in total.
[0045]
Group C: A total of 0.2% or less of one or more of Nb, Ti and V
Group C: Nb, Ti, and V are all carbide-forming elements, and contribute to increasing the strength by fine dispersion of carbides. Therefore, they can be selected and contained as necessary. However, if one or more of Nb, Ti, and V are contained in total exceeding 0.2%, press formability is lowered. For this reason, it is preferable to limit one or more of Nb, Ti, and V to 0.2% or less in total.
[0046]
  In the present invention, instead of Cu,W: 0.05 ~ 2.0 % Or evenMo: 0.05-2.0%, Cr: 0.05-2.0%ofOne or two selected from our houseSeed W, Mo , Cr ofYou may contain 2.0% or less in total.
  W: 0.05 ~ 2.0 % Or evenMo: 0.05-2.0%, Cr: 0.05-2.0%ofOne or two selected from our houseSeed2.0% or less in total
  Mo, Cr and W are all elements that remarkably increase the strain age hardening of the steel sheet, and are the most important elements in the present invention.W is required or even Mo , Cr TheCan be selected and contained.W or evenMo,Cr of1 or 2 of themSeedFurther, by adding a composite structure of ferrite and martensite, fine carbides are strain-induced finely precipitated during pre-deformation and heat treatment, and an increase in tensile strength of ΔTS: 80 MPa or more can be obtained. If the content of these elements is less than 0.05%, even if the predeformation-heat treatment conditions and the steel sheet structure are changed, an increase in tensile strength of ΔTS: 80 MPa or more cannot be obtained. On the other hand, even if the content of these elements exceeds 2.0%, the effects described above are saturated, an effect commensurate with the content cannot be expected, and it is economically disadvantageous, and the press formability is deteriorated. For this reason,W: 0.05 ~ 2.0 %,Mo: 0.05-2.0%, Cr: 0.05-2.0%ofLimited to range. From the viewpoint of press formability, the total content of Mo, Cr, and W was limited to 2.0% or less.
[0047]
2.0% or less of one or more of Nb, Ti and V in total
Nb, Ti, and V are all carbide-forming elements, and can be selected and contained as needed when one or more of Mo, Cr, and W are contained. By containing one or more of these Nb, Ti, and V, and further forming a composite structure of ferrite and martensite, fine carbides are strain-induced finely precipitated during pre-deformation and heat treatment, and ΔTS: 80 MPa or more An increase in tensile strength is obtained. However, if one or more of Nb, Ti, and V are contained in a total exceeding 2.0%, the press formability deteriorates. For this reason, Nb, Ti, and V are preferably limited to 2.0% or less in total.
[0048]
In addition to the elements described above, the present invention may contain one or two of Ca: 0.1% or less and REM: 0.1% or less. Both Ca and REM are elements that contribute to the improvement of ductility through shape control of inclusions. However, if the content exceeds Ca: 0.1% and REM: 0.1%, the cleanliness is lowered and ductility is lowered.
From the viewpoint of martensite formation, one or more of B: 0.1% or less, Zr: 0.1% or less, and Sb: 0.1% or less may be contained.
[0049]
The balance other than the above components is composed of Fe and inevitable impurities. As unavoidable impurities, Pb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less, Co: 0.1% or less are acceptable.
Below, the manufacturing method of the hot dip galvanized steel plate of this invention is demonstrated.
The hot dip galvanized steel sheet of the present invention is a line for performing continuous hot dip galvanizing on the steel sheet having the above composition._ThreeTransformation point ~ Ac₁After annealing to the two-phase region of ferrite + austenite in the temperature range of the transformation point, hot dip galvanizing treatment is performed to form a hot dip galvanized layer on the steel sheet surface.
[0050]
As the steel sheet to be used, both a hot rolled steel sheet and a cold rolled steel sheet are suitable.
Although the suitable manufacturing method of the steel plate to be used is demonstrated below, it cannot be overemphasized that it is not limited to this in this invention.
First, the suitable manufacturing method of a hot-rolled steel plate (hot-rolled plate) is demonstrated.
The material (steel slab) to be used is preferably manufactured by a continuous casting method in order to prevent macro segregation of components, but may be manufactured by an ingot forming method or a thin slab continuous casting method. In addition to the conventional method in which a steel slab is manufactured and then cooled to room temperature and then reheated, it is not cooled and inserted into a heating furnace as it is, or rolled immediately after a slight heat retention. Energy saving processes such as direct rolling and direct rolling can be applied without problems.
[0051]
The above-described material (steel slab) is heated and subjected to a hot rolling process to obtain a hot rolled sheet. The hot-rolling step is usually a known condition with no particular problem as long as a hot-rolled sheet having a desired thickness can be produced. Preferred hot rolling conditions are as follows.
Slab heating temperature: 900 ℃ or more
If the slab heating temperature is less than 900 ° C., the rolling load increases and the risk of trouble during hot rolling increases. However, when the slab heating temperature contains Cu, it is desirable that the slab heating temperature be low in order to prevent Cu-induced surface defects. Note that the slab heating temperature is desirably 1300 ° C. or less because of an increase in scale loss accompanying an increase in oxidized weight.
[0052]
In addition, it goes without saying that using a so-called sheet bar heater for heating the sheet bar from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling is an effective method. .
Finishing rolling finish temperature: 700 ℃ or more
By setting the finish rolling finish temperature FDT to 700 ° C. or higher, a uniform hot-rolled mother board structure can be obtained. On the other hand, when the finish rolling finish temperature is less than 700 ° C., the hot-rolled base metal structure becomes non-uniform, the rolling load during hot rolling increases, and the risk of occurrence of trouble during hot rolling increases. For these reasons, the FDT in the hot rolling process is preferably 700 ° C. or higher.
[0053]
Winding temperature: 800 ℃ or less
The coiling temperature is preferably 800 ° C. or lower, more preferably 200 ° C. or higher. When the winding temperature exceeds 800 ° C., the scale increases and the yield tends to decrease due to the scale loss. When the coiling temperature is less than 200 ° C., the shape of the steel sheet is significantly disturbed, increasing the risk of causing problems in actual use.
[0054]
As described above, the hot-rolled steel sheet that can be suitably used in the present invention, after heating the slab having the above composition to 900 ° C. or higher, is subjected to hot rolling at a finish rolling finish temperature of 700 ° C. or higher, preferably 800 ° C. or lower. Is preferably a rolled hot-rolled sheet at a winding temperature of 200 ° C. or higher.
In the hot rolling step, part or all of the finish rolling may be lubricated rolling in order to reduce the rolling load during hot rolling. Performing lubrication rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. In addition, it is preferable to make the wear coefficient in the case of lubrication rolling into the range of 0.25-0.10. Moreover, it is preferable to set it as the continuous rolling process which joins the sheet | seat bars which precede and follow, and finish-rolls continuously. The application of the continuous rolling process is also desirable from the viewpoint of the operational stability of hot rolling.
[0055]
Further, the hot rolled sheet with the scale attached may be subjected to hot rolled sheet annealing to form an internal oxide layer on the steel sheet surface layer. The formation of the internal oxide layer improves hot dip galvanizing properties to prevent surface enrichment of Si, Mn, P and the like.
Although the hot-rolled sheet manufactured by the above-described method may be used as a plating original sheet, a cold-rolled sheet obtained by subjecting the above-described hot-rolled sheet to a cold rolling process may be used as the plating original sheet.
[0056]
In the cold rolling process, the hot rolled sheet is cold rolled. The cold rolling conditions are not particularly limited as long as the cold rolled sheet having a desired size and shape can be obtained, but the rolling reduction during cold rolling is preferably 40% or more. When the rolling reduction is less than 40%, recrystallization hardly occurs during annealing, which is a subsequent process.
In the present invention, the above hot-rolled sheet or cold-rolled sheet (steel plate) is subjected to continuous hot-dip galvanization in a line.₁Transformation point ~ Ac_ThreeIt is preferable to perform annealing to heat the two-phase region of ferrite (α) + austenite (γ) in the temperature range of the transformation point.
[0057]
Heating temperature is Ac₁Below the transformation point, a ferrite single-phase structure is formed, whereas Ac_ThreeAt a high temperature exceeding the transformation point, the crystal grains become coarse and become an austenite single phase region, and the press formability deteriorates remarkably. Further, by annealing in the (α + γ) two-phase region, a composite structure of ferrite and martensite is obtained, and a high ΔTS is obtained.
[0058]
In order to obtain a composite structure of ferrite and martensite, it is preferable to set a cooling rate of 5 ° C./s or more from the heating temperature in the two-phase region to the temperature of the galvanizing treatment. When the cooling rate is less than 5 ° C./s, martensitic transformation is difficult to occur, and it becomes difficult to obtain a composite structure of ferrite and martensite.
The hot dip galvanizing treatment may be performed under the treatment conditions (zinc bath temperature: 450 to 500 ° C.) usually performed in a continuous hot dip galvanizing line, and is not particularly limited. However, plating at an extremely high temperature is inferior in plating characteristics, so that the temperature is preferably 500 ° C. or lower. Further, plating at less than 450 ° C. has a problem of deterioration of plating characteristics.
[0059]
From the viewpoint of martensite formation, the cooling rate from the temperature of the hot dip galvanizing treatment to 300 ° C. is preferably 5 ° C./s or more.
Further, after the plating treatment, wiping may be performed to adjust the basis weight as necessary.
Further, after the hot dip galvanizing treatment, an alloying treatment of the hot dip galvanized layer may be performed. The alloying treatment of the hot dip galvanized layer is preferably performed by reheating to a temperature range of 460 to 560 ° C. after the hot dip galvanizing treatment. Alloying at temperatures above 560 ° C degrades the plating properties. On the other hand, in the alloying treatment at a temperature lower than 460 ° C., the progress of alloying is slow and the productivity is lowered.
[0060]
In the present invention, before annealing in the continuous hot dip galvanizing line, preheating treatment in which heating is performed at a temperature of 700 ° C. or higher in the continuous annealing line, and subsequently, steel formed on the steel sheet surface by the preheating treatment. It is preferable to perform a pretreatment step for performing pickling treatment for removing the concentrated layer of the middle component in order to improve plating properties.
On the surface of the steel sheet preheated in the continuous annealing line, a surface concentrated layer is formed in which P as a component in the steel is concentrated and Si, Mn, Cr, etc. are concentrated as oxides. It is advantageous for improving the plating property that this surface concentrated layer is removed by pickling treatment and then annealed in a reducing atmosphere in a continuous hot dip galvanizing line. If the temperature of the preheating treatment is less than 700 ° C., the formation of the surface concentrated layer is not promoted, and the improvement of the plating property is not promoted. Further, the temperature of the preheating treatment is preferably 1000 ° C. or less from the viewpoint of press formability.
[0061]
Further, after the hot dip galvanizing process or after the alloying process, temper rolling of 10% or less may be added in order to adjust the shape correction, surface roughness and the like.
Further, the steel sheet of the present invention may be subjected to special treatment after hot dip galvanization to improve chemical conversion property, weldability, press formability, corrosion resistance, and the like.
[0062]
【Example】
Example 1
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. These slabs were made into hot-rolled steel strips (hot-rolled sheets) by hot rolling under the conditions shown in Table 2. After pickling these hot-rolled steel strips (hot-rolled sheets), they are annealed under the conditions shown in Table 2 in a continuous hot-dip galvanizing line (CGL), then hot-dip galvanized, and hot-dip galvanized on the steel sheet surface. A layer was formed. Subsequently, the galvanized layer was alloyed under the conditions shown in Table 2. Note that some of the steel plates were kept hot dip galvanized.
[0063]
Further, the hot-rolled steel strip (hot-rolled sheet) was further pickled, and then made into a cold-rolled steel strip (cold-rolled sheet) by the cold rolling process under the conditions shown in Table 2. These cold-rolled steel strips (cold-rolled sheets) were annealed under the conditions shown in Table 2 in a continuous hot-dip galvanizing line (CGL), then hot-dip galvanized, and a hot-dip galvanized layer was formed on the steel sheet surface. . Subsequently, the galvanized layer was alloyed under the conditions shown in Table 2. Note that some of the steel plates were kept hot dip galvanized.
[0064]
In addition, prior to annealing in a continuous hot dip galvanizing line (CGL), some steel sheets are pre-treated in the continuous annealing line (CAL) under the conditions shown in Table 2 and then pickling. The process was applied. The pickling in the pretreatment step was performed in a pickling tank on the CGL entry side.
The galvanizing bath temperature was in the range of 460 to 480 ° C., and the temperature of the immersed steel plate was not less than the plating bath temperature and not more than (bath temperature + 10 ° C.). The alloying treatment was reheated to the alloying treatment temperature and held at that temperature for 15 to 28 s. These plated steel sheets were further subjected to temper rolling of 1.0%.
[0065]
About the hot dip galvanized steel sheet (steel strip) obtained by the above-mentioned process, the microstructure, the tensile property, the strain age hardening property, and the hole expansion rate were determined. The press formability was evaluated from the elongation El (ductility), the yield strength, and the hole expansion rate.
(1) Microscopic tissue
A specimen is collected from the obtained steel strip, and the cross section (C cross section) orthogonal to the rolling direction is used to image the microstructure using an optical microscope or a scanning electron microscope, and in the main phase using an image analyzer. The structure fraction of a certain ferrite and the type and structure fraction of the second phase were determined.
[0066]
(2) Tensile properties
From the obtained steel strip, a JIS No. 5 tensile test piece was collected and subjected to a tensile test in accordance with the provisions of JIS Z 2241 to determine the yield strength YS, tensile strength TS, elongation El, and yield ratio YR. .
(3) Strain age hardening characteristics
JIS No. 5 test specimen was taken from the obtained steel strip (plated steel sheet) in the rolling direction, subjected to 5% plastic deformation as pre-deformation (tensile pre-strain), and then heat-treated at 250 ° C for 20 min. , Tensile test, tensile properties after heat treatment (yield stress YS_HT, Tensile strength TS_HT), ΔYS = YS_HT-YS, ΔTS = TS_HT-TS was calculated. YS_HT, TS_HTIs the yield stress (strength) and tensile strength after pre-deformation-heat treatment, and YS and TS are the yield stress and tensile strength of the steel strip (plated steel sheet).
[0067]
(4) Hole expansion rate
A test piece taken from the obtained steel strip (plated steel plate) was punched out with a 10mmφ punch, and then a hole was formed. Hole expansion was performed until a penetrating crack was generated, and the hole expansion ratio λ was obtained. The hole expansion ratio λ is λ (%) = {(d−d₀) / D₀} × 100. D₀: Initial hole diameter, d: Inner hole diameter when cracking occurs.
[0068]
These results are shown in Table 3.
[0069]
[Table 1]

[0070]
[Table 2]

[0071]
[Table 3]

[0072]
Each of the examples of the present invention has a low yield strength YS, a high elongation El, a low yield ratio YR, shows a larger hole expansion ratio λ, and is excellent in press formability including stretch flangeability, It exhibits a large ΔYS and an extremely large ΔTS, and is a plated steel sheet having excellent strain age hardening characteristics. On the other hand, in the comparative example outside the scope of the present invention, the yield strength YS is high, the elongation El is low, the hole expansion ratio λ is small, ΔTS is small, press formability, and strain age hardening characteristics are obtained. It is a lowered plated steel sheet.
[0073]
(Example 2)
Molten steel having the composition shown in Table 4 was melted in a converter and made into a slab by a continuous casting method. These slabs were formed into hot-rolled steel strips (hot-rolled sheets) having a thickness of 1.6 mm and 4.0 mm by hot rolling under the conditions shown in Table 5. After pickling these 1.6 mm thick hot-rolled steel strips (hot-rolled sheets), they were annealed under the conditions shown in Table 5 in a continuous hot-dip galvanizing line (CGL), and then hot-dip galvanized, A hot dip galvanized layer was formed. Subsequently, the galvanized layer was alloyed under the conditions shown in Table 5. Note that some of the steel plates were kept hot dip galvanized.
[0074]
Further, the 4.0 mm thick hot-rolled steel strip (hot-rolled sheet) was further pickled, and then cold-rolled steel strip (cold-rolled sheet) was formed by the cold rolling process under the conditions shown in Table 5. These cold-rolled steel strips (cold-rolled sheets) were annealed under the conditions shown in Table 5 in a continuous hot-dip galvanizing line (CGL), then hot-dip galvanized, and a hot-dip galvanized layer was formed on the steel sheet surface. . Subsequently, the galvanized layer was alloyed. Note that some of the steel plates were kept hot dip galvanized.
[0075]
In addition, prior to annealing in a continuous hot dip galvanizing line (CGL), some steel sheets are pre-treated in the continuous annealing line (CAL) under the conditions shown in Table 5 and then pickling. The process was applied. The pickling in the pretreatment step was performed in a pickling tank on the CGL entry side.
The galvanizing bath temperature was in the range of 460 to 480 ° C., and the temperature of the immersed steel plate was not less than the plating bath temperature and not more than (bath temperature + 10 ° C.). The alloying treatment was reheated to the alloying treatment temperature and held at that temperature for 15 to 28 s. These plated steel sheets were further subjected to temper rolling with an elongation of 1.0%.
[0076]
About the hot dip galvanized steel sheet (steel strip) obtained by the above process, the microstructure, tensile characteristics, strain age hardening characteristics, and hole expansion ratio were determined in the same manner as in Example 1. The press formability was evaluated from the elongation El (ductility), the yield strength, and the hole expansion rate.
These results are shown in Table 6.
[0077]
[Table 4]

[0078]
[Table 5]

[0079]
[Table 6]

[0080]
Each of the examples of the present invention has a low yield strength YS, a high elongation El, a low yield ratio YR, shows a larger hole expansion ratio λ, and is excellent in press formability including stretch flangeability, It exhibits a large ΔYS and an extremely large ΔTS, and is a plated steel sheet having excellent strain age hardening characteristics. On the other hand, in the comparative example outside the scope of the present invention, the yield strength YS is high, the elongation El is low, the hole expansion ratio λ is small, ΔTS is small, press formability, and strain age hardening characteristics are obtained. It is a lowered plated steel sheet.
[0081]
【The invention's effect】
According to the present invention, it is possible to stably produce a hot-dip galvanized steel sheet whose tensile strength is remarkably increased by heat treatment after press forming while maintaining excellent press formability. There is an effect. When the hot dip galvanized steel sheet of the present invention is applied to automobile parts, press molding is easy, and the characteristics of the parts after completion can be stably increased, and there is an effect that it can sufficiently contribute to weight reduction of the automobile body.
[Brief description of the drawings]
FIG. 1 is a graph showing the effect of Cu content on the relationship between ΔTS after pre-deformation and heat treatment and the recrystallization annealing temperature.
FIG. 2 is a graph showing the effect of Cu content on the relationship between ΔTS after pre-deformation and heat treatment and the heat treatment temperature.
FIG. 3 is a graph showing the influence of Cu content on the relationship between λ and YR.

Claims (12)

A hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the steel sheet surface, wherein the steel sheet is in mass%,
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
N: 0.02% or less, Cu: 1.28 to 3.0 %
And the balance is a composition composed of Fe and inevitable impurities, and the structure is a composite structure composed of a ferrite phase and a martensite phase with an area ratio of 2% or more. And a hot-dip galvanized steel sheet with excellent strain age hardening characteristics such that ΔTS: 80 MPa or more. The hot-dip galvanized steel sheet according to claim 1, further comprising one group or two or more groups of the following groups A to C in mass% in addition to the composition.
Record
Group A: Ni: 2.0% or less
Group B: One or two of Cr and Mo in total 2.0% or less
Group C: A total of 0.2% or less of one or more of Nb, Ti and V A hot-dip galvanized steel sheet having a hot-dip galvanized layer or an alloyed hot-dip galvanized layer on the steel sheet surface, wherein the steel sheet is in mass%,
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
N: 0.02% or less, W: 0.05-2.0%, the balance is composed of Fe and inevitable impurities, the structure is a ferrite phase and martensite with an area ratio of 2% or more A hot-dip galvanized steel sheet excellent in press formability and strain age hardening characteristics of ΔTS: 80 MPa or more, characterized by being a composite structure composed of phases.   In addition to the above composition, the composition further contains one or two selected from Mo: 0.05 to 2.0% and Cr: 0.05 to 2.0% in terms of mass% in total of W, Mo and Cr of 2.0% or less. The hot-dip galvanized steel sheet according to claim 3.   5. The hot dip galvanized steel sheet according to claim 3, further comprising, in addition to the above composition, one or more of Nb, Ti, and V in total by mass of 2.0% or less. . % By mass
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
N: 0.02% or less, Cu: 1.28 to 3.0 %
In a line for continuous hot-dip galvanizing to a steel sheet having a composition comprising Fe and the inevitable impurities in the balance, heat the ferrite to austenite in the temperature range from the Ac ₃ transformation point to the Ac ₁ transformation point. Hot-dip zinc with excellent press formability and strain age hardening characteristics of ΔTS: 80 MPa or more, characterized by forming a hot-dip galvanized layer on the steel sheet surface after annealing. Manufacturing method of plated steel sheet. The method for producing a hot-dip galvanized steel sheet according to claim 6, further comprising one group or two or more groups of the following groups A to C in mass% in addition to the composition.
Record
Group A: Ni: 2.0% or less
Group B: One or two of Cr and Mo in total 2.0% or less
Group C: A total of 0.2% or less of one or more of Nb, Ti and V   The hot dip galvanizing according to claim 6 or 7, wherein a pretreatment comprising a preheating treatment at a temperature of 700 ° C or higher and a pickling treatment is performed in a continuous annealing line before the annealing. A method of manufacturing a steel sheet.   The hot dip galvanization according to any one of claims 6 to 8, wherein the hot dip galvanization treatment is performed to form a hot dip galvanization layer on a steel sheet surface, and then the hot dip galvanization layer is alloyed. A method of manufacturing a steel sheet. In place of the steel sheet,
C: 0.15% or less, Si: 2.0% or less,
Mn: 3.0% or less, P: 0.1% or less,
S: 0.02% or less, Al: 0.1% or less,
10. The steel sheet according to claim 6, wherein the steel sheet contains N: 0.02% or less, further contains W: 0.05-2.0%, and the balance is composed of Fe and inevitable impurities. Manufacturing method of hot-dip galvanized steel sheet.   In addition to the above composition, the composition further contains one or two selected from Mo: 0.05 to 2.0% and Cr: 0.05 to 2.0% in terms of mass% in total of 2.0% or less in total of W, Mo, and Cr. 11. The method for producing a hot dip galvanized steel sheet according to claim 10, wherein:   12. The hot dip galvanized steel sheet according to claim 10 or 11, further comprising 2.0% or less of one or more of Nb, Ti, and V in total in addition to the composition. Manufacturing method.

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