JP3631710B2 - Si-containing high-strength hot-dip galvanized steel sheet with excellent corrosion resistance and ductility and method for producing the same - Google Patents

Description

【００６３】
その後、各鋼の成分（質量％）から下記式にしたがってＡｃ_１ とＡｃ_３ 変態温度を計算により求めた。
【００６４】
Ａｃ_１ ＝７２３−１０．７×Ｍｎ％＋２９．１×Ｓｉ％、
Ａｃ_３ ＝９１０−２０３×（Ｃ％）^１／２＋４４．７×Ｓｉ％＋３１．５×Ｍｏ％−３０×Ｍｎ％−１１×Ｃｒ％＋４００×Ａｌ％
これらのＡｃ_１ およびＡｃ_３ 変態温度から計算される焼鈍温度に１０％Ｈ_２ −Ｎ_２ 雰囲気中で昇温・保定したのち、０．１〜１０℃／秒の冷却速度で６５０〜７００℃温度域に冷却し、引き続いて０．１〜２０℃／秒の冷却速度でめっき浴温度にまで冷却し、浴組成を種々変化させた４６０〜４７０℃の亜鉛めっき浴に３秒間浸漬することでめっきを行った。
【００６５】
また、一部の鋼板については、Ｆｅ−Ｚｎ合金化処理として、めっき後の鋼板を４００〜５５０℃の温度域で１５秒〜２０分保持し、めっき層中のＦｅ含有率が質量％で５〜２０％となるよう調節した。めっき表面外観のドロス巻き込み状況の目視観察および不めっき部面積の測定によりめっき外観を評価した。作製しためっきはめっき層をインヒビターを含有した５％塩酸溶液で溶解し化学分析に供し組成を求め表２に示した。
【００６６】
表２および表３より、本発明鋼は、外観評点が耐食試験前後ですべて５〜４で、かつ強度・伸びバランスにも優れる。一方、本発明の範囲を満たさない比較例は、いずれも外観評点が低く、強度・伸びバランスに劣る。また、本願発明の請求項の範囲で製造した鋼板は、ミクロ組織も上述した組織になっており外観及び強度・伸びバランスに優れている。
【００６７】
【表２】

【００６８】
【表３】

【００６９】
【表４】

【００７０】
【発明の効果】
本発明の高強度溶融亜鉛めっき鋼板は耐食性に極めて優れ、加工性が良好であり、建材、家電製品、自動車車体用途等に極めて有効である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a Si-containing high-strength galvannealed steel sheet and hot-dip galvanized steel sheet that are excellent in corrosion resistance and ductility suitable for building materials, home appliances, automobiles, and the like, and a method for producing the same.
[0002]
[Prior art]
Hot dip galvanizing is applied for the purpose of corrosion protection of steel sheets, and is widely used in building materials, home appliances, automobiles and the like. As a manufacturing method thereof, in a continuous line, after degreasing and cleaning, heating in a non-oxidizing atmosphere, ₂ And N ₂ There is a Sendzimer method in which after cooling in a reducing atmosphere containing, cooled to near the plating bath temperature, immersed in a molten zinc bath, cooled, or reheated to produce an Fe-Zn alloy phase, the steel plate method It is frequently used for processing.
[0003]
For annealing before plating, after degreasing and washing, immediately after heating in a non-oxidizing atmosphere, H ₂ And N ₂ In some cases, an all-reducing furnace method in which annealing is performed in a reducing atmosphere containing selenium is also performed. In addition, a flux method is also performed in which a steel sheet is degreased and pickled, and then flux treatment is performed using ammonium chloride and the like, soaking in a plating bath, and then cooling.
[0004]
A small amount of Al is added to the plating bath used in these plating processes for deoxidation of molten zinc. In the Sendzimer method, the Zn plating bath contains about 0.1% Al by mass%. Since Al in this bath has a stronger affinity for Fe than Fe-Zn, when steel is immersed in the plating bath, an Fe-Al alloy phase, that is, an Al concentrated layer, is formed on the surface of the steel. It is known to suppress the reaction. Due to the presence of the Al concentrated layer, the Al content in the obtained plating layer is usually higher than the Al content in the plating bath.
[0005]
In recent years, the demand for high-strength steel sheets having high ductility has been increasing from the viewpoint of reducing the weight of a vehicle body for the purpose of improving the fuel efficiency particularly in the automobile body. As an inexpensive strengthening method, Si is added to the steel, and the high ductility and high strength steel sheet may contain 1% by mass or more.
[0006]
On the other hand, from the viewpoint of plating, if the Si content in the steel exceeds 0.1% by mass, plating wettability is greatly reduced in the Sendzimer method using a plating bath containing ordinary Al, Since non-plating occurs, the corrosion resistance of the non-plated part becomes a problem depending on the use environment. Moreover, also about the plating adhesion of a process part, the corrosion resistance of a peeling part becomes a problem depending on use environment like a plating peeling part like a non-plating part.
[0007]
As means for solving the problems with these Si-added steels, as shown in JP-A-3-28359, JP-A-3-64437, etc., the plating property is improved by applying specific plating. However, this method has a problem in that a new plating facility must be provided in front of the hot dipping line annealing furnace or a plating process must be performed in advance in the electroplating line, resulting in a significant increase in cost. Moreover, in these inventions, the corrosion resistance of the plating peeling part at the time of a process cannot be improved.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-mentioned problems, and to provide a Si-containing high-strength galvannealed steel sheet, a galvanized steel sheet, and a method for producing the same, in which non-plating is suppressed and corrosion resistance and ductility are excellent.
[0009]
[Means for Solving the Problems]
As a result of various investigations, the inventors have made the plating layer contain an appropriate concentration of a specific element and combine it with the components of the steel sheet, so that the hot-dip galvanizing wettability and alloying plating of the high-strength steel sheet can be achieved. In addition to finding acceleration of alloying, it was also found that corrosion resistance during plating peeling can be secured. This effect appears by controlling the amount of Fe, Si and Mo in the plating layer. That is, assuming that the Fe content in the plating layer is X, the Si content in the plating layer is Y, and the Mo content in the plating layer is Z in mass%, X, Y, and Z are expressed by the following formula (1).
X / 10− (Y + Z) + 1 ≧ 0 (1)
It is achieved by satisfying. The details of the reason for improving the corrosion resistance are unknown, but it is considered that Mo contained in the plating layer is effective for improving the corrosion resistance of the plating layer itself. It is estimated that this is because the basic zinc chloride, which is a corrosion product of plating, is stabilized, and the protective action of the base steel plate by the corrosion product is improved.
[0010]
The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%
C: 0.0001 to 0.3%,
Si: 0.1 to 3.0%,
Mn: 0.01 to 3%
Al: 0.001 to 4%
Mo: 0.001 to 1%,
P: 0.0001 to 0.3%,
S: 0.0001 to 0.1%,
In the surface of the steel plate made of the balance Fe and inevitable impurities,
Al: 0.001 to 4%,
Mo: 0.0001 to 1%,
Si: 0.0001 to 0.1%,
Fe: less than 20%,
A hot-dip galvanized steel sheet having a plating layer consisting of Zn and inevitable impurities, the balance being X, the Fe content in the plating layer being X, the Si content in the plating layer being Y, and the Mo in the plating layer being Mo. A Si-containing high-strength hot-dip galvanized steel sheet excellent in corrosion resistance and ductility, wherein X, Y, and Z satisfy the formula (1) where the content is Z.
[0011]
X / 10− (Y + Z) + 1 ≧ 0 (1)
[2] The plating layer is further mass%,
Mn: 0.0001 to 3%
Ni: 0.001 to 3%
The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to [1], characterized by containing one or two of the following:
[3] The plating layer is further mass%,
Ca: 0.001 to 0.1%,
Mg: 0.001 to 3%,
W: 0.001 to 0.1%,
Zr: 0.001 to 0.1%,
Cs: 0.001 to 0.1%,
Rb: 0.001 to 0.1%,
K: 0.001 to 0.1%,
Ag: 0.001 to 5%,
Na: 0.001 to 0.05%,
Cd: 0.001 to 3%
Cu: 0.001 to 3%,
Co: 0.001-1%,
La: 0.001 to 0.1%,
Tl: 0.001-8%
Nd: 0.001 to 0.1%,
Y: 0.001 to 0.1%
In: 0.001 to 5%,
Be: 0.001 to 0.1%,
Cr: 0.001 to 0.05%,
Pb: 0.001 to 1%,
Hf: 0.001 to 0.1%,
Tc: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
Ge: 0.001 to 5%,
Ta: 0.001 to 0.1%,
V: 0.001 to 0.2%,
B: 0.001 to 0.1%,
The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to [1] or [2].
[4] Steel is further mass%,
Cr: 0.001 to 25%,
Ni: 0.001 to 10%,
Cu: 0.001 to 5%,
Co: 0.001-5%
W: 0.001 to 5%,
The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to any one of [1] to [3].
[5] The steel further contains 0.001 to 1% in total of one or more of Nb, Ti, V, Zr, Hf, and Ta in mass%. 4] The Si-containing high-strength hot-dip galvanized steel sheet excellent in corrosion resistance and ductility.
[6] The steel is further mass% and contains B: 0.0001 to 0.1%, which is excellent in corrosion resistance and ductility according to any one of [1] to [5] High strength galvanized steel containing Si.
[7] Any one of [1] to [6], wherein the steel further contains 0.0001 to 1% of one or more of Y, Rem, Ca, Mg, and Ce in mass%. 2. A Si-containing high-strength hot-dip galvanized steel sheet excellent in corrosion resistance and ductility according to item 1.
[8] The microstructure of the steel has a ferrite phase or ferrite phase and bainite phase with a volume fraction of 50 to 97%, and the balance is one or both of martensite phase and residual austenite phase. The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to any one of [1] to [7], which is a composite structure containing 3 to 50% in total.
[9] The microstructure of the steel sheet is composed of 70 to 97% ferrite as a main phase in volume fraction, its average particle size is 20 μm or less, and as a second phase, austenite and / or 3 to 30% in volume fraction. Alternatively, the Si-containing high-strength molten zinc having excellent corrosion resistance and ductility according to any one of [1] to [8], which comprises martensite and has an average particle size of the second phase of 10 μm or less. Plated steel sheet.
[10] The second phase of the steel sheet is austenite, carbon content in steel: C (mass%), Mn content in steel: Mn (mass%), volume ratio of austenite: Vγ (%), ferrite and bainite The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to any one of [1] to [9], wherein Vα (%) satisfies the formula (2):
[0012]
(Vγ + Vα) / Vγ × C + Mn / 8 ≧ 2,000 (2)
[11] The microstructure of the steel sheet is mainly composed of ferrite with a volume fraction of 50 to 95%, the average grain size is 20 μm or less, and the second phase is austenite with a volume fraction of 3 to 30%. Or it contains martensite, those average particle diameters are 10 micrometers or less, and also consists of bainite of 2-47% by volume fraction, It is any one of [1]-[10] characterized by the above-mentioned High strength hot-dip galvanized steel sheet with excellent corrosion resistance and ductility.
[12] A method for producing the high-strength hot-dip galvanized steel sheet according to any one of [1] to [11], wherein the steel sheet component according to any one of claims 1, 4 to 7 is used. The cast slab comprising .1x (Ac ₃ -Ac ₁ ) + Ac ₁ (℃) or more Ac ₃ After annealing at a temperature range of +50 (° C.) or less for 10 seconds to 30 minutes, it is cooled to a temperature range of 650 to 700 ° C. at a cooling rate of 0.1 to 10 ° C./second, and subsequently 0.1 to 100 ° C. / After cooling to a plating bath temperature of −50 ° C. to a plating bath temperature of +50 (° C.) at a cooling rate of 2 seconds, it is immersed in the plating bath and includes the immersion time. A method for producing a Si-containing high-strength hot-dip galvanized steel sheet excellent in corrosion resistance and ductility, characterized by being held in the temperature range of 2 to 200 seconds and then cooled to room temperature.
[13] The high strength of Si-containing high strength excellent in corrosion resistance and ductility according to [12], wherein the alloying treatment is performed in a temperature range of 400 to 550 ° C. after the plating bath immersion and holding treatment, and is cooled to room temperature. Manufacturing method of hot dip galvanized steel sheet.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0014]
Inventors are mass%,
C: 0.0001 to 0.3%,
Si: 0.1 to 3.0%,
Mn: 0.01 to 3%
Al: 0.001 to 4%,
Mo: 0.001 to 1%,
P: 0.0001 to 0.3%,
S: 0.0001 to 0.1%,
The steel sheet comprising the remainder Fe and inevitable impurities is annealed, immersed in a Zn plating bath at a temperature of 450 to 470 ° C. for 3 seconds, and some samples are heated at 500 to 550 ° C. for 10 to 60 seconds. It was. Thereafter, the appearance before the corrosion resistance test was evaluated based on five levels based on the defect occurrence rate on the surface of the plated steel sheet. In addition, in the corrosion resistance test, the surface of the sample after plating was scratched with a cutter knife with a length of 1 cm, repeated dry and wet cycle tests were performed up to 100 cycles, and the appearance was evaluated again. Further, the mechanical properties were evaluated together by a tensile test. As a result, on the surface of the steel sheet,
Al: 0.001 to 4%,
Mo: 0.0001 to 1%,
Si: 0.0001 to 0.1%,
Fe: less than 20%,
A hot-dip galvanized steel sheet having a plating layer consisting of Zn and inevitable impurities, the balance being X, the Fe content in the plating layer being X, the Si content in the plating layer being Y, and the Mo in the plating layer being Mo. If the content rate is Z, X, Y and Z satisfy the formula (1),
X / 10− (Y + Z) + 1 ≧ 0 (1)
It was found that a score of 5 to 4 was obtained with almost no appearance defects or rusting before and after the corrosion test.
[0015]
In each of the grades 1 to 5, the appearance of plating was visually evaluated for the state of occurrence of non-plating, the state of occurrence of defects of scratches and patterns, and the corrosive biological form. The evaluation index is as follows.
Score 5: No plating, scratches and patterns, almost no rust after corrosion test (0.1% or less in area ratio)
Score 4: Non-plating, scratches and patterns, rusting after corrosion test is very small (over 0.1% in area ratio and 3% or less)
Score 3: Non-plating, scratches and patterns, rusting after corrosion test is small (over 3% and less than 50% in area ratio)
Score 2: Unplated, scratches and patterns, many rusting after corrosion test (area ratio exceeds 50%)
Rating 1: No plating wet or rust on the front after corrosion test.
[0016]
There are no particular restrictions on the amount of plating deposited, but the amount on one side is 5 g / m from the viewpoint of corrosion resistance. ² The above is desirable. For the purpose of improving the paintability and weldability on the hot-dip Zn plated steel sheet of the present invention, various treatments such as chromate treatment, phosphate treatment, lubricity improvement treatment, weldability improvement treatment, etc. However, the present invention does not depart from the present invention.
[0017]
The reason why the amount of Al in the plating layer is in the range of 0.001 to 4% by mass is that if less than 0.001%, dross generation is remarkable and a good appearance cannot be obtained, and if adding Al exceeding 4% This is because the alloying reaction is remarkably suppressed and it becomes difficult to form an alloyed hot-dip galvanized layer.
[0018]
The reason why the amount of Mo in the plating layer is in the range of 0.0001 to 1% by mass is that non-plating is suppressed in this range, plating with a good appearance can be obtained, and corrosion resistance can be improved. When the amount of Mo exceeds the upper limit of 1% by mass, the appearance of the plating is remarkably deteriorated due to the generation of dross containing Mo.
[0019]
The reason why the Si amount in the plating layer is set to 0.0001 to 0.1% by mass is that non-plating is suppressed and plating having a good appearance is obtained, and corrosion resistance is improved. On the other hand, when the upper limit is exceeded, the appearance of plating is remarkably deteriorated due to generation of dross containing each element.
[0020]
In particular, in order to suppress non-plating and further improve the adhesion, Mn and / or Ni in the plating layer was Mn: 0.0001 to 3% by mass and Ni: 0.001 to 3% by mass. This is because non-plating does not occur in this range, and plating with a good appearance can be obtained. When the amount of Mn and the amount of Ni exceed the upper limit of 3% by mass, the Zn compound is precipitated in the plating bath and taken into the plating layer, so that the appearance is remarkably deteriorated or dross is noticeable. Cause damage.
[0021]
Further, Mg, Ca, W, Zr, Cs, Rb, K, Ag, Na, Cd, Cu, Co, La, Tl, Nd, Y, In, Be, Cr, Pb, Hf, Tc, Ti are contained in the plating layer. It has been found that non-plating is suppressed and alloying is promoted by containing one or more of Ge, Ta, V, and B.
[0022]
0.001 to 0.1% Ca content, 0.001 to 3% Mg content, 0.001 to 0.1 mass% W content, 0.001 to 0.1 mass% Zr content, Cs content 0.001 to 0.1 mass%, Rb content 0.001 to 0.1 mass%, K content 0.001 to 0.1 mass%, Ag content 0.001 to 5 mass%, Na content 0.001 to 0.05 mass%, Cd content is 0.001 to 3 mass%, Cu content is 0.001 to 3 mass%, Co content is 0.001 to 1 mass%, and La content is 0.001. -0.1 mass%, Tl content is 0.001-8 mass%, Nd content is 0.001-0.1 mass%, Y content is 0.001-0.1 mass%, and In content is 0.001. -5 mass%, Be amount 0.001-0.1 mass%, Cr amount 0.001-0.05 mass%, Pb amount 0.001-1 mass%, Hf amount 0.001-0. .1 quality %, Tc amount is 0.001 to 0.1% by mass, Ti amount is 0.001 to 0.1% by mass, Ge amount is 0.001 to 5% by mass, Ta amount is 0.001 to 0.1% by mass. %, V amount is 0.001 to 0.2 mass%, and B content is within the range of 0.001 to 0.1 mass%. Is obtained. When the amount of each element exceeds the upper limit, the appearance of plating is remarkably deteriorated due to generation of dross containing each element.
[0023]
Fe is taken into the plating layer by the alloying treatment, and a high-strength hot-dip galvanized steel sheet excellent in paintability and spot weldability can be obtained. In the invention according to the above (1), if the amount of Fe in the plating layer exceeds 20% by mass, the adhesion of the plating layer itself is impaired, and the plating layer breaks and falls off during processing and adheres to the mold. Causes wrinkles. On the other hand, in order to improve spot weldability, it is preferable that the amount of Fe is 5% by mass or more. Therefore, the range of the amount of Fe in the plating layer when performing the alloying treatment is 5 to 20% by mass.
[0024]
In addition, even when the alloying treatment is not performed, even if the amount of Fe in the plating layer is less than 5% by mass, the corrosion resistance, ductility, workability, etc., which are effects other than the effects such as paintability and spot weldability obtained by alloying are good is there.
[0025]
Next, the reasons for limiting the steel plate components in the present invention will be described.
[0026]
C: In order to ensure strength, the lower limit of the C amount was 0.0001% by mass. In particular, the retained austenite is an indispensable additive element necessary to ensure a sufficient amount and stability. On the other hand, the upper limit for maintaining weldability was set to 0.3% by mass.
[0027]
Si: 0.1% or more in order to ensure manufacturability and material strength. Further, excessive addition leads to hardening of ferrite and deteriorates ductility and weldability.
[0028]
The range of Mn: 0.01 to 3% by mass is that a strengthening effect appears at 0.01% by mass or more, and 3% by mass is the upper limit because addition exceeding this has an adverse effect on elongation. It is.
[0029]
Al: The range of 0.001 to 4% by mass is set to 0.001% by mass or more for the purpose of deoxidation because of low Si. It also has the effect of improving the strength ductility balance and promoting the alloying behavior of the plating. On the other hand, excessive addition adversely affects weldability, plating wettability and manufacturability, so 4% was made the upper limit.
[0030]
Mo: Strengthening element. 0.001% or more is added to suppress pearlite and carbide precipitation that adversely affect the strength and ductility balance. On the other hand, excessive addition is accompanied by a decrease in ductility because it generates and stabilizes retained austenite and hardens ferrite, so the upper limit was made 1%.
[0031]
The amount of P was set in the range of 0.0001 to 0.3% by mass because the strengthening effect appears at 0.0001% by mass or more and the extremely low is economically disadvantageous, so this was set as the lower limit. . The reason why the upper limit is 0.3% by mass is that the addition exceeding this amount adversely affects weldability and manufacturability during casting and hot rolling.
[0032]
The reason why the S amount is in the range of 0.0001 to 0.1% by mass is that the extremely low reduction is economically disadvantageous, so 0.0001% by mass is set as the lower limit, and 0.1% by mass is also required. The upper limit is because addition of an amount exceeding this adversely affects weldability, manufacturability during casting and hot rolling.
[0033]
Furthermore, the steel targeted by the present invention can contain one or more of Cr, Ni, Cu, Co, and W for the purpose of further improving the strength.
[0034]
The Cr content in the range of 0.001 to 25% by mass means that a strengthening effect appears at 0.001% by mass or more, and the upper limit is 25% by mass. This is to adversely affect
[0035]
The amount of Ni in the range of 0.001 to 10% by mass is that the strengthening effect appears at 0.001% or more, and the upper limit of 10% by mass is the workability when the amount exceeds this. This is to have an adverse effect.
[0036]
The amount of Cu in the range of 0.001 to 5% by mass is that the strengthening effect appears at 0.001% by mass or more, and the upper limit of 25% by mass is the workability when the amount exceeds this. This is to adversely affect
[0037]
The Co amount in the range of 0.001 to 5% by mass is that the strengthening effect appears at 0.001% by mass or more, and the upper limit of 5% by mass is the workability when the amount exceeds this. This is to adversely affect
[0038]
The amount of W in the range of 0.001 to 5% by mass is that the strengthening effect appears at 0.001% by mass or more, and the upper limit of 5% by mass is the workability when the amount exceeds this. This is to adversely affect
[0039]
Furthermore, the steel targeted by the present invention can contain one or more of Nb, Ti, V, Zr, Hf, and Ta, which are strong carbide forming elements, for the purpose of further improving the strength.
[0040]
These elements form fine carbides, nitrides, or carbonitrides, and are extremely effective in strengthening the steel sheet. Therefore, if necessary, one or more elements may be added in total to 0.001% by mass or more. Was added. On the other hand, since it inhibits ductility deterioration and concentration of C in retained austenite, the upper limit of the total addition amount of one kind or two or more kinds is set to 1% by mass.
[0041]
B can also be added as needed. B is effective for strengthening grain boundaries and increasing the strength of steel by adding 0.0001% or more, but when the amount of addition exceeds 0.1% by mass, the effect is saturated and more than necessary. Therefore, the upper limit was set to 0.1% by mass.
[0042]
Y, Rem, Ca, Mg, Ce, are added for the purpose of suppressing the formation of an Si-based internal grain boundary oxidation phase that deteriorates the wettability of plating. Grain boundary oxides are not formed like Si-based oxides, but relatively fine oxides can be dispersed and formed. From these element groups, one or more elements were added in an amount of 0.0001% or more. On the other hand, excessive addition reduces the manufacturability such as castability and hot workability, and the ductility of the steel sheet product, so the upper limit was made 1 mass%.
[0043]
Next, a preferable microstructure of the base steel sheet will be described.
In order to ensure sufficient workability, it is desirable that the main structure is a ferrite phase with a volume fraction of 50% or more, preferably 70% or more, but a bainite phase may be included in view of increasing the strength. Further, in order to achieve both high strength and high ductility, a composite structure including a retained austenite phase and / or a martensite phase is formed. For high strength and high ductility, the residual austenite phase and martensite phase were made 3% or more in total by volume. When the volume ratio exceeds 50% in total, an embrittlement tendency is exhibited, so 50% or less is desirable.
[0044]
An increase in the volume fraction of ferrite increases ductility but leads to a decrease in strength. Therefore, the upper limit is 97% in the case of containing no bainite phase and 95% in the case of containing a bainite phase. In order to achieve both high strength and high ductility, a composite structure containing retained austenite and / or martensite is used. Due to high strength and high ductility, the residual austenite phase and / or martensite was 3% or more in total in terms of volume fraction. When the volume fraction exceeds 30% in total, an embrittlement tendency is shown, so this was made the upper limit.
[0045]
In order to ensure the high ductility of the steel sheet itself, the average grain size of ferrite is set to 20 μm or less, and the average grain size of austenite and / or martensite as the second phase is specified to be 10 μm or less. In addition, here, it is desirable that the second phase is austenite and / or martensite and a ratio of 0.7 or less is ensured with respect to the average particle diameter of the ferrite as the main phase. On the other hand, the average particle size of the austenite and / or martensite, which is the second phase, is less than 0.01 times the average particle size of ferrite, and it is difficult to actually manufacture, so it may be 0.01 times or more. preferable.
[0046]
Furthermore, in order to improve the balance between plating adhesion and high strength ductility / ductility, when the second phase of the steel sheet is austenite, the carbon content in the steel: C, the Mn content in the steel: Mn, and the austenite The volume ratio: Vγ, the volume ratio of ferrite and bainite: Vα, and the formula (2) is satisfied.
[0047]
(Vγ + Vα) / Vγ × C + Mn / 8 ≧ 2,000 (2)
By satisfying this formula, a steel sheet having a particularly excellent balance between strength and ductility and good plating adhesion can be obtained.
[0048]
The volume fraction and the like in the case of including bainite will be described as follows. By containing 2% or more of the bainite phase in terms of volume fraction, it helps to increase the strength, and when it coexists with the austenite phase, it contributes to stabilization of the austenite and consequently increases the n value. In addition, this phase is basically fine and contributes to plating adhesion during high processing. In particular, when the second phase is austenite, the balance between plating adhesion and ductility is further improved when the volume fraction of bainite is 2% or more. On the other hand, when it produces | generates excessively, a ductility fall will be caused, and a bainite phase shall be 47% or less by a volume fraction.
[0049]
In addition to the above, the case of containing one or more of carbides, nitrides, sulfides, and oxides as the remaining structure of the microstructure is also within the category of the steel sheet of the present invention. Is preferably 1% or less in volume fraction. In addition, the ferrite, bainite, austenite, martensite, and the remaining structure of the above microstructure, the observation of the existing position, and the measurement of the average particle size (average equivalent circle diameter) and the space factor were made by using the Nital reagent and It can be quantified by observing an optical microscope at 500 to 1000 times by corroding a cross section in the rolling direction of the steel sheet or a cross section perpendicular to the rolling direction with the reagent disclosed in Japanese Patent No. 219473. Here, measurement of the particle size of martensite may be difficult when using an optical microscope. In this case, the average equivalent circle diameter is obtained by observing and measuring the martensite block boundary, packet boundary, or a set thereof using a scanning electron microscope.
[0050]
The average particle size is defined as a value obtained by JIS based on the result of 20 field observations or more by the above method.
[0051]
A method for producing a high-strength hot-dip galvanized steel sheet having such a structure will be described below.
[0052]
When the steel sheet of the present invention is manufactured by cold rolling and annealing after hot rolling, the slab adjusted to a predetermined component is cast or once cooled and then reheated to 1180 ° C. or higher to form a uniform scale. Increases descalability by forming on the surface. On the other hand, since heating above 1250 ° C. promotes local abnormal oxidation, this was set as the upper limit of the heating temperature. In addition, for the purpose of suppressing excessive internal oxidation, hot rolling is finished at 880 ° C. or higher, then pickling, and annealing after cold rolling to obtain a final product. At this time, the hot rolling completion temperature is determined by the chemical composition of the steel. ₃ Generally, it is carried out at the transformation temperature or higher, but Ar ₃ If it is from about 10 degreeC to about 10 degreeC low temperature, the characteristic of the final steel plate will not be deteriorated. On the other hand, in order to avoid the formation of a large amount of oxide scale, the hot rolling completion temperature is set to 1100 ° C. or lower.
[0053]
In addition, if the coiling temperature after cooling is higher than the bainite transformation start temperature determined by the chemical composition of the steel, it is possible to avoid unnecessarily increasing the load during cold rolling, but when the total rolling reduction of cold rolling is small Is not limited to this, and even if the steel sheet is wound at a temperature lower than the bainite transformation temperature of the steel, the properties of the final steel sheet are not deteriorated. Further, the total rolling reduction ratio of the cold rolling is set based on the relationship between the final sheet thickness and the cold rolling load, but if it is 40% or more, the final steel sheet characteristics are not deteriorated.
[0054]
Temperature Ac determined by the chemical composition of steel when annealing after cold rolling ₁ And Ac ₃ 0.1 × (Ac) expressed by temperature (for example, “steel material science”: W. C. Leslie, translated by Koyasu Naruyasu, Maruzen P273) ₃ -Ac1) + Ac ₁ When the temperature is less than (° C.), since the amount of austenite obtained at the annealing temperature is small, the residual austenite phase or martensite phase cannot be left in the final steel sheet, so this was made the lower limit of the annealing temperature. Also, the annealing temperature is Ac ₃ Even if the temperature exceeds +50 (° C.), the upper limit of the annealing temperature is set to Ac in order to improve the manufacturing cost without improving the properties of the steel sheet. ₃ +50 (° C.). The annealing time at this temperature requires 10 seconds or more to make the temperature of the steel plate uniform and to secure austenite. However, if it exceeds 30 minutes, the effect is not only saturated but also the cost is increased.
[0055]
Subsequent primary cooling is important for promoting the transformation from the austenite phase to the ferrite phase and concentrating C in the untransformed austenite phase to stabilize the austenite. If the cooling rate is less than 0.1 ° C./second, a production disadvantage such as lengthening the required production line length or extremely slowing the production rate is caused. The temperature was 1 ° C./second. On the other hand, when the cooling rate exceeds 10 ° C./sec, ferrite transformation does not occur sufficiently, and it becomes difficult to secure the residual austenite phase in the final steel sheet, or the hard phase such as martensite phase becomes large. Therefore, this is the upper limit.
[0056]
When this primary cooling is performed to less than 650 ° C., pearlite is not generated during cooling, and sufficient ferrite is not generated. However, since the ferrite transformation does not progress sufficiently even when cooling exceeds 700 ° C., this is set as the upper limit.
[0057]
The subsequent rapid cooling of the secondary cooling requires a cooling rate of at least 0.1 ° C./second or more so as not to cause pearlite transformation or precipitation of iron carbide during cooling. However, since it is difficult to increase the cooling rate above 100 ° C./second in terms of equipment capacity, the cooling rate range was set to 0.1-100 ° C./second.
[0058]
If the cooling stop temperature of this secondary cooling is lower than the plating bath temperature −50 ° C., it becomes a serious problem in operation, and if it exceeds the plating bath temperature +50 (° C.), carbide precipitation occurs in a short time, resulting in the retained austenite and martensite. The amount of can not be secured. For this reason, the secondary cooling stop temperature was set to a plating bath temperature of −50 ° C. or higher and a plating bath temperature of +50 (° C.). In order to stabilize the austenite phase remaining in the steel sheet at room temperature, it is essential to further increase the carbon concentration in the austenite by transforming a part thereof into the bainite phase. In order to allow the bainite transformation to proceed in a short time together with the alloying treatment, it was held for 2 to 200 seconds including the dipping time in the temperature range from the plating temperature −50 ° C. to the plating temperature + 50 ° C.
If the plating temperature is lower than −50 ° C., the bainite transformation hardly occurs, and if it exceeds the plating temperature + 50 ° C., carbides are generated and it is difficult to leave a sufficient residual austenite phase.
[0059]
Unlike the retained austenite phase, it is not necessary to cause the bainite transformation to produce the martensite phase. On the other hand, since it is necessary to suppress the formation of carbides and pearlite phases in the same manner as the retained austenite phase, alloying treatment is performed in a temperature range of 400 to 550 ° C. in order to perform sufficient alloying treatment after secondary cooling. To do.
[0060]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
[0061]
A steel strip having a composition as shown in Table 1 was heated to 1180 to 1250 ° C., completed hot rolling at 880 to 1100 ° C., and wound up at a temperature higher than the bainite transformation start temperature determined by the chemical composition of each steel after cooling. After pickling, it was cold rolled to a thickness of 1.0 mm.
[0062]
[Table 1]

[0063]
Then, according to the following formula from the component (mass%) of each steel, Ac ₁ And Ac ₃ The transformation temperature was determined by calculation.
[0064]
Ac ₁ = 723-10.7 × Mn% + 29.1 × Si%,
Ac ₃ = 910-203 × (C%) ^1/2 + 44.7 * Si% + 31.5 * Mo% -30 * Mn% -11 * Cr% + 400 * Al%
These Ac ₁ And Ac ₃ 10% H in the annealing temperature calculated from the transformation temperature ₂ -N ₂ After raising the temperature and holding in the atmosphere, it is cooled to a temperature range of 650 to 700 ° C. at a cooling rate of 0.1 to 10 ° C./second, and subsequently to the plating bath temperature at a cooling rate of 0.1 to 20 ° C./second. Then, the plating was carried out by immersing in a galvanizing bath at 460 to 470 ° C. with various changes in the bath composition for 3 seconds.
[0065]
Moreover, about some steel plates, as a Fe-Zn alloying process, the steel plate after plating is hold | maintained for 15 seconds-20 minutes in the temperature range of 400-550 degreeC, and the Fe content rate in a plating layer is 5 by mass%. Adjusted to ˜20%. The plating appearance was evaluated by visual observation of the dross entrainment situation of the plating surface appearance and measurement of the non-plated area. The prepared plating was dissolved in a 5% hydrochloric acid solution containing an inhibitor and subjected to chemical analysis, and the composition was determined and shown in Table 2.
[0066]
From Tables 2 and 3, the steel of the present invention has an appearance score of 5 to 4 before and after the corrosion resistance test, and is excellent in strength and elongation balance. On the other hand, all the comparative examples not satisfying the scope of the present invention have low appearance scores and are inferior in strength / elongation balance. Further, the steel sheet produced in the scope of the claims of the present invention has the microstructure described above, and is excellent in appearance, strength and elongation balance.
[0067]
[Table 2]

[0068]
[Table 3]

[0069]
[Table 4]

[0070]
【The invention's effect】
The high-strength hot-dip galvanized steel sheet of the present invention is extremely excellent in corrosion resistance, has good workability, and is extremely effective for building materials, home appliances, automobile body applications, and the like.

Claims (13)

% By mass
C: 0.0001 to 0.3%,
Si: 0.1 to 3.0%,
Mn: 0.01 to 3%
Al: 0.001 to 4%,
Mo: 0.001 to 1%,
P: 0.0001 to 0.3%,
S: 0.0001 to 0.1%,
In the surface of the steel plate made of the balance Fe and inevitable impurities,
Al: 0.001 to 4%,
Mo: 0.0001 to 1%,
Si: 0.0001 to 0.1%,
Fe: less than 20%,
A hot-dip galvanized steel sheet having a plating layer consisting of Zn and inevitable impurities, the balance being X, the Fe content in the plating layer being X, the Si content in the plating layer being Y, and the Mo in the plating layer being Mo. A Si-containing high-strength hot-dip galvanized steel sheet excellent in corrosion resistance and ductility, wherein X, Y, and Z satisfy the formula (1) where the content is Z.
X / 10− (Y + Z) + 1 ≧ 0 (1) The plating layer is further mass%,
Mn: 0.0001-3%
Ni: 0.001 to 3%,
The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to claim 1, wherein one or two of the above are contained. The plating layer is further mass%,
Ca: 0.001 to 0.1%,
Mg: 0.001 to 3%,
W: 0.001 to 0.1%,
Zr: 0.001 to 0.1%,
Cs: 0.001 to 0.1%,
Rb: 0.001 to 0.1%,
K: 0.001 to 0.1%,
Ag: 0.001 to 5%,
Na: 0.001 to 0.05%,
Cd: 0.001 to 3%
Cu: 0.001 to 3%,
Co: 0.001-1%,
La: 0.001 to 0.1%,
Tl: 0.001-8%
Nd: 0.001 to 0.1%,
Y: 0.001 to 0.1%
In: 0.001 to 5%,
Be: 0.001 to 0.1%,
Cr: 0.001 to 0.05%,
Pb: 0.001 to 1%,
Hf: 0.001 to 0.1%,
Tc: 0.001 to 0.1%,
Ti: 0.001 to 0.1%,
Ge: 0.001 to 5%,
Ta: 0.001 to 0.1%,
V: 0.001 to 0.2%,
B: 0.001 to 0.1%,
The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to claim 1 or 2, characterized by containing one or more of the following. Steel is more mass%,
Cr: 0.001 to 25%,
Ni: 0.001 to 10%,
Cu: 0.001 to 5%,
Co: 0.001-5%
W: 0.001 to 5%,
The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to any one of claims 1 to 3, wherein one or more of the above are contained. The steel further contains 0.001 to 1% in total of one or more of Nb, Ti, V, Zr, Hf, and Ta in mass%. A Si-containing high-strength hot-dip galvanized steel sheet excellent in corrosion resistance and ductility according to item 1. The steel further contains B: 0.0001 to 0.1% by mass%, and the Si-containing high-strength melt excellent in corrosion resistance and ductility according to any one of claims 1 to 5 Galvanized steel sheet. 7. The steel according to claim 1, wherein the steel further contains 0.0001 to 1% of one or more of Y, Rem, Ca, Mg, and Ce in mass%. High strength hot-dip galvanized steel sheet with excellent corrosion resistance and ductility. The microstructure of the steel has a ferrite phase of 50 to 97% by volume fraction, or a ferrite phase and a bainite phase as the main phase, and the balance is one or both of the martensite phase and residual austenite phase in a total volume of 3 to 3. The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to any one of claims 1 to 7, wherein the composite structure contains 50%. The microstructure of the steel sheet is mainly composed of 70 to 97% ferrite in volume fraction, the average particle size is 20 μm or less, and the second phase is austenite and / or martensite in volume fraction of 3 to 30%. The Si-containing high-strength hot-dip galvanized steel sheet excellent in corrosion resistance and ductility according to any one of claims 1 to 8, wherein the second phase has an average particle size of 10 µm or less. The second phase of the steel sheet is austenite, carbon content in steel: C (mass%), Mn content in steel: Mn (mass%), volume ratio of austenite: Vγ (%), volume ratio of ferrite and bainite Vα (%) satisfies the formula (2). The Si-containing high-strength hot-dip galvanized steel sheet having excellent corrosion resistance and ductility according to any one of claims 1 to 9.
(Vγ + Vα) / Vγ × C + Mn / 8 ≧ 2,000 (2) The microstructure of the steel sheet is austenite and / or martensite with a volume fraction of 50 to 95% of ferrite as the main phase, an average particle size of 20 μm or less, and a volume fraction of 3 to 30% as the second phase. The average particle size thereof is 10 μm or less, and further consists of bainite having a volume fraction of 2 to 47%, and the corrosion resistance and ductility according to claim 1, Excellent Si-containing high-strength hot-dip galvanized steel sheet. A method for producing the high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 11, comprising a cast slab comprising the steel sheet component according to any one of claims 1 and 4 to 7. As cast or once cooled, it is heated again to 1180 to 1250 ° C., and after hot rolling is finished at 880 to 1100 ° C., the wound hot-rolled steel sheet is pickled and cold-rolled, and then 0.1 × (Ac ₃₋ Ac ₁ ) + Ac ₁ (° C.) to Ac ₃ +50 (° C.) and after annealing for 10 seconds to 30 minutes, a cooling rate of 0.1 to 10 ° C./second and a temperature range of 650 to 700 ° C. Then, after cooling to a plating bath temperature of −50 ° C. to a plating bath temperature of +50 (° C.) at a cooling rate of 0.1 to 100 ° C./second, the plate is immersed in the plating bath, and includes the immersion time. 2 to 2 in the temperature range of bath temperature −50 ° C. to plating bath temperature +50 (° C.) A method for producing a Si-containing high-strength hot-dip galvanized steel sheet excellent in corrosion resistance and ductility, characterized by cooling to room temperature after holding for 00 seconds. 13. The high strength hot-dip galvanized steel sheet containing Si having excellent corrosion resistance and ductility according to claim 12, wherein the alloying treatment is performed in a temperature range of 400 to 550 ° C. after the plating bath immersion and holding treatment, and cooling to room temperature. Manufacturing method.