JP3898924B2 - High-strength hot-dip galvanized steel sheet excellent in appearance and workability and its manufacturing method - Google Patents

Description

【００６１】
その後、各鋼の成分（質量％）から下記式にしたがってＡｃ₁ とＡｃ₃ 変態温度を計算により求めた。
【００６２】

これらのＡｃ₁ およびＡｃ₃ 変態温度から計算される焼鈍温度に１０％Ｈ₂ −Ｎ₂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。
【００６３】
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。
【００６４】
表２および表３より、本発明鋼は、外観評点がすべて５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。
【００６５】
【表２】

【００６６】
【表３】

【００６７】
【表４】

【００６８】
【発明の効果】
本発明の高強度溶融亜鉛めっき鋼板はめっき外観に極めて優れ、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 high-strength alloyed hot-dip galvanized steel sheet and hot-dip galvanized steel sheet that are excellent in appearance and workability 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,₂And N₂There is a Sendzimer method of cooling after annealing in a reducing atmosphere containing, cooling to near the plating bath temperature, immersing in a molten zinc bath, cooling or reheating to form an Fe-Zn alloy phase, It is frequently used for processing.
[0003]
For annealing before plating, after degreasing and washing, immediately after heating in a non-oxidizing atmosphere, H₂And N₂In 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 steel surface, and Fe—Zn 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 fuel consumption especially 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, when the content of Si in the steel exceeds 0.3% by mass, plating wettability is greatly reduced in the Sendzimer method using a normal Al-containing plating bath, Appearance quality deteriorates because non-plating occurs. It is said that this is because Si oxide is concentrated on the surface of the steel sheet during reduction annealing, and the wettability of Si oxide to molten zinc is poor.
[0007]
As a means for solving this problem, as shown in JP-A-3-28359, JP-A-3-64437 and the like, the plating property is improved by applying specific plating. Then, it is necessary to provide a new plating facility in front of the hot dipping line annealing furnace, or to perform a plating process in the electroplating line in advance, resulting in a significant increase in cost.
[0008]
Further, Si scale scratches generated during hot rolling also cause the appearance of subsequent plating to be damaged. In order to suppress this, it is indispensable to reduce the amount of Si in the steel. However, in the retained austenitic steel and the duplex steel sheets, which are representative of high ductility type high-strength steel sheets, the balance between strength and ductility is enhanced. Si is a very effective additive element.
[0009]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-mentioned problems, and to provide a high-strength galvannealed steel sheet, a galvanized steel sheet, and a method for producing the same, in which non-plating is suppressed and appearance and workability are excellent.
[0010]
[Means for Solving the Problems]
  As a result of various investigations, the inventors have made the plating layer contain a specific element at an appropriate concentration and combine it with the components of the steel sheet to thereby improve the hot dip galvanizing wettability and alloying plating of the high strength steel sheet. Found alloying promotion. This effect is mainly due to platinglayerIt can be made to appear by controlling the concentration of medium Al and Mn in steel.
[0011]
  Mn content in steel is X (mass%), Si content in steel is Y (mass%), platinglayerAssuming that the content of Al in the middle is Z (mass%), it has been found that extremely good plating can be obtained when X, Y and Z satisfy the formula (1).
[0012]
  0.6− (X / 18 + Y + Z) ≧ 0 (1)
  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: 0.001 to less than 0.1%
        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%
From the balance Fe and inevitable impuritiesThe steel microstructure is a composite structure composed of a main phase and a second phase, and the main phase is a ferrite phase or a ferrite phase and a bainite phase with a volume fraction of 50 to 97%, and the average of the main phase Particle size is 20μ m The second phase is composed of one or both of a martensite phase and a retained austenite phase, the volume fraction of the second phase is 3 to 50% in total, and the average particle size of the second phase is 10 μm. m The average particle size of the second phase is 0.01 to 0.7 times the average particle size of the main phase.On the surface of the steel plate
        Mn: 0.001 to 3%
        Al: 0.001 to 4%
        Mo: 0.0001 to 1%
        Fe: 5 to 20%
A hot-dip galvanized steel sheet having a plating layer consisting of Zn and inevitable impurities, the balance being Mn content in steel X (mass%), Si content in steel Y (mass%), during plating High-strength hot-dip galvanizing excellent in appearance and workability, characterized in that X, Y, and Z satisfy the formula (1) where the Al content is Z (mass%)Cold rollingsteel sheet.
[0013]
    0.6− (X / 18 + Y + Z) ≧ 0 (1)
[2] By mass%
        C: 0.0001 to 0.3%
        Si: 0.001 to less than 0.1%
        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%
From the balance Fe and inevitable impuritiesThe steel microstructure is a composite structure composed of a main phase and a second phase, and the main phase is a ferrite phase or a ferrite phase and a bainite phase with a volume fraction of 50 to 97%, and the average of the main phase Particle size is 20μ m The second phase is composed of one or both of a martensite phase and a retained austenite phase, the volume fraction of the second phase is 3 to 50% in total, and the average particle size of the second phase is 10 μm. m The average particle size of the second phase is 0.01 to 0.7 times the average particle size of the main phase.On the surface of the steel plate
        Mn: 0.001 to 3%
        Al: 0.001 to 4%
        Mo: 0.0001 to 1%
        Fe: less than 5%
A hot-dip galvanized steel sheet having a plating layer consisting of Zn and inevitable impurities, the balance being Mn content in steel X (mass%), Si content in steel Y (mass%), during plating High-strength hot-dip galvanizing excellent in appearance and workability, characterized in that X, Y, and Z satisfy the formula (1) where the Al content is Z (mass%)Cold rollingsteel sheet.
    0.6− (X / 18 + Y + Z) ≧ 0 (1)
[3] The plating layer is further mass%,
        Si: 0.0001 to 0.1%,
        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%,
        Ni: 0.001 to 0.5%,
        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%,
High-strength hot-dip galvanizing excellent in appearance and workability described in [1] or [2], characterized by containing one or more ofCold rollingsteel sheet.
[4] Steel is further mass%,
        Cr: 0.001 to 25%,
        Ni: 0.001 to 10%,
        Cu: 0.001 to 5%,
        Co: 0.001 to 5%
        W: 0.001 to 5%
The high-strength hot-dip galvanizing excellent in appearance and workability according to any one of [1] to [3], characterized by containing one or more ofCold rollingsteel sheet.
[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] High-strength hot-dip galvanizing excellent in appearance and workabilityCold rollingsteel sheet.
[6] Steel is further contained by mass%, and B: 0.0001 to 0.1%, and is excellent in appearance and workability according to any one of [1] to [5] High strength hot dip galvanizingCold rollingsteel sheet.
[7] The steel further contains 0.0001 to 1% of one or more of Y, Rem, Ca, Mg, and Ce in total by mass% [1] to [6] High-strength hot-dip galvanizing with excellent appearance and workabilityCold rollingsteel sheet.
[8]The main phase isFerai with volume fraction of 70-97%In phaseYes, second phaseofVolume fractionBut3-30%so[1]-[7] High-strength hot-dip galvanizing excellent in appearance and workabilityCold rollingsteel sheet.
[9]FirstTwo phasesResidualAustenitephaseThe amount of carbon in steel: C (mass%), the amount of Mn in steel: Mn (mass%),Said residueAustenitephaseVolume ratio: Vγ (%), ferritephaseAnd bainitephaseVolume ratio: Vα (%) satisfies the formula (2) [1] to [1]8] High-strength hot-dip galvanizing excellent in appearance and workabilityCold rollingsteel sheet.
(Vγ + Vα) /Vγ×C+Mn/8≧2.000 (2)
[10]The main phase isFerrite with volume fraction of 50-95%2 to 47% bainite phase by volume and volume fractionAnd the second phaseofVolume fractionBut3-30%In[1]-[9] High-strength hot-dip galvanizing excellent in appearance and workabilityCold rollingsteel sheet.
[11] [2] ~ [10] High-strength hot-dip galvanizing according to any one ofCold rollingA method of manufacturing a steel plate, wherein the cast slab comprising the steel plate component according to any one of [1], [4] to [7] is heated to 1180 to 1250 ° C. as cast or once cooled. The hot-rolled steel sheet wound after the hot rolling at 880 to 1100 ° C. is cold-rolled after pickling, and then 0.1 × (Ac3-Ac1) + Ac1 (° C.) or more and Ac3 + 50 (° C.) or less. After annealing in the temperature range for 10 seconds to 30 minutes, cooling to a temperature range of 650 to 700 ° C. at a cooling rate of 0.1 to 10 ° C./secondDo the primary coolingAnd thenGreater than the primary cooling and 5Cooling to plating bath temperature -50 ° C to plating bath temperature + 50 (° C) at a cooling rate of ~ 100 ° C / secondDo secondary coolingAfter being immersed in a plating bath and after being kept in a temperature range of plating bath temperature −50 ° C. to plating bath temperature +50 (° C.) for 2 to 200 seconds including the immersion time., RoomHigh-strength hot-dip galvanizing with excellent workability characterized by cooling to temperatureCold rollingA method of manufacturing a steel sheet.
[12] A method for producing a high-strength hot-dip galvanized cold-rolled steel sheet according to any one of [1] and [3] to [10], wherein the alloying treatment is performed after immersion and holding in the plating bath. The method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent appearance and workability according to [11], wherein the method is performed in a temperature range of ˜550 ° C. and cooled to room temperature.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in detail.
[0015]
  Inventors are mass%,
    C: 0.0001 to 0.3%,
    Si: 0.001 to less than 0.1%,
    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%
Steel, the balance Fe and inevitable impurities steel plate 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 530 ° C. for 10 to 60 seconds. It was. Thereafter, the external appearance was evaluated in five stages based on the defect occurrence rate on the surface of the plated steel sheet. In addition, mechanical properties were evaluated in a tensile test. As a result, the Mn content in steel is X (mass%), the Si content in steel is Y (mass%), platinglayerIt was found that when the Al content is Z (mass%), X, Y, and Z satisfy the formula (1), and a rating of 5 with almost no appearance defect is obtained.
0.6− (X / 18 + Y + Z) ≧ 0 (1)
  In each of the grades 1 to 5, the appearance of plating was visually evaluated for the occurrence of unplating and the occurrence of scratches and pattern defects. The evaluation index is as follows.
Score 5: almost no plating, scratches or patterns (area ratio of 1% or less)
Score 4: Unplated, scratches and patterns are very small (over 1% in area ratio and less than 10%)
Score 3: Unplated, small scratches and patterns (over 10% and less than 50% in area ratio)
Score 2: Unplated, many scratches and patterns (over 50% in area ratio)
Score 1: The details of why the plating does not get wet and the occurrence of non-plating and other defects are unknown, but the wettability between Al added to the plating bath and SiO2 formed on the steel sheet surface is poor. It is thought that non-plating occurs. That is, it is possible to suppress the occurrence of non-plating by adding an element that removes the adverse effect of Al added to the Zn bath. As a result of intensive studies by the present inventors, it was found that the notation purpose can be achieved by adding Mn in an appropriate concentration range. Moreover, it is thought that it is effective for the improvement of external appearance that the generation | occurrence | production of the damage | wound by Si scale produced at the time of hot rolling by suppressing the amount of Si in steel was suppressed. In addition, with regard to material deterioration due to the reduction of Si, it has also been found that ductility can be ensured by adding manufacturing conditions and other components: Al and Mo, and that the reduction of Si and addition of Al are effective in promoting alloying. It was.
[0016]
Further, in the plating layer, W, Zr, Cs, Rb, K, Ag, Na, Cd, Cu, Ni, Co, La, Tl, Nd, Y, In, Be, Cr, Pb, Hf, Tc, Ti, Ge It has been found that by containing one or more of Ta, V, and B, non-plating is suppressed and alloying is promoted.
[0017]
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.
[0018]
The reason why the amount of Mn in the plating layer is within the range of 0.001 to 3% by mass is that non-plating does not occur in this range and plating with a good appearance can be obtained. When the amount of Mn exceeds 3 mass% of an upper limit, a Mn-Zn compound will precipitate in a plating bath and it will take in in a plating layer, and an external appearance will fall remarkably.
[0019]
The reason why the amount of Al in the plating layer is in the range of 0.001 to 4% by mass is that when less than 0.001% by mass, dross generation is remarkable and a good appearance cannot be obtained, and Al exceeds 4% by mass. If added, the alloying reaction is remarkably suppressed, and it becomes difficult to form an alloyed hot-dip galvanized layer.
[0020]
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 and plating with a good appearance is obtained. When the amount of Mo exceeds the upper limit of 1% by mass, the appearance of plating is significantly deteriorated due to generation of dross containing Mo.
[0021]
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 [1], the spot weldability is insufficient when the Fe content of the plating layer is less than 5% by mass. On the other hand, if the amount of Fe exceeds 20% by mass, the adhesion of the plating layer itself is impaired, and the plating layer breaks and drops during processing and adheres to the mold, thereby causing defects during molding. Therefore, the range of the amount of Fe in the plating layer when performing the alloying treatment is 5 to 20% by mass.
[0022]
When the alloying treatment is not performed, the amount of Fe in the plating layer may be less than 5% by mass. That is, in the invention according to the above [2], even if the Fe content of the plating layer is less than 5% by mass, the effects such as appearance, workability and corrosion resistance other than the effects of the alloying according to the present invention according to [1] are good. .
[0023]
The amount of Si in the plating layer is 0.0001 to 0.1 mass%, the amount of W is 0.001 to 0.1 mass%, the amount of Zr is 0.001 to 0.1 mass%, and the amount of Cs is 0.001 to 0. 0.1 mass%, Rb content is 0.001 to 0.1 mass%, K content is 0.001 to 0.1 mass%, Ag content is 0.001 to 5 mass%, and Na content is 0.001 to 0. 0.05 mass%, Cd content 0.001-3 mass%, Cu content 0.001-3 mass%, Ni content 0.001-0.5 mass%, Co content 0.001-1 mass% The La amount is 0.001 to 0.1% by mass, the Tl amount is 0.001 to 8% by mass, the Nd amount is 0.001 to 0.1% by mass, and the Y amount is 0.001 to 0.1% by mass. In amount 0.001 to 5 mass%, Be amount 0.001 to 0.1 mass%, Cr amount 0.001 to 0.05 mass%, Pb amount 0.001 to 1 mass%, Hf 0.001 to 0.1 mass%, Tc content 0.001 to 0.1 mass%, Ti content 0.001 to 0.1 mass%, Ge content 0.001 to 5 mass%, Ta content In the range of 0.001 to 0.1% by mass, V content in the range of 0.001 to 0.2% by mass, and B content in the range of 0.001 to 0.1% by mass in this range, respectively. This is because plating with a good appearance can be 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.
[0024]
Next, the reasons for limiting the steel plate components in the present invention will be described.
[0025]
  C: In order to ensure strength, the lower limit of the C amount was 0.0001% by mass. Also especially retained austenitephaseIs 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.
[0026]
Si: 0.001% or more to ensure manufacturability and material strength, and Si was made less than 0.1% to reduce scale scratches. Addition exceeding this often causes scale damage, leading to deterioration of the plating appearance and reduction of the yield of the steel sheet.
[0027]
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.
[0028]
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.
[0029]
  Mo: Strengthening element. Perlite that adversely affects strength ductility balance due to low SiphaseFurther, 0.001 mass% or more is added to suppress precipitation of carbides instead of Si. On the other hand, excessive addition is retained austenitephaseGeneration and stabilization of ferritephase1% was made the upper limit because it causes a decrease in ductility.
[0030]
The amount of P is 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 is 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.
[0031]
The reason why the amount of S is in the range of 0.0001 to 0.1% by mass is that the extremely low is economically disadvantageous, so 0.0001% by mass is set as the lower limit, and 0.1% by mass is set. The upper limit is because addition of an amount exceeding this adversely affects weldability, manufacturability during casting and hot rolling.
[0032]
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.
[0033]
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
[0034]
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.
[0035]
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
[0036]
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
[0037]
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
[0038]
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.
[0039]
  These elements form fine carbides, nitrides or carbonitrides, and the strengthening of the steel sheet is extremely effective. Therefore, if necessary, one or more elements may be added in total to 0.001% by mass or more. It was set as addition. On the other hand, ductile deterioration and retained austenitephaseSince the concentration of C into the inside is inhibited, the upper limit of the total amount of one or more types is set to 1% by mass.
[0040]
B can also be added as needed. B is effective for strengthening grain boundaries and increasing the strength of steel by adding 0.0001% by mass or more. However, when the amount of addition exceeds 0.1% by mass, the effect is not only saturated but also necessary. Since the steel sheet strength is increased and the workability is lowered as described above, the upper limit is set to 0.1% by mass.
[0041]
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%.
[0042]
  Next, a preferable microstructure of the base steel sheet will be described. Mainly to ensure sufficient workabilityphaseIs preferably 50% or more, and preferably 70% or more in terms of volume fraction, but a bainite phase may be included in consideration of increasing the strength. Meanwhile, ferritephaseThe increase in volume fraction increases the ductility but leads to a decrease in strength, so the upper limit is 97% in the volume fraction when not containing the bainite phase, and 95% in the volume fraction when containing the bainite phase. In order to achieve both high strength and high ductility, a composite structure containing a retained austenite phase and / or 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. Since the embrittlement tendency is exhibited when the volume ratio exceeds 50% in total, it is set to 50% or less, and preferably 30% or less.
[0043]
  In order to ensure the high ductility of the steel plate itself,phaseThe average particle size of 20 μm or less is the second phaseResidualAustenitephaseAnd / or martensitephaseThe average particle size is defined as 10 μm or less. Also here the second phaseResidualAustenitephaseAnd / or martensitephaseAnd ferrite as the main phasephaseIt is desirable to ensure a ratio of 0.7 or less with respect to the average particle diameter. On the other hand, it is the second phaseResidualAustenitephaseAnd / or martensitephaseThe average particle size of ferritephaseSince it is difficult for actual production to be less than 0.01 times the average particle size, it is preferably 0.01 times or more.
[0044]
  Furthermore, in order to achieve a good balance between plating adhesion and high strength ductility / ductility, the second phase of the steel sheetResidualAustenitephaseWhen the amount of carbon in steel: C (mass%), the amount of Mn in steel: Mn (mass%),ResidualAustenitephaseVolume ratio: Vγ (%), ferritephaseAnd bainitephaseThe volume ratio of Vα (%) was set to satisfy the formula (2).
  (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.
[0045]
  Bay nightphaseThe volume fraction and the like in the case of including are described as follows. By containing 2% or more of the bainite phase in terms of volume fraction, it helps to increase the strength, and if it coexists with the austenite phaseResidualAustenitephaseAs a result, it helps to increase the n value. In addition, this phase is basically fine and contributes to plating adhesion during high processing. Especially the second phaseResidualAustenitephaseIn the case of bainitephaseIf the volume fraction of is 2% or more, the balance between plating adhesion and ductility is further improved. 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.
[0046]
  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, ferrite of the above microstructurephase, Bay nightphase,ResidualAustenitephase,MartensitephaseIn addition, the identification of the remaining structure, the observation of the existing position, and the measurement of the average particle diameter (average equivalent circle diameter) and the space factor were carried out by using the Nital reagent and the reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473 in the direction of rolling in the steel plate. The cross section perpendicular to the rolling direction can be corroded and quantified by observation with an optical microscope of 500 to 1000 times. Where martensitephaseMeasurement of the particle size may be difficult when using an optical microscope. In this case, use a scanning electron microscope to martensite.phaseThe average circle equivalent diameter is obtained by observing the block boundaries, packet boundaries, or a set of them and measuring the particle size.
[0047]
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.
[0048]
A method for producing a high-strength hot-dip galvanized steel sheet having such a structure will be described below.
[0049]
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. Moreover, in order to suppress excessive internal oxidation production, the hot rolling is finished at 880 ° C. or higher, and then pickled and annealed 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._Three Generally, it is carried out at the transformation temperature or higher, but Ar_Three 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.
[0050]
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 increasing the load during cold rolling more than necessary, 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.
[0051]
  When annealing after cold rolling, the annealing temperature is expressed by the temperature Ac1 and Ac3 temperature determined by the chemical composition of the steel (for example, “Steel Material Science” written by W.C. Leslie, translated by Narita Koda, Maruzen P273). When the temperature is less than 1 × (Ac3−Ac1) + Ac1 (° C.), the amount of austenite obtained at the annealing temperature is so small that the retained austenite phase or martensite phase cannot be left in the final steel sheet. Was the lower limit of the annealing temperature. Further, even if the annealing temperature exceeded Ac3 +50 (° C), the upper limit of the annealing temperature was set to Ac3 +50 (° C) in order to improve the manufacturing cost without improving the properties of the steel sheet. The annealing time at this temperature is equal to the temperature of the steel sheet and austenitephaseIt takes 10 seconds or more to secure the above. However, if it exceeds 30 minutes, the effect is not only saturated but also the cost is increased.
[0052]
  Subsequent primary cooling promotes transformation from the austenite phase to the ferrite phase, concentrating C in the untransformed austenite phase.ResidualAustenitephaseIt is important to achieve stabilization. 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, and the hard phase such as martensite phase becomes large. Therefore, this is the upper limit.
[0053]
  When this primary cooling is performed to a temperature lower than 650 ° C., the pearlite is cooled during the cooling.phaseProduces enough ferritephaseThis is set as the lower limit since does not generate. However, if the cooling is finished at a temperature higher than 700 ° C., the ferrite transformation does not progress sufficiently, so this was made the upper limit.
[0054]
  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.In addition, secondary cooling rate of production number 1, 2, 4, 5, 7, 10-17, 19-26, 28, 30 which is invention steel of Table 4 of an Example must be larger than a primary cooling rate. Based on the above, it is assumed that the cooling rate of the secondary cooling is larger than the cooling rate of the primary cooling, and the lower limit of the cooling rate of the secondary cooling is 2 of production numbers 4, 5, 11 to 13 in Table 4 of Examples. Based on the fact that the next cooling rate was 5 ° C./s, the cooling rate was 5 ° C./s or more.
[0055]
  If the cooling stop temperature of this secondary cooling is lower than the plating bath temperature of −50 ° C., it becomes a serious problem in operation, and if it exceeds the plating bath temperature of +50 (° C.), carbide precipitation occurs in a short time, resulting in retained austenite.phaseAnd martensitephaseThe 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, a part of the austenite phase is transformed into a bainite phase.phaseIt is essential to further increase the carbon concentration inside. 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.
[0056]
If the plating temperature is less than −50 ° C., the bainite transformation hardly occurs, and if the plating temperature exceeds + 50 ° C., carbides are generated and it is difficult to leave a sufficient residual austenite phase.
[0057]
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.
[0058]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples.
[0059]
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.
[0060]
[Table 1]

[0061]
Then, according to the following formula from the component (mass%) of each steel, Ac₁ And Ac_Three The transformation temperature was determined by calculation.
[0062]

These Ac₁ And Ac_Three 10% H in the annealing temperature calculated from the transformation temperature₂ -N₂After 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. The plating was performed by immersing in a galvanizing bath at 460 to 470 ° C. with various changes in the bath composition for 3 seconds.
[0063]
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 condition 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.
[0064]
From Tables 2 and 3, the steel according to the present invention has an appearance score of 5 and is excellent in strength / 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. Moreover, the steel sheet manufactured in the scope of the claims of the present invention has the microstructure described above, and is excellent in appearance, strength and elongation balance.
[0065]
[Table 2]

[0066]
[Table 3]

[0067]
[Table 4]

[0068]
【The invention's effect】
The high-strength hot-dip galvanized steel sheet of the present invention is extremely excellent in plating appearance, has good workability, and is extremely effective for building materials, home appliances, automobile body applications, and the like.

Claims (12)

% By mass
C: 0.0001 to 0.3%
Si: 0.001 to less than 0.1% 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%
Containing, Ri Do the balance Fe and unavoidable impurities, the steel microstructure is a composite structure consisting of the main phase and the second phase, 50 to 97% of ferrite phase or a ferrite phase the main phase at a volume fraction and a bainitic phase, the average grain size of the main phase is less 20 [mu] m, the second phase is martensite phase, made from one or both of retained austenite phase, the volume fraction of the second phase is a total of 3 was 50%, an average particle size of the second phase is equal to or less than 10 [mu] m, an average particle size of the second phase Ru 0.01 to 0.7 Baidea an average particle size of the main phase steel plate On the surface of the
Mn: 0.001 to 3%
Al: 0.001 to 4%
Mo: 0.0001 to 1%
Fe: 5 to 20%
Is a hot dip galvanized steel sheet having a plating layer consisting of Zn and inevitable impurities,
Assuming that the Mn content in steel is X (mass%), the Si content in steel is Y (mass%), and the Al content in plating is Z (mass%), X, Y and Z satisfy the formula (1). A high-strength hot-dip galvanized cold-rolled steel sheet with excellent appearance and workability.
0.6− (X / 18 + Y + Z) ≧ 0 (1) % By mass
C: 0.0001 to 0.3%
Si: 0.001 to less than 0.1% 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%
Containing, Ri Do the balance Fe and unavoidable impurities, the steel microstructure is a composite structure consisting of the main phase and the second phase, 50 to 97% of ferrite phase or a ferrite phase the main phase at a volume fraction and a bainitic phase, the average grain size of the main phase is less 20 [mu] m, the second phase is martensite phase, made from one or both of retained austenite phase, the volume fraction of the second phase is a total of 3 was 50%, an average particle size of the second phase is equal to or less than 10 [mu] m, an average particle size of the second phase Ru 0.01 to 0.7 Baidea an average particle size of the main phase steel plate On the surface of the
Mn: 0.001 to 3%
Al: 0.001 to 4%
Mo: 0.0001 to 1%
Fe: a hot dip galvanized steel sheet having a plating layer containing less than 5%, the balance being Zn and inevitable impurities,
Assuming that the Mn content in steel is X (mass%), the Si content in steel is Y (mass%), and the Al content in plating is Z ( mass%), X, Y and Z satisfy the formula (1). A high-strength hot-dip galvanized cold-rolled steel sheet with excellent appearance and workability.
0.6− (X / 18 + Y + Z) ≧ 0 (1) The plating layer is further mass%,
Si: 0.0001 to 0.1%,
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%,
Ni: 0.001 to 0.5%,
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 high-strength hot-dip galvanized cold-rolled steel sheet having excellent appearance and workability 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 high-strength hot-dip galvanized cold-rolled steel sheet excellent in appearance and workability according to any one of claims 1 to 3, characterized by containing one or more of the following. 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 high-strength hot-dip galvanized cold-rolled steel sheet excellent in appearance and workability as described in item 1. The high-strength molten zinc excellent in appearance and workability according to any one of claims 1 to 5, wherein the steel further contains B: 0.0001 to 0.1% by mass. Plated cold-rolled 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 galvanized cold-rolled steel sheet excellent in appearance and workability. A 70 to 97% of ferrite phase main phase volume fraction, according to any one of claims 1 to 7, the volume fraction of the second phase is characterized by a 3% to 30% High-strength hot-dip galvanized cold-rolled steel sheet with excellent appearance and workability. The second phase is the retained austenite phase , the carbon content in the steel: C (mass%), the Mn content in the steel: Mn (mass%), the volume fraction of the retained austenite phase : Vγ (%), the ferrite phase and The volume ratio of bainite phase : Vα (%) satisfies the formula (2). 9. The high-strength hot-dip galvanized cold-rolled steel sheet having excellent appearance and workability according to any one of claims 1 to 8. .
(Vγ + Vα) / Vγ × C + Mn / 8 ≧ 2,000 (2) A 2-47% bainite phase in a ferrite phase and a volume fraction of 50% to 95% in the main phase volume fraction, wherein the volume fraction of the second phase is characterized by 3% to 30% der Rukoto Item 10. A high-strength hot-dip galvanized cold-rolled steel sheet excellent in appearance and workability according to any one of items 1 to 9 . A method for producing the high-strength hot-dip galvanized cold-rolled steel sheet according to any one of claims 2 to 10 , wherein the casting comprises the steel sheet component according to any one of claims 1 and 4 to 7. After the slab is cast or once cooled, it is heated again to 1180 to 1250 ° C., and after hot rolling is completed at 880 to 1100 ° C., the wound hot-rolled steel sheet is pickled and cold-rolled. × (Ac3-Ac1) + Ac1 (° C.) to Ac3 + 50 (° C.) and after annealing for 10 seconds to 30 minutes, cooling to 650 to 700 ° C. at a cooling rate of 0.1 to 10 ° C./second perform primary cooling to, greater than said primary cooling subsequently, and subjected to secondary cooling to cool down to 5 to 100 ° C. / sec cooling rate in the plating bath temperature -50 ° C. ~ plating bath temperature +50 (° C.) After immersion in the plating bath, including the immersion time After holding 2-200 seconds to a temperature range of the plating bath temperature -50 ° C. ~ plating bath temperature +50 (° C.), the high-strength galvanized cold-rolled steel sheet excellent in workability and appearance, characterized in that cooling to room temperature Manufacturing method. A method for producing the high-strength hot-dip galvanized cold-rolled steel sheet according to any one of claims 1, 3 to 10 , wherein the alloying treatment is performed in a temperature range of 400 to 550 ° C after immersion and holding of the plating bath. The method for producing a high-strength hot-dip galvanized cold-rolled steel sheet having excellent appearance and workability according to claim 11 , wherein the method is performed and cooled to room temperature.