JP4837604B2 - Alloy hot-dip galvanized steel sheet - Google Patents

Description

The present invention relates to a steel plate-out galvannealed plating, more specifically tensile strength relates to high strength galvannealed steel sheet at about 390～690MPa, particularly with yield point is excellent in moldability lowered, excellent The present invention relates to an galvannealed steel sheet having plating adhesion.

  Alloyed hot-dip galvanized steel sheets are widely used in automobiles, home appliances, building materials, etc. because of their excellent paint adhesion, corrosion resistance, and weldability. An alloyed hot-dip galvanized steel sheet forms a Zn-Fe alloy by coating hot-dip zinc on the surface of the steel sheet and immediately holding it at a temperature equal to or higher than the melting point of zinc and diffusing Fe from the steel sheet into the zinc. However, since the alloying speed varies greatly depending on the composition and structure of the steel sheet, its control requires a fairly advanced technique. On the other hand, steel sheets for automobiles that are pressed into complex shapes are required to have very high formability, and in recent years, alloyed hot dip galvanizing has been applied due to an increase in demand for rust prevention performance of automobiles. Increasing cases.

In the alloyed hot-dip galvanized steel sheet, as a result of alloying reaction between the steel sheet and the zinc layer, the plated layer changes to a Zn—Fe intermetallic compound called ζ phase, δ ₁ phase, and Γ phase. While these alloy phases improve paintability, paint adhesion, and weldability, the alloy layer itself has high hardness, and the Γ phase is particularly fragile. It becomes easy to produce what is called a powdering phenomenon which peels and becomes. In addition to impairing the soundness of the plating, powdering causes the peeled-off powder-like plating to accumulate on the press mold and significantly deteriorate the appearance of the pressed product. In recent years, thickened alloyed hot-dip galvanized steel sheets are becoming popular for the purpose of enhancing the anti-corrosion performance of automobile bodies. However, the above-mentioned powdering phenomenon is more likely to occur as the amount of plated coating increases. There is a strong demand for improvement of sex.

  Further, in the automobile field, it is necessary to increase the strength of the plated steel sheet in order to secure the function of protecting the occupant in the event of a collision and simultaneously reduce the weight for the purpose of improving the fuel consumption. In particular, recently, the strength of steel plates applied to automobile outer plates and difficult-to-form members has been increasing, and these steel plates are required to have both excellent workability and high plating adhesion.

  As one method for increasing the strength of a steel sheet without degrading workability, a method is known in which the structure of the steel sheet is a composite structure. In order to make the steel sheet structure a composite structure, it is effective to add elements such as Mn, Cr, and Mo, and there are many inventions of high-strength plated steel sheets having a composite structure prepared by adding such elements. It is disclosed.

  For example, in Patent Document 1, a steel sheet containing C: 0.005 to 0.15%, Mn: 0.3 to 2.0%, Cr: 0.03 to 0.8% is plated and alloyed. A method for producing a high-strength composite structure alloyed hot-dip galvanized steel sheet is disclosed.

  Moreover, in patent document 2, it plated on the steel plate containing C: 0.04-0.15%, Mn: 1.0-2.5%, Cr: 0.1-2.0%, and alloyed A method for producing a high-strength composite structure alloyed hot-dip galvanized steel sheet is disclosed.

  In Patent Document 3, C: 0.02 to 0.06%, Mn: 1.5 to 2.5%, Cr: 0.03 to 0.5%, Mo: 0 to 0.5% A method for producing a high-strength, complex-structure alloyed hot-dip galvanized steel sheet, in which the steel sheet contained is plated and alloyed, is disclosed.

  In general, high strength steel sheets with a high C content are known to have a low alloying rate, and thus it is known that powdering phenomenon is less likely to occur. In steel plates with a large C content, the ferrite structure of the main phase becomes extremely low C when C concentrates in the second phase, so the alloying speed in alloying of hot dip galvanizing becomes very fast and alloying progresses too much. As a result, the Γ phase grows thick and the powdering performance tends to decrease.

  For such a steel plate, it is possible to cope with from the operational aspect such as lowering the plating bath temperature, increasing the Al concentration in the plating bath, lowering the alloying temperature and increasing the heating time. However, since insufficient alloying tends to cause strip-like plating peeling called flaking, stable plating adhesion is not always obtained. Moreover, since these changes in operating conditions are accompanied by a stop of the production line, productivity is lowered and costs are increased.

  In addition, as a method for producing an alloyed hot-dip galvanized steel sheet having excellent powdering resistance, Patent Document 1 and Patent Document 5 describe a technique for prescribing soaking conditions after alloying together with alloying heat treatment conditions and cooling conditions. Has been proposed. Furthermore, as a method for producing an alloyed hot-dip galvanized steel sheet having excellent deep drawability and plating adhesion, for example, in Patent Document 6, in addition to the composition of the steel sheet, hot rolling conditions and cooling conditions, and annealing conditions after cold rolling, A technique for limiting the relational expression between the P and Ti contents of the steel sheet and the effective Al concentration in the plating bath has been proposed.

JP 55-122821 JP-A-6-73497 JP 2001-303184 A Japanese Patent Laid-Open No. 2-310352 Japanese Patent Laid-Open No. 2-310353 Japanese Patent Laid-Open No. 5-331612

  In order to increase the strength while maintaining high workability in a high strength steel sheet having a composite structure, it is effective to concentrate C in the second phase containing martensite and bring the ferrite structure of the main phase closer to pure Fe. is there. On the other hand, bringing the ferrite structure of the main phase closer to pure Fe increases the alloying rate in alloying hot dip galvanizing and lowers the powdering performance. For this reason, the above-mentioned and other high-strength plated steel sheets disclosed so far cannot achieve both excellent workability and high plating adhesion that can be used as automobile outer plates and difficult-to-form members.

  Furthermore, plating methods such as JP-A-2-310352 and JP-A-2-310353 require a soaking process for a long time, which reduces the productivity of the plating line and is not economical. In addition, these plating methods are all technologies for ultra-low carbon IF steel, and even when applied to a high-strength steel sheet having a composite structure in which the cooling condition after annealing affects the steel sheet structure, the intended steel sheet structure is obtained. Therefore, excellent workability and high plating adhesion cannot be achieved at the same time.

  Moreover, in the plating method as disclosed in JP-A-5-331612, the productivity of the plating line is reduced and the cost is increased due to the change or adjustment of the operation condition of the plating line by controlling the Al concentration.

  Furthermore, it is known that the addition of Si or P to steel reduces the alloying rate in alloying of hot dip galvanizing. However, the addition of such elements increases the strength of the ferrite structure and decreases the workability of the high-strength steel sheet having a composite structure, so that excellent workability and high plating adhesion cannot be achieved at the same time.

  Therefore, in view of the above situation, the present invention improves the plating adhesion of a hot-dip galvanized steel sheet with a high-strength composite-structure alloyed steel with a ferrite structure as the main phase, and forms a high-strength composite-structure alloy that can be used as an automobile outer plate. The object is to provide a hot-dip galvanized steel sheet.

  As a result of various studies on means for improving the plating adhesion without reducing the workability of the steel sheet and the productivity of the hot dip galvanizing line, the present inventor has controlled the weight of the steel sheet by controlling the Al weight added to the steel sheet to be plated. The present inventors have found that both workability and plating adhesion can be achieved, leading to the present invention.

  That is, the gist of the present invention is as follows.

(1) In mass%,
% By mass
C: 0.02-0.3%,
Si: 0.1% or less,
Mn: 1.0 to 3.5%
P: 0.02% or less,
S: 0.02% or less,
Al: 0.008 % or less,
N: 0.001 to 0.004 %,
Containing steel sheet balance of Fe and inevitable impurities ing is Al: 0.05 to 0.5 wt%, Fe: 7 to 15 wt%, the galvannealed layer balance consisting of Zn and unavoidable impurities Having a tensile strength of 390 MPa or more, a yield ratio of 0.55 or less, and a relationship between tensile strength F (MPa) and elongation L (%),
L ≧ 57.5−0.0467 × F
Alloyed molten galvanized steel plate, characterized in der Rukoto.

(2) Furthermore, in mass%,
Cr: 0.01 to 1.5%
Co: 0.01 to 1%
Mo: 0.01 to 1.5%,
One or alloyed hot-dip galvanized steel sheet according to (1), characterized by containing two or more.

(3) the plating alloying molten galvanized steel plate d = 1.26, d = 1.222 in the X-ray diffraction intensity Aizeta, X-ray diffraction intensity of the d = 3.13 for IΓ and Si standard plate The alloyed hot dip galvanizing as described in (1) or (2) above , wherein the ratios Iζ / ISi and IΓ / ISi with ISi are Iζ / ISi ≦ 0.004 and IΓ / ISi ≦ 0.004 steel sheet.

( 4 ) The alloyed hot-dip galvanized steel sheet according to any one of ( 1 ) to ( 3 ), wherein a ferrite grain size of the metal structure of the steel sheet is 5 to 40 μm.

( 5 ) The metal structure of the steel sheet contains a martensite structure in which the fraction of the ferrite structure which is the main phase is 70% or more in area ratio, 1% or more and 15% or less in area ratio, and contains the martensite. The alloyed hot-dip galvanized steel sheet according to any one of ( 1 ) to ( 4 ), wherein the sum of the fractions of the two phases is 30% or less in terms of area ratio.

  The present invention relates to a steel sheet to be plated that can produce a high-strength, high-strength composite structure alloyed hot-dip galvanized steel sheet that is high in strength and excellent in both workability and plating adhesion, and a high-strength composite structure alloy that can be used as an outer panel for automobiles It is possible to provide a hot-dip galvanized steel sheet, which greatly contributes to industrial development.

  The present invention is described in detail below. First, the reason why the range of each component is limited in the present invention will be described. In the present invention, “%” means “% by mass” unless otherwise specified.

  C: C is added for the purpose of controlling the fraction of the main phase (the phase with the largest area ratio) and the second phase in order to control the martensite amount within an appropriate range, to ensure strength and to reduce yield strength. It is an element. It will also affect the fine uniformity of the substrate. In order to secure the strength and the area ratio of each second phase, 0.02% or more is required. If it exceeds 0.3%, the weldability is remarkably deteriorated, so this is the upper limit. 0.025 to 0.18% is a more preferable range.

  Si: When Si is added in a large amount, it causes unplating and pattern defects that occur after plating. Further, the addition of Si increases the strength of the ferrite structure and leads to a decrease in workability, so the upper limit is made 0.1%. When particularly excellent workability is required, the Si content is less than 0.05%. Although the lower limit of Si is not limited, since extremely low Si leads to an increase in manufacturing cost, it usually contains 0.001% or more.

  Mn: Mn is an austenite stabilizing element, and is an element effective for producing a transformation product and increasing the mechanical strength of the steel sheet. In the present invention, it is necessary to contain 1.0% or more in order to generate an appropriate amount of martensite and to reduce the yield strength and yield ratio. However, when the Mn content exceeds 3.5%, not only is melting difficult, but workability deteriorates, so the upper limit of Mn is set to 3.5%. In order to further reduce the yield strength and improve the formability, the Mn content is set to 2.5% or less.

  P: P is an element that increases the mechanical strength of the steel sheet at a low cost. However, when the P content exceeds 0.02%, the workability is reduced, so the upper limit of P is 0.02%. Although the minimum of P is not limited, since extremely low is also economically disadvantageous, it usually contains 0.001 mass% or more.

  S: Since S is an element that lowers the hot workability and corrosion resistance of steel, it is preferably as small as possible, and the upper limit content is 0.02%. However, reducing the amount of S is costly, and excessively reducing S tends to cause surface defects such as streaks. Therefore, S is reduced to the required level from hot workability and corrosion resistance. Just do it. Although a minimum is not limited, Usually, 0.001% or more is contained. In order to allow fine sulfides to be present in the steel and control the crystal grain size, it is preferable to contain 0.002% or more of S. Further, when the content of S exceeds 0.012%, the crystal grain size of the steel becomes too fine and the yield strength increases and the formability decreases. Is preferably 0.012% or less.

  Al: Al is generally added as a deoxidizing element for steel. However, in a high-strength composite steel sheet, the content of Al is 0.014 for the purpose of reducing the alloying speed in alloying of hot dip galvanizing as described above. % Or less. In addition, when the Al content is reduced, the alloying rate can be further reduced, and the formation of Al-based precipitates is suppressed to improve ductility. Therefore, in order to further improve workability and plating adhesion, the upper limit of Al amount Is preferably 0.008%. More preferably, it is 0.005% or less, whereby the yield strength is reduced and the occurrence of surface strain is suppressed. Although a minimum is not limited, Usually, 0.0005% or more is contained.

  The reason why the plating adhesion can be improved by reducing the amount of Al added in the steel is not clear, but the following reasons can be considered.

  If the amount of Al added in the steel is reduced, the amount of solute Al in the ferrite structure decreases, and the reactivity of Al and ferrite in the bath in the plating bath is considered to increase. For this reason, the Fe-Al barrier layer produced | generated in a plating bath becomes thick uniformly, and it becomes possible to make the alloying speed | rate in the alloying of hot dip galvanization small. The alloying of hot dip galvanization is known to start diffusion of Fe from the steel sheet into the plating after the disappearance of the Fe-Al barrier layer formed at the plating / steel interface, and the Fe-Al barrier layer is uniform. As the thickness increases, alloying is delayed, and the generation of the Γ phase, which is the main cause of the powdering phenomenon, can be suppressed.

  N: N is an important additive element for increasing mechanical strength and imparting BH property (seizure hardening). If the addition amount of N is less than 0.001%, the effect of dent resistance cannot be sufficiently obtained. On the other hand, if it exceeds 0.008%, the yield ratio increases, workability deteriorates, and non-aging at room temperature. It becomes difficult to ensure the property. Therefore, the range of N content is limited to 0.001 to 0.008%. From the viewpoint of securing higher workability, the preferable upper limit of the N amount is 0.006% or less.

  Furthermore, the steel which this invention makes object can contain the 1 type (s) or 2 or more types of Cr, Co, and Mo for the purpose of the further improvement of workability.

  Cr: Cr is added in order to generate an appropriate amount of martensite and achieve both improvement in tensile strength and reduction in yield strength. Cr is an element indispensable for improving the non-aging property at room temperature. If the Cr content is less than 0.01%, these effects are insufficient. On the other hand, if it exceeds 1.5%, the tensile strength becomes too high and the formability is impaired. Therefore, the Cr content is set in the range of 0.01% to 1.5%. In order to further reduce the yield ratio, addition of 0.3% or more is preferable.

  Co: Co was added in an amount of 0.01% by mass or more in order to provide a good balance between strength and hole expansibility by controlling bainite transformation. On the other hand, although the upper limit of addition is not particularly provided, it is an expensive element, and addition of a large amount impairs economic efficiency.

  Mo: Mo is an element added for the purpose of strengthening and suppressing the formation of carbides and the formation of bainite and bainitic ferrite, and the effect is obtained at 0.01% or more. However, since an increase in cost becomes a problem when it exceeds 1.5%, the upper limit is made 1.5%. In addition, Mo also has the effect of preventing softening in the heat affected zone during welding.

  In the present invention, the reason why the Al composition of the galvannealed layer is limited to 0.05 to 0.5% by mass is that when it is less than 0.05% by mass, Zn-Fe alloying proceeds too much during the alloying treatment. This is because a brittle alloy layer develops too much at the iron interface and plating adhesion deteriorates. If it exceeds 0.5 mass%, an Fe-Al-Zn-based barrier layer is formed too thick and alloying occurs during the alloying process. This is because the desired iron content plating cannot be obtained because it does not progress. Desirably, it is 0.1-0.3 mass%.

  The reason why the Fe composition is limited to 7 to 15% by mass is that if it is less than 7% by mass, a soft Zn—Fe alloy is formed on the plating surface and press formability is deteriorated. This is because a brittle alloy layer develops too much at the iron interface and the plating adhesion deteriorates. Desirably, it is 9-12 mass%.

Next, the alloyed hot-dip galvanized layer will be described. In the present invention, the alloyed hot-dip galvanized layer is a plated layer mainly composed of an Fe—Zn alloy formed by diffusion of Fe in steel during Zn plating by an alloying reaction. This plating layer forms alloy layers called ζ phase, δ ₁ phase, and Γ phase due to the difference in Fe content. Among them, the ζ phase has a high coefficient of friction because it is soft to be plated and easily adheres to the press mold, and tends to cause plate breakage when severe pressing is performed. Further, since the Γ phase is hard and brittle, plating peeling called powdering is liable to occur during processing. Therefore, press workability and plating adhesion can be improved by reducing the ζ phase and the Γ phase as much as possible and making the plating layer a δ ₁ phase. Here, it is known that a hard and brittle phase called Γ ₁ phase is also present in the plating layer. However, since the Γ phase and the Γ ₁ phase cannot be distinguished from the X-ray diffraction intensity, the Γ phase And Γ ₁ phase are combined and treated as Γ phase.

  Specifically, the ratio between the X-ray diffraction intensity Iζ and IΓ of d = 1.26 and d = 1.222 indicating the ζ phase and the Γ phase and the X-ray diffraction intensity ISi of d = 3.13 of the Si standard plate. Let Iζ / ISi and IΓ / ISi be Iζ / ISi ≦ 0.004 and IΓ / ISi ≦ 0.004.

  The reason why Iζ / ISi is limited to 0.004 or less is that when Iζ / ISi is 0.004 or less, the amount of ζ phase is extremely small, and the press workability is not deteriorated.

  Further, the reason why IΓ / ISi is limited to 0.004 or less is that when IΓ / ISi is 0.004 or less, the Γ phase is extremely small, and the plating adhesion is not deteriorated.

  In addition to the above components, the steel sheet of the present invention may contain other alloy elements for the purpose of further improving the corrosion resistance and hot workability of the steel sheet itself, or as an inevitable impurity from secondary materials such as scrap. Even if other alloy elements are contained, it does not depart from the scope of the present invention. Such alloy elements include Ti, Nb, B, Cu, Ni, W, Ca, Y, V, Zr, Ta, Hf, Pb, Sn, Zn, Mg, As, Sb, and Bi.

In addition, the steel sheet of the present invention is made of Pb, Sb, Si, Sn, Mg, Mn, Mo, W, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi during hot dip galvanizing bath or galvanizing. Even if one kind or two or more kinds of rare earth elements are contained or mixed in, the effect of the present invention is not impaired, and depending on the amount, the corrosion resistance may be improved. There are no particular restrictions on the amount of galvannealed coating, but it is preferably 20 g / m ² or more from the viewpoint of corrosion resistance and 150 g / m ² or less from the viewpoint of economy. In particular, the powdering phenomenon is more likely to occur as the plating deposition amount is larger, and is more likely to be a problem when the deposition amount exceeds 30 g / m ² .

  In the present invention, the thickness of the steel sheet does not impose any restrictions on the present invention, and the present invention can be applied as long as it is a commonly used sheet thickness.

  In addition, the steel sheet of the present invention can be applied to a normal hot dip galvanized steel sheet production line to obtain a high-strength composite structure alloyed hot dip galvanized steel sheet having excellent appearance and formability.

The production method is not particularly limited, but in order to achieve both high strength and good workability, an appropriate annealing condition is selected, the tensile strength is 390 MPa or more, the yield ratio is 0.55 or less, and the tensile strength F ( MPa) and elongation L (%) are L ≧ 57.5−0.0467 × F
Is desirable.

  There are no other restrictions on the manufacturing process, and the process may be selected as appropriate in consideration of cost and productivity.

  For example, the slab to be used for hot rolling can be one produced by a continuously cast slab or a thin slab caster, and is not particularly limited. It is also suitable for processes such as continuous casting-direct rolling (CC-DR) in which hot rolling is performed immediately after casting.

  The hot rolling finishing temperature is preferably not less than the Ar3 transformation point from the viewpoint of ensuring the press formability and appearance of the steel sheet. The cooling conditions and coiling temperature after hot rolling are not particularly limited, but the coiling temperature is to avoid large material variations at both ends of the coil and to avoid pickling deterioration due to increased scale thickness. Is not more than 750 ° C., and if bainite or martensite is partially formed, it is easy to cause an ear crack during cold rolling, and in extreme cases, the plate may be broken. Cold rolling may be performed under ordinary conditions, and the rolling rate is set to 50% or more for the purpose of finely dispersing the hard second phase so that the ferrite is easily work-hardened and maximizing workability improvement. On the other hand, it is not realistic to perform cold rolling at a rolling rate exceeding 90% because a large cold rolling load is required.

  In order to ensure a tensile strength of 390 MPa or more and a yield ratio of 0.55 or less using a continuous hot dip galvanizing facility, annealing is performed in the two-phase coexisting temperature range of 750 ° C. or more and 880 ° C. or less. By cooling from the ultimate temperature to 650 ° C. at an average cooling rate of 0.5 to 10 ° C./second, and subsequently cooling from 650 ° C. to the plating bath at an average cooling rate of 3 ° C./second or more, and then performing hot dip galvanizing treatment Forming a hot-dip galvanized layer on the surface of the cold-rolled steel sheet, and then subjecting the steel sheet on which the hot-dip galvanized layer has been formed to an alloyed hot-dip zinc on the surface of the steel sheet. It is desirable to form a plating layer.

  If the annealing temperature is less than 750 ° C., recrystallization is insufficient and the press workability necessary for the steel sheet cannot be achieved. Annealing at a temperature exceeding 880 ° C. is not preferable because the production cost is increased and the deterioration of the equipment is accelerated.

  After annealing, the average of 0.5 to 10 ° C./second up to 650 ° C. is increased by increasing the volume fraction of ferrite to improve workability and at the same time increasing the C concentration of austenite. The purpose is to lower the temperature at which the martensitic transformation starts to be below the plating bath temperature. In order to make the average cooling rate up to 650 ° C. less than 0.5 ° C./second, it is necessary to lengthen the line length of the continuous hot dip galvanizing equipment, resulting in high costs. 5 ° C / second or more.

  In order to make the average cooling rate up to 650 ° C. less than 0.5 ° C./second, it is possible to lower the maximum temperature and to anneal at a temperature at which the volume fraction of austenite is small. If the appropriate temperature range is narrower than the allowable temperature range, and even if the annealing temperature is low, austenite is not formed and the purpose is not achieved.

  On the other hand, if the average cooling rate up to 650 ° C. exceeds 10 ° C./second, the increase in the volume fraction of ferrite is not sufficient, and the increase in the C concentration in austenite is small, so the steel strip is immersed in the plating bath. A part of it is transformed into martensite before it is heated, and then martensite is tempered by heating for plating alloying treatment and precipitates as cementite, making it difficult to achieve both high strength and good workability.

  The reason why the average cooling rate from 650 ° C. to the plating bath is 3 ° C./second or more is to avoid the transformation of austenite to pearlite during the cooling, and the cooling rate is less than 3 ° C./second. Even if it is annealed at a temperature specified in (1) and cooled to 650 ° C., the formation of pearlite cannot be avoided. The upper limit of the average cooling rate is not particularly defined, but it is difficult to cool the steel strip so that the average cooling rate exceeds 50 ° C./second in a dry atmosphere.

  The plating alloying treatment conditions are not particularly defined, but a treatment temperature of 460 to 550 ° C. and a treatment time of 10 to 40 seconds are appropriate in actual operation. Since martensitic transformation occurs during cooling after alloying treatment, cooling after hot dip galvanizing or alloying treatment is performed from the viewpoint of stably securing workability and surface strain resistance by lowering the yield ratio. Is preferably performed at a temperature of at least 200 ° C. at a cooling rate of 5 ° C./s or more. A more preferable cooling rate for obtaining excellent surface strain resistance and strength-ductility balance is 10 ° C./s or more.

  The temper rolling is performed to correct the shape and secure the surface properties, and is preferably performed within a range of elongation of 2% or less. This is because if the elongation exceeds 2%, the amount of BH may decrease.

  The steel sheet of the present invention thus produced has a composite structure composed of ferrite and a hard second phase.

The tensile strength is 390 MPa or more, the yield ratio is 0.55 or less, and the relationship between tensile strength F (MPa) and elongation L (%) is L ≧ 57.5−0.0467 × F
In order to achieve this, it is necessary to use ferrite as the main phase and the ferrite area ratio to be 70% or more in terms of the area ratio with respect to the entire structure. If a higher stretch flangeability is required, a ferrite area ratio of 85% or more is desirable. The term “ferrite” as used herein refers to a so-called polygonal ferrite structure that does not include strain due to processing.

  Martensite is an extremely important structure for obtaining an excellent balance of strength, ductility, and yield ratio, which is a target of the present invention. When the martensite fraction is less than 1% in terms of the area ratio with respect to the entire structure, a sufficient effect of improving ductility cannot be obtained, and the tensile strength is low. However, if the fraction exceeds 15% in terms of area ratio, although the strength is greatly increased, the stretch flangeability is remarkably lowered, which is not desirable. Further, when higher stretch flangeability is required, an area ratio of 10% or less is desirable.

  The hard second phase containing martensite can be bainite, retained austenite, etc. in addition to martensite. Furthermore, bainitic ferrite and acicular ferrite that do not include carbide precipitation are also included in the category of the hard second phase in the present application.

  By setting the area ratio of the hard second phase to a range of 30% or less, all of strength, yield strength, yield ratio, and strength-ductility balance can be made good ranges. Further, when higher stretch flangeability is required, an area ratio of 15% or less is desirable.

  The measurement of the fraction of these structures can be evaluated as an area ratio by observing the cross-sectional structure of the steel sheet with an optical microscope and SEM.

Here, if the ferrite grain size is less than 5 μm, the yield ratio increases and the surface distortion resistance deteriorates. On the other hand, if it exceeds 40 μm, the surface appearance after molding deteriorates. It is not preferable. Therefore, the ferrite particle size is preferably in the range of 5 to 40 μm.
Of course, it is of course possible to apply various types of upper plating, especially electroplating, on the galvannealed steel sheet obtained by using the steel sheet of the present invention in order to improve the paintability and weldability. And does not depart from the present invention. Further, it is of course possible to add various treatments to the alloyed hot-dip galvanized steel sheet obtained by the method of the present invention. For example, chromate treatment, phosphate treatment, and phosphate treatment properties can be achieved. Even if the treatment for improving, the lubricity improving treatment, the weldability improving treatment, the resin coating treatment, etc. are performed, it does not depart from the scope of the present invention, and various types are added depending on the required additional characteristics. Can be applied.

  Hereinafter, the present invention will be described specifically by way of examples.

  Steels having the components shown in Table 1-1 were melted, and then the slab was heated to 1150 ° C. to form a 4 mm hot-rolled steel strip at a finishing temperature of 910 to 930 ° C., and wound at 580 to 680 ° C. After pickling and cold rolling to obtain a 1 mm cold rolled steel strip, an alloyed hot dip galvanized steel sheet was produced using an in-line annealing continuous hot dip galvanizing facility. Table 1 shows chemical component values of the test material, that is, the steel plate before plating.

Annealing in the continuous hot dip galvanizing equipment is carried out at 800 ° C., from the maximum reached temperature to 650 ° C. at an average cooling rate of 5 ° C./second, and subsequently from 650 ° C. to the plating bath at an average cooling rate of 10 ° C./second. It cooled and the hot dip galvanization process was performed. The molten zinc bath was molten zinc containing 0.12% Al, and the basis weight of zinc was adjusted to 50 g / m ² with a gas wiper. Heating for alloying was performed at a temperature of 530 ° C. using induction heating type heating equipment.

  After plating, temper rolling was performed at a rolling reduction of 0.3%, and a JIS No. 5 tensile test piece was collected and subjected to a tensile test. The tensile test results are shown in Table 1-2.

  The ferrite grain size of the steel sheet was determined by analyzing the structure photograph taken with an optical microscope in accordance with JIS G 0551.

  In addition, the area ratio of the steel sheet structure is that the test piece is subjected to Nital corrosion, and the central part of the plate thickness is continuously observed at a magnification of 2000 times by SEM observation of a field of length 100 μm × width 200 μm, and the ferrite phase fraction and martensite phase fraction are determined. It was measured. As the hard second phase fraction, a value obtained by subtracting the ferrite phase fraction from 100 was used.

  Fe% and Al% during plating were obtained by dissolving the plating with hydrochloric acid containing an inhibitor and measuring by ICP.

  The stretch flangeability was evaluated by cutting a 200 mm square test piece and performing a hole expansion test using a flat bottom cylindrical punch (diameter 100 mm) on a hole having a hole diameter of 25 mm. For evaluation of stretch flangeability, after the hole expansion test, measure the hole diameter when a crack occurs in the hole edge, and ((hole diameter when crack occurs in the hole edge−initial hole diameter) ÷ initial hole The hole expansion ratio λ defined by the diameter was obtained. These test results are listed in Table 1-2.

  The plating adhesion was evaluated by performing a square tube drawing test under the following conditions, and measuring the mass of the plating peeled from the mass difference before and after the test.

Square tube drawing test conditions Blank size: 150 x 110 mm
Punch size: 80 × 40mm
Punch shoulder r: 5mm
Dice shoulder r: 5mm
Molding depth: 25mm

Adhesion was evaluated according to the following classification, and x was rejected.
◎: Plating layer peeling amount of 50 mg or less ○: Plating layer peeling amount of more than 50 mg and 150 mg or less
Δ: The amount of peeling of the plating layer exceeds 150 mg and is 300 mg or less
X: The amount of peeling of the plating layer exceeds 300 mg

  The evaluation results are as shown in Table 1-2. Nos. 32, 33, and 34 were inferior in plating adhesion to the products of the present invention because the Al content was outside the scope of the present invention. In No. 35, since the Mn content was outside the range of the present invention, the values of YR and EL were inferior to those of the present invention, and the workability was insufficient.

  The products of the present invention other than these were high-strength composite alloyed hot-dip galvanized steel sheets that have both excellent workability and high plating adhesion and can be used as automobile outer plates.

Steel of the component shown in Table 1-1 was melted, and then the slab was heated to 1150 ° C. to form a hot-rolled steel strip of 4 mm at a finishing temperature of 910 to 930 ° C., and wound at 580 to 680 ° C. After pickling and cold rolling to produce a 1mm cold rolled steel strip, an alloyed hot dip galvanized steel sheet is manufactured using a vertical hot dipping machine capable of simulating the CGL thermal cycle and atmosphere. did. During plating, the annealing atmosphere was 5% hydrogen + 95% nitrogen mixed gas, the annealing temperature was 800 ° C., and the annealing time was 90 seconds. The molten zinc bath was molten zinc containing Al, and the basis weight of zinc was adjusted to 50 g / m ² by gas wiping. The alloying was heated using an induction heating system so that the Fe content in the alloyed hot dip galvanizing was the value shown in Table 2. The Al concentration in the plating bath was adjusted so that the Al content in the alloyed hot dip galvanizing was a value shown in Table 2.

  After plating, temper rolling was performed at a rolling reduction of 0.3%, and a JIS No. 5 tensile test piece was collected and subjected to a tensile test. As a result, all samples were YS ≦ 220 MPa, TS440 to 450 MPa, EL 38 to 39%. It was. Moreover, the ferrite grain size of the steel plate was 8 to 9 μm in any sample.

  The Fe content and Al content of the plating were measured by ICP after dissolving the coating with hydrochloric acid containing an inhibitor.

  X-ray diffraction is the ratio of the X-ray diffraction intensity Iζ and IΓ of d = 1.26 and d = 1.222 indicating the ζ phase and Γ phase and the X-ray diffraction intensity ISi of d = 3.13 of the Si standard plate. Iζ / ISi and IΓ / ISi were measured.

  The obtained plated steel sheet was examined for press formability and plating adhesion.

As for press formability, a bead pull-out test was conducted in order to investigate the sliding property of plating in press working. Test conditions are shown below.
・ Sample drawing width: 30mm
-Mold: Square bead with one side shoulder R1mmR (convex part is 4x4mm) convex, the other side is concave shape with shoulder R1mmR-Pressing load: 800, 1000kg
・ Pullout speed: 200mm / min
-Oil coating: The evaluation of press moldability with rust-preventing oil was evaluated according to the following classification, and x was rejected.
◎: Pulled out with a pressing load of 1000 kgf ○: Pulled out with a pressing load of 900 kgf, but broken with a load of 1000 kgf Δ: Pulled out with a pressing load of 800 kgf, but broken with a load of 900 kgf Broken

The plating adhesion was evaluated by performing a square tube drawing test under the following conditions, and measuring the mass of the plating peeled from the mass difference before and after the test.
Square tube drawing test condition blank size: 150 × 110mm
Punch size: 80 × 40mm
Punch shoulder r: 5mm
Dice shoulder r: 5mm
Molding depth: 25mm

Adhesion was evaluated according to the following classification, and x was rejected.
：: Plating layer peeling amount of 50 mg or less ○: Plating layer peeling amount of more than 50 mg and 150 mg or less Δ: Plating layer peeling amount of more than 150 mg and 300 mg or less ×: Plating layer peeling Amount exceeding 300mg

  The evaluation results are as shown in Table 2. In No. 11, since Fe% during plating is out of the range of the present invention, press formability was rejected. In No. 12, since the Fe% during plating was outside the range of the present invention, the plating adhesion was not acceptable. In No. 13, since Al% during plating was less than 0.05, plating adhesion was rejected.

  The products of the present invention other than these were high-strength composite alloyed hot-dip galvanized steel sheets that have both excellent workability and high plating adhesion and can be used as automobile outer plates.

  Steels having the components shown in Table 3-1 were melted, and then the slab was heated to 1150 ° C. to form a 4 mm hot-rolled steel strip at a finishing temperature of 910 to 930 ° C., and wound at 580 to 680 ° C. After pickling and cold rolling to obtain a 1 mm cold rolled steel strip, an alloyed hot dip galvanized steel sheet was produced using an in-line annealing continuous hot dip galvanizing facility. Table 3 shows chemical component values of the test material, that is, the steel plate before plating.

For annealing in a continuous hot dip galvanizing facility, hot dip galvanizing treatment was performed after various changes were made to the maximum temperature reached and the average cooling rate. The molten zinc bath was molten zinc containing 0.12% Al, and the basis weight of zinc was adjusted to 50 g / m ² with a gas wiper. Heating for alloying was performed at a temperature of 530 ° C. using induction heating type heating equipment.

  After plating, temper rolling was performed at a rolling reduction of 0.3%, and a JIS No. 5 tensile test piece was collected and subjected to a tensile test. The tensile test results are shown in Table 3-2.

  The ferrite grain size of the steel sheet was determined by analyzing the structure photograph taken with an optical microscope in accordance with JIS G 0551.

  In addition, the area ratio of the steel sheet structure is that the test piece is subjected to Nital corrosion, and the central part of the plate thickness is continuously observed at a magnification of 2000 times by SEM observation of a field of length 100 μm × width 200 μm, and the ferrite phase fraction and martensite phase fraction are determined. It was measured. As the hard second phase fraction, a value obtained by subtracting the ferrite phase fraction from 100 was used.

  Fe% and Al% during plating were obtained by dissolving the plating with hydrochloric acid containing an inhibitor and measuring by ICP.

  The stretch flangeability was evaluated by cutting a 200 mm square test piece and performing a hole expansion test using a flat bottom cylindrical punch (diameter 100 mm) on a hole having a hole diameter of 25 mm. For evaluation of stretch flangeability, after the hole expansion test, measure the hole diameter when a crack occurs in the hole edge, and ((hole diameter when crack occurs in the hole edge−initial hole diameter) ÷ initial hole The hole expansion ratio λ defined by the diameter was obtained.

The plating adhesion was evaluated by performing a square tube drawing test under the following conditions, and measuring the mass of the plating peeled from the mass difference before and after the test.
Square tube drawing test condition blank size: 150 × 110mm
Punch size: 80 × 40mm
Punch shoulder r: 5mm
Dice shoulder r: 5mm
Molding depth: 25mm

Adhesion was evaluated according to the following classification, and x was rejected.
◎: Plating layer peeling amount of 50 mg or less ○: Plating layer peeling amount of more than 50 mg and 150 mg or less
Δ: The amount of peeling of the plating layer exceeds 150 mg and is 300 mg or less
X: The amount of peeling of the plating layer exceeds 300 mg

  The evaluation results are as shown in Table 3-2. The product of the present invention was a high-strength composite alloyed hot-dip galvanized steel sheet that has both excellent workability and high plating adhesion and can be used as an automobile outer plate. In particular, those with a martensite fraction of 1 to 10% showed good workability.

Claims (5)

% By mass
C: 0.02-0.3%,
Si: 0.1% or less,
Mn: 1.0 to 3.5%
P: 0.02% or less,
S: 0.02% or less,
Al: 0.008 % or less,
N: 0.001 to 0.004 %,
Containing steel sheet balance of Fe and inevitable impurities ing is Al: 0.05 to 0.5 wt%, Fe: 7 to 15 wt%, the galvannealed layer balance consisting of Zn and unavoidable impurities Having a tensile strength of 390 MPa or more, a yield ratio of 0.55 or less, and a relationship between tensile strength F (MPa) and elongation L (%),
L ≧ 57.5−0.0467 × F
Alloyed molten galvanized steel plate, characterized in der Rukoto. Furthermore, in mass%,
Cr: 0.01 to 1.5%
Co: 0.01 to 1%
Mo: 0.01 to 1.5%,
One or alloyed hot-dip galvanized steel plate according to claim 1, characterized in that it contains two or more. The alloyed hot-dip galvanized plating d = 1.26 for steel plate, d = 1.222 in the X-ray diffraction intensity Aizeta, the X-ray diffraction intensity ISi of d = 3.13 for IΓ and Si standard plate 3. The alloyed hot-dip galvanized steel sheet according to claim 1 , wherein the ratios Iζ / ISi and IΓ / ISi are Iζ / ISi ≦ 0.004 and IΓ / ISi ≦ 0.004. The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 3 , wherein the metal grain structure has a ferrite grain size of 5 to 40 µm. The metal structure of the steel sheet includes a martensite structure in which the fraction of the ferrite structure, which is the main phase, is 70% or more in area ratio, 1% or more and 15% or less in area ratio, and the second phase containing the martensite. The alloyed hot-dip galvanized steel sheet according to any one of claims 1 to 4 , wherein the sum of the fractions is 30% or less in terms of area ratio.