TW201942386A - Alloyed hot-dip galvanized steel sheet having excellent characteristics in extensibility, hole expandability, and anti-fatigue - Google Patents

Abstract

Disclosed is steel sheet having its surface containing hot-dip galvanized layer or an alloyed hot-dip galvanized layer, wherein the steel sheet comprises, by mass%, C: 0.06% to 0.22%, Si: 0.50% to 2.00%, Mn: 1.50% to 2.80%, Al: 0.01% to 1.00%, P: 0.001% to 0.100%, S: 0.0005% to 0.0100%, N: 0.0005% to 0.0100%, and the balance: Fe and impurities, and having a hot-dip galvanized plating layer or an alloyed hot-dip galvanized plating layer on a surface of the steel sheet, wherein: a 1/8 to 3/8 sheet thickness region centered around the 1/4 sheet thickness in the sheet thickness direction from the surface of the steel sheet has a microstructure comprising, by area ratio, ferrite: 15% to 85%, residual austenite: less than 5%, martensite: 15% to 75%, pearlite: 5% or less, and the balance (including 0%): bainite; the number of ferrite agglomerates having a thickness of 20 [mu]m or less in the sheet thickness direction is 50% or more of the total number of ferrite agglomerates; a decarburized layer having a thickness of 10 [mu]m to 150 [mu]m is formed in a surface layer part of the steel sheet; ferrite agglomerates in the decarburized layer have a grain size of 30 [mu]m or smaller; and the ratio of martensite agglomerates having an aspect ratio of 5 or larger among martensite agglomerates is 50% orless.

Description

Alloyed hot-dip galvanized steel sheet

The present invention relates to a high-strength alloyed hot-dip galvanized steel sheet having a strength of 590 MPa (preferably 980 MPa) or more suitable as a press-processed automotive steel sheet, and has excellent elongation, hole expandability, and fatigue characteristics.

In recent years, with the increase in awareness of environmental issues, the automotive industry has become more important in lightening fuel consumption due to rising fuel costs. On the other hand, in order to ensure safety during a collision, the strength of the car body also needs to be increased. In order to achieve both weight reduction and safety of the car body, a high-strength material may be used, but the higher the strength, the more difficult the press forming is. In addition, in order to improve the pressing performance, it is preferable that the characteristics in the width direction of the steel plate be homogeneous.

This is because, generally speaking, as the strength of the steel becomes higher, the drop strength increases, which leads to a decrease in elongation or hole expandability. In addition, the higher the strength, the lower the fatigue ratio, so it is difficult to increase the strength.

The high-strength hot-dip galvanized steel sheet generates toughened iron during slow cooling in a conventional annealing step. For this reason, for example, a steel sheet containing ferritic iron composed mainly of Asada loose iron as disclosed in Patent Document 1 is well known, but a hot-dip galvanized steel sheet having sufficient formability has not been realized.

Patent Document 2 discloses a technique for correcting the low-temperature metamorphic phase size of Vosstian iron to improve the elongation and flangeability, but fails to have both strength and elongation. Regarding the improvement of extensibility, for example, Patent Documents 3 and 4 disclose a steel sheet (hereinafter, a TRIP steel) or the like which induces deformation induced by processing of residual Vostian iron.

However, a large amount of Si is required in the TRIP steel sheet in order to suppress the production of cis-carbon iron. However, when a large amount of Si is added, the hot-dip galvanizing properties of the steel sheet are deteriorated, so the usable steel is limited. In addition, in order to ensure high strength, a large amount of C is required, but when a large amount of C is added, problems such as cracks in gold nuggets will occur.

Regarding the hot-dip galvanizing property of the steel sheet surface, Patent Document 5 discloses a method for reducing Si in TRIP steel. Although it is desired to improve the hot-dip galvanizing property and ductility, the problem of the aforementioned weldability remains.

In a DP steel that can be produced with a smaller amount of C than a TRIP steel, when the amount of Si is large, the ductility will increase as described later. However, similar to the TRIP steel, the problem of plating properties remains. For example, Patent Document 6 discloses a method for controlling the problem of a decrease in plating properties caused by high Si, and a method of controlling the ambient gas during annealing to de-crust the surface layer of the steel sheet. If the surface layer of the steel sheet is de-C, plating can be performed even if Si exceeds 1% by mass, but the problem that the fatigue characteristics are greatly deteriorated due to the softening of the surface layer of the steel sheet. In addition, since the ultra-high-strength grade steel plate having a strength of 980 MPa or higher has a high microstructure strength, it is susceptible to variations in the width direction such as cooling or rolling during manufacture, and has a problem that it is difficult to produce a steel plate having uniform characteristics in the width direction. Prior Art Literature Patent Literature

Patent Literature 1: Patent No. 5305149 Patent Literature 2: Patent No. 4730056 Patent Literature 3: Japanese Patent Laid-Open No. 61-157625 Patent Literature 4: Japanese Patent Laid-Open No. 2007-063604 Patent Literature 5: Japan Patent Publication No. 2000-345288 Patent Document 6: Patent No. 5370104 Patent Document 7: International Publication No. 2013/047755

SUMMARY OF THE INVENTION Problems to be Solved by the Invention In view of the current state of the art, the present invention aims to provide an alloyed hot-dip galvanized steel sheet that can solve the following problems. The problem is an alloy having a strength of 590 MPa or higher (preferably 980 MPa or higher). In the hot-dip galvanized steel sheet, elongation, hole expandability, and fatigue characteristics are improved, and the properties in the width direction of the uniform steel sheet are improved. Means to solve the problem

The present inventors have worked hard to find a solution to the aforementioned problems, and have obtained the following observations.

(w) After cold rolling, after heat treatment in the two-phase domain or single-phase domain, cool down, or keep at a temperature higher than the temperature at which the toughened iron is generated, suppress the deformation of the toughened iron and form a low toughened iron fraction, It is a composite structure of fat iron and loose iron in Asada, which can improve ductility.

(x) By adding Si, the iron content of fertilizer particles can be steadily increased, and ductility is improved, and solid solution strengthening can be expected to increase strength, so it can ensure excellent strength-ductility balance.

(y) Conventional ambient gas control treatment is used to reduce the plating performance caused by the addition of Si. However, as long as the Asada scattered iron in the de-C layer which is generated under the control of the ambient gas and obstructs the fatigue characteristics is changed to the Asada scattered iron with a small aspect ratio. If so, the fatigue characteristics can be improved. If the hot-rolled steel sheet is leveled before and after pickling the hot-rolled steel sheet, the Asada loose iron in the de-C layer can be made into the Asada loose iron with a small aspect ratio.

(z) With the implementation of leveling, in the annealing step after the cold rolling, the heating rate in the required temperature range is controlled to the required range, and the segregation of iron in the fertilizer particles is suppressed, and the iron particles in the fertilizer particles will accumulate. If the ferrous grain iron is homogenized to a harmless form, the hole expandability can be improved with both extensibility and hole expandability, and the characteristics of the plating state and the width direction of the steel plate can be uniformized.

This invention is based on the knowledge obtained by the said observation, The summary is as follows.

(1) An alloyed hot-dip galvanized steel sheet having an alloyed hot-dip galvanized layer on the surface of the steel sheet, and characterized in that the composition of the steel sheet is composed of the following: in mass%, C: 0.06% or more, 0.22 % Or less, Si: 0.50% or more, 2.00% or less, Mn: 1.50% or more, 2.80% or less, Al: 0.01% or more, 1.00% or less, P: 0.001% or more, 0.100% or less, S: 0.0005% or more, 0.0100 % Or less, N: 0.0005% or more, 0.0100% or less, Ti: 0% or more, 0.10% or less, Mo: 0% or more, 0.30% or less, Nb: 0% or more, 0.050% or less, Cr: 0% or more, 1.00 % Or less, B: 0% or more, 0.0050% or less, V: 0% or more, 0.300% or less, Ni: 0% or more, 2.00% or less, Cu: 0% or more, 2.00% or less, W: 0% or more, 2.00 % Or less, Ca: 0% or more, 0.0100% or less, Ce: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, Zr: 0% or more, 0.0100% or less, La: 0% or more, 0.0100 % Or less, REM: 0% or more, 0.0100% or less, Sn: 0% or more, 1.000% or less, Sb: 0% or more, 0.200% or less, the remainder: Fe and impurities: of which, The microstructure in the range of 1/8 plate thickness to 3/8 plate thickness centered on the 1/4 plate thickness from the surface of the steel plate toward the plate thickness direction is composed of the following: in terms of area ratio, ferrous iron: 15% or more , Less than 85%, residual Vostian iron: less than 5%, Asada loose iron: more than 15%, less than 75%, bolai iron: less than 5%, and the rest (including 0%): toughened iron; the aforementioned board In the thick direction, the number of ferrous iron ingots with a thickness of 20 mm or less is more than 50% of the total number of ferrous iron ingots; a de-C layer with a thickness of 10 mm and under 150 mm is formed on the surface layer of the steel plate; Below 30mm, the proportion of Asada scattered iron with an aspect ratio of 5 or more in Asada scattered iron is 50% or less.

(2) The alloyed hot-dip galvanized steel sheet according to (1) above, further comprising a micronized layer having an average thickness of 0.1 mm to 5.0 mm between the alloyed hot-dip galvanized layer and the de-C layer.

(3) The alloyed hot-dip galvanized steel sheet according to (1) or (2) above, wherein in the aforementioned alloyed hot-dip galvanized layer, the difference in Fe concentration in the width direction is less than 1.0% by mass%, and in the width direction The difference in the proportion of Asada scattered iron having an aspect ratio of 5 or more is 10% or less.

(3) The alloyed hot-dip galvanized steel sheet according to any one of (1) to (3), wherein the aforementioned component composition includes one or more of the following: in terms of mass%, Ti: 0.01% or more and 0.10 % Or less, Mo: 0.01% or more, 0.30% or less, Nb: 0.005% or more, 0.050% or less, Cr: 0.01% or more, 1.00% or less, B: 0.0002% or more, 0.0050% or less, V: 0.001% or more, 0.300 % Or less, Ni: 0.01% or more, 2.00% or less, Cu: 0.01% or more, 2.00% or less, W: 0.01% or more, 2.00% or less, Ca: 0.0001% or more, 0.0100% or less, Ce: 0.0001% or more, 0.0100 % Or less, Mg: 0.0001% or more, 0.0100% or less, Zr: 0.0001% or more, 0.0100% or less, La: 0.0001% or more, 0.0100% or less, REM: 0.0001% or more, 0.0100% or less, Sn: 0.001% or more, 1.000 % Or less, Sb: 0.001% or more and 0.200% or less. Invention effect

According to the present invention, it is possible to provide a high-strength alloyed hot-dip galvanized steel sheet which is excellent in extensibility, hole expandability, and fatigue characteristics, and has uniform properties in the width direction of the steel sheet.

Form for Carrying Out the Invention The alloyed hot-dip galvanized steel sheet of this embodiment has an alloyed hot-dip galvanized layer on the surface of the steel sheet, and is characterized in that the composition of the steel sheet is composed of the following: in mass%, C : 0.06% or more, 0.22% or less, Si: 0.50% or more, 2.00% or less, Mn: 1.50% or more, 2.80% or less, Al: 0.01% or more, 1.00% or less, P: 0.001% or more, 0.100% or less, S : 0.0005% or more, 0.0100% or less, N: 0.0005% or more, 0.0100% or less, Ti: 0% or more, 0.10% or less, Mo: 0% or more, 0.30% or less, Nb: 0% or more, 0.050% or less, Cr : 0% or more, 1.00% or less, B: 0% or more, 0.0050% or less, V: 0% or more, 0.300% or less, Ni: 0% or more, 2.00% or less, Cu: 0% or more, 2.00% or less, W : 0% or more, 2.00% or less, Ca: 0% or more, 0.0100% or less, Ce: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, Zr: 0% or more, 0.0100% or less, La : 0% or more, 0.0100% or less, REM: 0% or more, 0.0100% or less, Sn: 0% or more, 1.000% or less, Sb: 0% or more, 0.200% or less, The rest: Fe and impurities; among them, the microstructure in the range of 1 / 8th to 3 / 8th of the thickness centered on the 1 / 4th thickness from the surface of the steel plate to the thickness direction is composed of: Granulated iron: 15% to 85%, Residual Vostian iron: Less than 5%, Asada loose iron: 15% to 75%, Pola iron: 5% or less, and the remainder (including 0%): Toughened iron; the number of ferrous iron ingots in the thickness direction below 20mm is more than 50% of the total number of ferrous iron ingots; a de-C layer with a thickness of 10 mm and under 150 mm is formed on the surface layer of the steel sheet; The grain size of fertilized iron is 30 mm or less, and the proportion of Asa loose iron in Asa loose iron with an aspect ratio of 5 or more is 50% or less.

Hereinafter, the alloyed hot-dip galvanized steel sheet of this embodiment and the manufacturing method of this embodiment will be described in order.

In the current DP (Dual Phase [Dual Phase]) ultra-high-strength steel (DP steel), the material of the steel can be controlled by appropriately adjusting the organizational fraction of the ferrous grain iron or the Mata loose iron. Although increasing ductile iron fraction can improve ductility, if the percentage of ferrous iron increases, the strength will decrease due to (a) increase in soft tissue fraction, and (b) reduction of ductility due to lumpy formation of iron grains. Failed to get the required strength-ductility equilibrium.

The present inventors, in order to improve the elongation, hole expansion, and fatigue characteristics of the alloyed hot-dip galvanized steel sheet according to this embodiment, focus on the characteristics and existence forms of ferrous iron, and the characteristics and existence states of other structures, and repeat Dedicated to research. As a result, it was found that in the DP steel (including the case where a small amount of residual Vostian iron is contained) by controlling the hardening and morphology of the soft ferrous iron, even if the strength is increased, the ductility and hole expandability are not reduced. This will be described below.

In order to reduce the degree of strength reduction (a) described above, when the amount of Si in the steel is set to 0.5%, it can be seen that the strength-ductility-bore expandability is improved in a balanced manner. The reason is not clear. It can be considered as (a1) solid solution strengthening of ferrous iron, which can reduce the loose iron fraction of Asada, reduce the origin of cracks, improve local ductility, or reduce the plasticity by (a2) solid solution strengthening of ferrous iron. Unstable areas, improving uniform extensibility, etc.

However, on the other hand, increasing Si will increase the scale of the surface layer, resulting in a decrease in plating properties and making it difficult to manufacture plated steel sheets. Therefore, a method of controlling the ambient gas and forming an oxide inside the steel sheet without using the surface layer of the steel sheet to ensure good plating properties is used. However, this method forms oxides inside the steel sheet and removes C from the surface layer of the steel sheet, so that the surface layer of the steel sheet is softened, and fatigue cracks are easily propagated, and the fatigue ratio is greatly reduced.

The present inventors endeavored to review the method for solving the problems of the present invention. After the hot rolling was completed, it was found that the leveled steel sheet or the steel sheet having the surface layer of the polished steel sheet had good fatigue characteristics. In addition, it was found that performing the bending process on the steel sheet in the annealing step would further improve the fatigue characteristics.

The reason is not clear, but it can be considered that the aspect ratio of Asada loose iron existing in the de-C layer formed on the surface of the steel sheet becomes smaller or the surface structure becomes finer, which is less likely to cause fatigue crack propagation.

As described above, the strength-ductility balance can be improved by adding Si, but even if the iron fraction of the fertilizer particles increases, the degree of improvement in the strength-ductility balance or the strength-hole expansion balance is still small.

Generally speaking, the ferrous grain iron in the soft phase has deformation in the DP steel, and the amount of deformation in the low strain region is large. However, the ferritic iron that exists near the loose iron in Asada is restrained by the loose iron in Asada when deformed, and the amount of deformation is small.

The present inventors paid attention to this phenomenon, and reviewed the optimal conditions for restraining the iron grains from deforming. In addition, by maintaining the ferritic iron fraction that can improve the elongation, and appropriately restricting the deformed state of the adhering hard phase (Matian loose iron), the elongation and hole expandability can be achieved.

So far, the mainstream of microstructure control has been to review the relationship between crystal grain boundaries and characteristics. In the case of single-phase steel, it can be seen that the influence of grain boundaries with different characteristics is large. The present inventor is reviewing the situation of improving the hole expandability in a composite structure where the characteristics of ferrous grain iron and Asada loose iron are greatly different. The crystalline particle size of each tissue is not significant, but the existence of the same phase is very helpful for the characteristics.

In addition, based on the foregoing thoughts, the present inventors have confirmed that an aggregate of fertile iron nuggets (a large number of ferrous iron iron cores surrounded by a hard phase) is adjacent to the hard iron phases (toughened iron and Asada loose iron). , Hereinafter referred to as "fertilized iron nuggets".) The importance of evaluation and found the most appropriate conditions for restraint of ferrous iron deformation.

The principle of material improvement under the aforementioned appropriate conditions has not yet been clarified, but the inventors believe that when the thickness of the ferrous iron in the plate thickness direction is thin, the deformation of the ferrous iron is more restricted due to the deformation constraint of the hard phase, and the pseudoferrous iron is hardened. It intentionally acts to maintain the strength, and suppresses the occurrence of cracks caused by local huge deformation, and effectively acts to improve the hole expandability.

The thickness of the fat iron nuggets is the maximum thickness of the iron iron nuggets surrounded by the hard phase.

In addition, the extensibility is related to the deformability of the ferrous iron, which is linearly related to the ferrous iron fraction. Therefore, by controlling the ferrous iron fraction and the existing form of the ferrous iron, it can have both extensibility and pore expandability. .

The alloyed hot-dip galvanized steel sheet according to this embodiment was created by the inventors who have discovered the knowledge obtained through the above observations. The features and characteristics of the alloyed hot-dip galvanized steel sheet according to this embodiment will be described below.

First, the reasons for limiting the component composition will be explained. Hereinafter,% of the component composition means mass%.

Composition C: 0.06% or more and 0.22% or less C is an element that increases the hardness of Asada's loose iron and helps to increase strength. When C is less than 0.06%, since the effect of addition cannot be obtained sufficiently, C is made 0.06% or more. Above 0.07% is preferred. On the other hand, if C exceeds 0.22%, it will promote the production of citronite, and the hole expandability or weldability will decrease. Therefore, C is set to 0.22% or less. It is preferable that C is 0.17% or less.

Si: 0.50% or more and 2.00% or less Si is a solid solution strengthening element that does not reduce ductility and contributes to improving strength and fatigue strength. When Si is less than 0.50%, since the effect of addition is not sufficiently obtained, Si is set to 0.50% or more. It is preferably 0.80% or more, and more preferably 1.00% or more. On the other hand, if Si exceeds 2.00%, the ductility and spot weldability are reduced, so Si is set to 2.00% or less. Si is preferably 1.80% or less, and more preferably 1.60% or less.

Mn: 1.50% or more and 2.80% or less Mn is an element that enhances solid solution strengthening and hardenability and contributes to strength improvement. When Mn is less than 1.50%, since the effect of addition is not sufficiently obtained, Mn is set to 1.50% or more. Above 1.80% is preferred. On the other hand, if Mn exceeds 2.80%, the weldability decreases and the production of ferrous iron is suppressed, the ductility decreases, and the segregation expansion and hole expansion properties also decrease. Therefore, Mn is set to 2.80% or less. It is preferable that Mn is 2.50% or less.

Al: 0.01% or more and 1.00% or less Al is an element required for deoxidation, and it can suppress the formation of harmful carbides, which is helpful for improving the elongation and hole expandability. In particular, it is an element that does not reduce ductility in a low Si-based component system, but contributes to improving the chemical conversion processability.

When Al is less than 0.01%, the effect of addition is not sufficiently obtained, so Al is set to 0.01% or more. On the other hand, if Al exceeds 1.00%, the effect of addition is saturated and the chemical conversion processability and spot weldability are reduced. Therefore, Al is set to 1.00% or less. From the perspective of improving the chemical conversion treatment, it is preferably 0.80% or less.

P: 0.001% or more and 0.100% or less P is an element that contributes to the improvement of strength and an element that improves the corrosion resistance when coexisted with Cu. When P is less than 0.001%, the effect of addition is not sufficiently obtained, and the cost of steelmaking increases significantly. Therefore, P is set to 0.001% or more. From the point of view of steel making cost, P is preferably 0.010% or more. On the other hand, if P exceeds 0.100%, the weldability or processability is reduced, so P is set to 0.100% or less. When the corrosion resistance is not a problem and the workability is important, P is preferably 0.050% or less.

S: 0.0005% or more and 0.0100% or less S is an element that forms sulfides (MnS, etc.) that become the origin of cracks, and hinders hole expansion and full elongation. S is preferably small, but if S is reduced to less than 0.0005%, the cost of steelmaking will increase significantly, so S is set to 0.0005% or more. On the other hand, if S exceeds 0.0100%, the hole expandability and full elongation are significantly reduced, so S is set to 0.0100% or less. S is preferably 0.0060% or less.

N: 0.0005% or more and 0.0100% or less N is an element that hinders workability. When N coexists with Ti and / or Nb, a nitride (TiN and / or NbN) that inhibits elongation and hole expansion is formed, and is an element that reduces the effective amount of Ti and / or Nb.

It is preferable that N is small. If N is reduced to less than 0.0005%, the steelmaking cost will increase significantly, so N is set to 0.0005% or more. On the other hand, when N exceeds 0.0100%, workability, elongation, and hole expandability are significantly reduced, so N is set to 0.0100% or less. N is preferably 0.0060% or less.

For the purpose of improving characteristics, the component composition of the plated steel sheet of the present invention may also suitably include Ti: 0.01% or more, 0.10% or less, Mo: 0.01% or more, 0.30% or less, Nb: 0.005% or more, 0.050% or less, Cr : 0.01% or more, 1.00% or less, B: 0.0002% or more, 0.0050% or less, V: 0.001% or more, 0.300% or less, Ni: 0.01% or more, 2.00% or less, Cu: 0.01% or more, 2.00% or less, W : 0.01% or more, 2.00% or less, Ca: 0.0001% or more, 0.0100% or less, Ce: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less, Zr: 0.0001% or more, 0.0100% or less, La : 0.0001% or more, 0.0100% or less, REM: 0.0001% or more, 0.0100% or less, Sn: 0.001% or more, 1.000% or less, Sb: 0.001% or more, and 0.200% or less.

Ti: 0.01% or more and 0.10% or less Ti is an element that retards recrystallization and contributes to the formation of unrecrystallized fertilizer iron, and also forms carbides and / or nitrides, which helps to improve strength.

When Ti is less than 0.01%, since the effect of containing is not sufficiently obtained, Ti is preferably 0.01% or more. On the other hand, if it exceeds 0.10%, the formability will be lowered. Therefore, Ti is set to 0.10% or less. The Ti content is preferably 0.05% or less.

Mo: 0.01% or more and 0.30% or less Mo is an element that retards recrystallization, contributes to the formation of unrecrystallized fertilizer iron, improves the hardenability, and controls the iron fraction of Asada. In addition, Mo segregates at the grain boundaries, suppresses zinc from entering the structure of the welded part during welding, helps prevent cracks during welding, and also helps to suppress the generation of elements during cooling during the annealing step.

When Mo is less than 0.01%, since the effect of containing is not sufficiently obtained, Mo is preferably 0.01% or more. Mo is preferably 0.04% or more. On the other hand, if Mo exceeds 0.30%, moldability will be deteriorated. Therefore, Mo is set to 0.30% or less. Mo is preferably 0.25% or less.

Nb: 0.005% or more and 0.050% or less Nb is an element that retards recrystallization, helps to form unrecrystallized fertilizer iron, and forms carbides and / or nitrides, which contributes to strength improvement. When Nb is less than 0.005%, since the effect of containing is not sufficiently obtained, it is preferable to set Nb to 0.005% or more. Preferably, Nb is 0.010% or more. On the other hand, if Nb exceeds 0.050%, moldability will be reduced, so Nb is set to 0.050% or less. Nb is preferably 0.030% or less.

Cr: 0.01% or more and 1.00% or less Cr is an element that retards recrystallization, helps to form unrecrystallized fertilizer iron, and helps to suppress the generation of boron iron during cooling in the annealing step. When the Cr content is less than 0.01%, the content effect cannot be sufficiently obtained. Therefore, the Cr content is preferably 0.01% or more. Preferably, Cr is 0.05% or more. On the other hand, if Cr exceeds 1.00%, the formability will decrease, so Cr is set to 1.00% or less. Cr is preferably 0.50% or less.

B: 0.0002% or more and 0.0050% or less. B is an element that retards recrystallization and contributes to the formation of unrecrystallized fertilizer iron. It can also improve the hardenability and control the iron fraction of Asada. In addition, B segregates at the grain boundaries, suppresses zinc from entering the structure of the welded part during welding, helps to prevent cracks during welding, and also helps to suppress the generation of elemental iron during cooling during the annealing step.

When B is less than 0.0002%, the content effect is not sufficiently obtained. Therefore, B is preferably 0.0002% or more. Preferably, B is 0.0010% or more. On the other hand, if B exceeds 0.0050%, moldability will be reduced, so B is set to 0.0050% or less. It is preferable that B is 0.0025% or less.

V: 0.001% or more and 0.300% or less V is an element that contributes to strength enhancement by strengthening with precipitates, fine grain strengthening that inhibits the growth of ferrous iron crystal grains, and translocation strengthening that suppresses recrystallization. When V is less than 0.001%, the strength-enhancing effect is not sufficiently obtained, so V is preferably 0.001% or more. Preferably, V is 0.010% or more. On the other hand, if V exceeds 0.300%, carbonitrides will be excessively precipitated, resulting in a decrease in formability. Therefore, V is set to 0.300% or less. V is preferably 0.150% or less.

Ni: 0.01% or more and 2.00% or less Ni is an element that suppresses phase transformation at high temperatures and contributes to strength improvement. When Ni is less than 0.01%, since the effect of containing is not sufficiently obtained, Ni is preferably 0.01% or more. Preferably, Ni is 0.10% or more. On the other hand, if Ni exceeds 2.00%, the weldability will be reduced. Therefore, Ni is set to 2.00% or less. Ni is preferably 1.20% or less.

Cu: 0.01% or more and 2.00% or less Cu is present as fine particles and is an element that contributes to strength improvement. When Cu is less than 0.01%, since the effect of containing is not sufficiently obtained, Cu is preferably 0.01% or more. Preferably, Cu is 0.10% or more. On the other hand, if Cu exceeds 2.00%, the weldability will decrease, so Cu is set to 2.00% or less. Cu is preferably 1.20% or less.

W: 0.01% or more and 2.00% or less W is an element that suppresses phase transformation at high temperatures and contributes to strength improvement. When W is less than 0.01%, the content effect is not sufficiently obtained. Therefore, W is preferably 0.01% or more. Preferably, W is 0.10% or more. On the other hand, if W exceeds 2.00%, productivity decreases due to a decrease in hot workability. Therefore, W is set to 2.00% or less. It is preferable that W is 1.20% or less.

Ca: 0.0001% or more, 0.0100% or less Ce: 0.0001% or more, 0.0100% or less Mg: 0.0001% or more, 0.0100% or less Zr: 0.0001% or more, 0.0100% or less La: 0.0001% or more, 0.0100% or less REM: 0.0001% or more , 0.0100% or less Ca, Ce, Mg, Zr, La, and REM are elements that help improve formability. When the contents of Ca, Ce, Mg, Zr, La, and REM are less than 0.0001%, respectively, the effect is not sufficiently obtained. Therefore, it is preferable that any element is 0.0001% or more. Preferably, any element is 0.0010% or more.

On the other hand, when Ca, Ce, Mg, Zr, La, and REM each exceed 0.0100%, there is a concern that the ductility is reduced, and therefore, any element is set to 0.0100% or less. It is preferable that any element is 0.0070% or less.

In addition, REM is an abbreviation of Rare Earth Metal, which refers to an element belonging to the lanthanum series. REM or Ce is mostly contained in the form of a rare earth metal alloy, but in addition to La or Ce, it may still contain a lanthanum series element in a complex manner. In terms of impurities, even if a lanthanum series element other than La or Ce is contained, the characteristics are not hindered. It may also contain metal La or Ce.

Sn: 0.001% or more and 1.000% or less Sn is an element that suppresses the coarsening of the tissue and contributes to the improvement of strength. When the content of Sn is 0.001% or more, the content effect cannot be sufficiently obtained. Therefore, the content of Sn is preferably 0.001% or more. Preferably, Sn is 0.010% or more. On the other hand, if Sn exceeds 1.000%, the steel sheet will be excessively brittle, which may cause the steel sheet to break during rolling delay. Therefore, Sn is set to 1.000% or less. Sn is preferably 0.500% or less.

Sb: 0.001% or more and 0.200% or less Sb is an element that suppresses the coarsening of the tissue and contributes to the improvement of strength. When Sb is less than 0.001%, since the effect of containing is not sufficiently obtained, Sb is preferably 0.001% or more. Preferably, Sb is 0.005% or more. On the other hand, when Sb exceeds 0.200%, the steel sheet will be excessively brittle, which may cause the steel sheet to break during rolling delay. Therefore, Sb is set to 0.200% or less. It is preferable that Sb is 0.100% or less.

In the component composition of the alloyed hot-dip galvanized steel sheet according to this embodiment, the remainder from which the foregoing elements are removed is Fe and impurities. The impurities are elements unavoidably mixed in from the steel raw material and / or the steel making process, and are elements that are allowed to exist within a range that does not hinder the characteristics of the alloyed hot-dip galvanized steel sheet according to this embodiment.

For example, Ti, Mo, Nb, Cr, B, V, Ni, Cu, W, Ca, Ce, Mg, Zr, La, REM, Sn, and Sb should be less than the components of the alloyed hot-dip galvanized steel sheet according to this embodiment. Any trace amount of the composition lower limit may be an unavoidable impurity.

In addition, impurities such as H, Na, Cl, Sc, Co, Zn, Ga, Ge, As, Se, Y, Tc, Ru, Rh, Pd, Ag, Cd, In, Te, Cs, Ta, Re, Os, Ir, Pt, Au, and Pb are in a range of 0.010% or less in total.

Next, the microstructure of the alloyed hot-dip galvanized steel sheet according to this embodiment will be described.

In the alloyed hot-dip galvanized steel sheet according to this embodiment, by controlling the ratio and shape of ferrous iron and loose iron in Mata, and controlling the surface structure, it is possible to obtain a high balance of strength, ductility, hole expansion, and fatigue characteristics.

Generally speaking, when the iron content of fertilizer grains is increased, the ductility is improved, but because the ferrous grain iron is soft, the strength and pore expandability are reduced. In this embodiment, the deformation of the soft phase is restricted by the hard phase, so the characteristics and functions of the ferrous iron can be effectively utilized.

Limited range of microstructure: 1/8 sheet thickness ~ 3/8 sheet thickness centered on 1/4 sheet thickness from the surface of the steel plate to the thickness direction The microstructure in the range of 1/8 sheet thickness to 3/8 sheet thickness mainly has the mechanical characteristics of the entire steel plate. Therefore, in the present embodiment, the range of the plate thickness direction in which the microstructure fraction is prescribed is set to “1/8 plate thickness to 3/8 plate thickness centered on 1/4 plate thickness”. In addition, the percentage of the tissue fraction is the area ratio.

Fertile iron: 15% or more and 85% or less When the ferrous iron is less than 15%, it is difficult to ensure the required elongation, so the ferrous iron is set to 15% or more. Fertilizer iron is more than 20%. On the other hand, if the amount of ferrous iron exceeds 85%, it is difficult to ensure the required strength. Therefore, the amount of ferrous iron is 85% or less. Fertilizer iron is preferably 75% or less.

Pole iron: 5% or less Pole iron is more than 5%, which will reduce the ductility and hole expandability, so it is set to be less than 5%. The lower limit contains 0%.

Residual Vastfield Iron: Less than 5% From the perspective of ensuring elongation, it is effective to use residual Vastfield iron supplementarily. However, the residual Vastfield iron will be the cause of hydrogen cracking due to different conditions of use. Staine is set to less than 5%. The lower limit contains 0%.

Asada scattered iron: 15% or more and 75% or less As it is difficult to ensure the required strength, the Asada scattered iron is set to 15% or more. More than 20% of Asada scattered iron is preferred. On the other hand, if the amount of loose iron in Mata exceeds 75%, it is difficult to ensure the required elongation, so the amount of loose iron in Asada is set to 75% or less. It is preferable that the amount of loose iron in Asada is 65% or less.

Toughened iron: The remaining part Toughened iron is used as the organization to adjust the fraction of loose iron in Asada. It can be generated as the remaining part of the tissue, and it can also be 0%. In order to ensure the fertilized iron and Asada loose iron at each lower limit fraction, the upper limit of the remaining portion is set to 70%.

Here, the calculation method of an area ratio is demonstrated.

A plate thickness cross section parallel to the rolling direction is used as an observation surface, and a sample is taken, the observation surface is polished, and the nitrate is etched, and then observed with an optical microscope or a scanning electron microscope (scanning electron microscopy: SEM). Use the image after shooting or the image analysis software in the machine to calculate the area ratio. The area ratio is defined as one field of view of 200 mm in length and 200 mm in width, and each image analysis is performed on 10 different fields of view, and the average value is calculated after calculating the area ratio of each tissue. The average value is used as the area ratio. .

Using the aforementioned image, the thickness of the fertile iron nuggets was measured. In the aforementioned field of view, the longest thickness in the plate thickness direction of the fertile iron ingot was taken as the thickness of the ferrous iron ingot.

The aspect ratio of the Asada loose iron in the de-C layer described later can be calculated using the aforementioned image. The long and short parts of the thickness of the Asada loose iron were measured, and the thickness of the long part was divided by the thickness of the short part, and this was used as the aspect ratio. In addition, the number ratio of the Asada scattered iron having an aspect ratio of 5 or more was calculated from the Asada scattered iron in the field of view having an area of 200 mm in length and 200 mm or more.

Furthermore, when it is not easy to use Nitrous etchant for discrimination of Asada scattered iron, LePera etching can also be used.

Next, a method for measuring the residual Vosted iron will be described.

The area ratio of the residual Vostian iron can be measured by an electron backscatter diffraction (EBSD) method or an X-ray diffraction method. When measuring by the X-ray diffraction method, the diffraction intensity (a (111)) of the (111) plane of the ferrous grain iron and the diffraction intensity ((200) plane of the residual Vostian iron) were measured using a Mo-Ka line. g (200)), the diffraction intensity (a (211)) of the (211) plane of the ferrous grain iron, and the diffraction intensity (g (311)) of the (311) plane of the residual Vostian iron, and can be used The area ratio (f _A ) of the residual Vosstian iron was calculated by the following formula. f _A = (2/3) {100 / (0.7 × a (111) / g (200) +1)} + (1/3) {100 / (0.78 × a (211) / g (311) +1 )}

Thickness of fertile iron ingot in plate thickness direction: 20mm or less The aforementioned number of ferrous iron ingots with a thickness of 20mm or less: 50% or more of the total number of ferrous iron ingots. From the perspective of porosity, the thickness and quantity of fertile iron nuggets are important.

If the thickness of the fertile iron in the plate thickness direction exceeds 20mm, the adjacent hard phases (Matian loose iron, toughened iron) will not sufficiently restrain the ferrous iron, and excessive deformation will occur in the center of the ferrous iron. It is easy to reach the deformation limit, local deformation occurs in the steel sheet, and the improvement effect of the hole expandability cannot be obtained. Therefore, the thickness of the ferrous iron plate in the thickness direction is set to 20 mm or less. It is preferably below 16mm.

If the number of ferrous iron nuggets with a thickness of 20 mm or less is less than 50% of the total number of ferrous iron nuggets, it is difficult to obtain the aforementioned effect of improving the hole expandability with an excellent difference, so the thickness in the thickness direction is 20 mm. The number of fat iron nuggets below is 50% or more of the total number of fat iron nuggets. Above 70% is preferred.

Thickness of the de-C layer on the surface layer of the steel plate: 10 mm to 150 mm The de-C layer is formed because the C of the steel surface layer reacts with oxygen in the ambient gas to become CO or CO ₂ and dissipates into the ambient gas. In the surface layer portion after reducing C, since hard structure is not easily obtained, it becomes a softer structure than that inside the steel plate.

The thickness of the de-C layer was set as follows.

In the plate thickness direction, the hardness in the range of 1/8 plate thickness to 3/8 plate thickness is measured with 1/4 plate thickness as the center, and the average value is used as the reference hardness of the steel plate hardness. The hardness is measured from the thickness of 1/8 of the steel plate toward the surface of the steel plate, the point where the reference hardness is 0.9 or less, and the distance from the point below 0.9 to the surface of the steel plate is the thickness of the de-C layer.

In the alloyed hot-dip galvanized steel sheet according to this embodiment, in order to ensure the required hole expandability and fatigue characteristics, it is important that the thickness of the surface layer portion of the steel sheet: 10 mm or more and 150 mm or less is the presence of a de-C layer. The formation of the de-C layer will be described later.

When the thickness of the C-removed layer is less than 10 mm, the plating properties and hole expandability are reduced, so the thickness of the C-removed layer is set to 10 mm or more. It is preferably at least 20 mm, and more preferably at least 30 mm. On the other hand, when the thickness of the de-C layer exceeds 150 mm, even if the morphology of Asada loose iron in the de-C layer is controlled, the fatigue characteristics have not been improved, and the fatigue characteristics and strength have decreased. Therefore, the thickness of the C-layer is set to 150 mm or less. . It is preferably 120 mm or less, and more preferably 100 mm or less.

From the viewpoint of ensuring the required fatigue characteristics in the aforementioned C-removed layer, the particle size of the ferrous iron is set to 30 mm or less, and the proportion of the Asada scattered iron having an aspect ratio of 5 or more in the Asada scattered iron is set to 50% or less. Explained below.

Particle size of the ferrous iron in the de-C layer: 30 mm or less When the particle size of the ferrous iron in the de-C layer exceeds 30 mm, the fatigue characteristics are reduced, so the particle size of the ferrous iron is 30 mm or less. The reason for the decline in fatigue characteristics is not clear, but it is thought that fatigue cracks are likely to spread because the fraction of loose iron in Asada, which is adjacent when the iron particle size of the fertilizer grains is large, decreases. The smaller the iron particle size, the better, preferably 25 mm or less, and more preferably 20 mm or less. Here, the ferrite grain iron particle size indicates an average particle size. For example, in an area with an observation area of 40,000 mm ² or more, a line segment parallel to the rolling direction is drawn, and the average value of the total length of the line segment divided by the number of intersections between the line segment and the grain boundary is taken as the grain size of the fertile grains.

The proportion of Asada loose iron with an aspect ratio of 5 or more: 50% or less The ratio of Asada loose iron with an aspect ratio of 5 or more is easy to cause fatigue cracks and spread along the Asada loose iron. Therefore, the alloyed molten zinc-plated steel sheet in this embodiment has a reduced ratio. The aspect ratio of Asada loose iron in the de-C layer is 5 or more, in order to further improve the fatigue characteristics.

If the ratio of the aspect ratio of Asada loose iron in the de-C layer is 5 or more than 50%, the fatigue characteristics will be significantly reduced, so the above ratio is set to 50% or less. 40% or less is preferable, and 30% or less is more preferable. In addition, in order to more uniform the characteristics in the width direction, it is preferable that the difference in the proportion of Asa loose iron in the width direction with an aspect ratio of 5 or more is 10% or less. It is preferably 6% or less.

Next, a micronized layer existing between the de-C layer and the alloyed hot-dip galvanized layer will be described. The micronized layer is a layer formed by performing an oxidation or decarburization reaction under conditions controlled under a specific ambient gas during annealing as described later. Therefore, the structure constituting the micronized layer is mainly composed of a ferrous iron phase after removing oxides or inclusion particles. The boundary between the micronized layer and the de-C layer is defined as a boundary where the average particle size of the ferrous iron in the micronized layer is smaller than 1/2 of the average particle size of the de-C layer fertilizer iron.

The average thickness of the micronized layer is preferably 0.1 mm to 5.0 mm. When the average thickness of the micronized layer is less than 0.1 mm, the effect of suppressing the occurrence of cracks and stretching cannot be obtained, and the effect of improving the plating adhesion cannot be obtained. If it exceeds 5.0 mm, alloying of the plating layer (generation of Zn-Fe alloy) proceeds, and the Fe content in the alloyed hot-dip galvanized layer increases, and the plating adhesion decreases. The average thickness of the preferred micronized layer is 0.2 mm to 4.0 mm, and more preferably 0.3 mm to 3.0 mm.

The average thickness of the micronized layer was measured by the method shown below. From the alloyed hot-dip galvanized steel sheet, a cross-section parallel to the rolling direction of the base material steel sheet was taken as a sample for observation. The observation surface of the sample was processed using a CP (Cross section polisher) device, and a reflected electron image was observed and measured at 5000 times using a FE-SEM (Field Emission Scanning Electron Microscopy).

The refinement layer contains one or more oxides of Si and Mn. The oxide may be exemplified by one or more selected from the group consisting of SiO ₂ , Mn ₂ SiO ₄ , MnSiO ₃ , Fe ₂ SiO ₄ , FeSiO ₃ , and MnO. This oxide is formed in the base material steel plate in a specific temperature range during annealing as described later. The oxide particles suppress the growth of the ferrous grain iron phase crystals on the surface layer of the base material steel sheet to form a finer layer.

Next, an alloyed hot-dip galvanized layer will be described.

The alloyed hot-dip galvanized layer is a plated layer obtained by alloying a hot-dip galvanized layer (also including a hot-dip zinc-plated layer formed by hot-dip plating) formed under ordinary plating conditions.

The plating adhesion amount of the alloyed hot-dip galvanized layer is not particularly limited to a specific amount, but from the viewpoint of ensuring the required corrosion resistance, the adhesion amount on one side is preferably 5 g / m ² or more.

In order to reduce unevenness in appearance, the difference in Fe concentration in the width direction of the alloyed hot-dip galvanized layer is preferably set to less than 1.0% in terms of mass%. It is preferably 0.7 or less.

In the alloyed hot-dip galvanized steel sheet of this embodiment, for the purpose of improving paintability or weldability, an upper-layer plating (for example, Ni plating) may be performed on the alloyed hot-dip galvanized layer. In addition, for the purpose of improving the surface properties of the alloyed hot-dip galvanized layer, various treatments such as chromic acid treatment, phosphate treatment, lubricity improvement treatment, and weldability improvement treatment can be performed.

The tensile strength of the alloyed hot-dip galvanized steel sheet in this embodiment is preferably 590 MPa or more. High-strength steel plates with a tensile strength of 590 MPa or more are suitable as material steel plates for automotive components.

The thickness of the alloyed hot-dip galvanized steel sheet according to this embodiment is not limited to a specific thickness range, but is preferably 0.1 to 11.0 mm. A high-strength thin steel plate having a thickness of 0.1 to 11.0 mm is suitable as a material steel plate for automotive components manufactured by press processing. In addition, the high-strength thin steel plate having the aforementioned plate thickness can be easily manufactured on a thin plate manufacturing line.

Next, a manufacturing method of this embodiment will be described.

The manufacturing method for manufacturing the alloyed hot-dip galvanized steel sheet according to this embodiment For example, (a) The cast flat steel slab of the composition of the alloyed hot-dip galvanized steel sheet according to this embodiment is heated to 1100 ° C or higher to be hot-rolled, and Ar3 points The above completion temperature ends hot rolling and coils the hot rolled steel sheet after the hot rolling is finished in a temperature range below 680 ° C; (b) before and / or after the pickled hot rolled steel sheet, the hot rolled steel sheet is executed. After leveling, cold rolling is carried out to obtain a cold rolled steel sheet with a rolling reduction of 30% or more and 70% or less. (C) The cold rolled steel sheet (c-1) is made of 1 to 10% by volume of H ₂ and N. ₂ , H ₂ O, and the remainder of one or more of O ₂ , the ratio of the water pressure to the hydrogen partial pressure of the pre-tropical zone and the soaking zone is -1.7 or more in terms of log (P _H2O / P _H2 ) In the ambient gas below -0.2; (c-2) Set the average heating rate in the temperature range of 500 ° C or higher and the maximum temperature of -50 ° C to 1 ° C / s or higher, and heat up to 720 ° C or higher and 900 ° C or lower. After reaching the temperature, keep it for more than 30 seconds and less than 30 minutes, and then keep it; (c-3) The temperature from the highest reaching temperature of -50 ° C to satisfy X (° C / ° C / ) The above average cooling rate is cooled to a cooling stop temperature T (° C) that satisfies the following formula (B), and annealing is performed for bending processing with a bending radius of 800 mm or more once; (d) annealing the steel plate. Hot-dip galvanizing is followed by alloying treatment.

X ≧ (Ar3-350) / 10 ^a ××× (A) a = 0.6 [C] +1.4 [Mn] +1.3 [Cr] +3.7 [Mo] -100 [B] -0.87 T ≧ 730-350 [ C] -90 [Mn] -70 [Cr] -83 [Mo] ××× (B) [Element]: mass% of element

Hereinafter, the process conditions of the manufacturing method of this embodiment will be described.

(a) Step Heating temperature for casting flat steel slab: 1100 ℃ or higher Finish hot rolling temperature: Ar3 or higher coiling temperature: 680 ℃ or lower

The cast flat steel blank of the alloy composition of the alloyed hot-dip galvanized steel sheet according to the present embodiment is prepared according to a usual method. The cast flat steel slab is temporarily cooled, and then heated to 1100 ° C or higher, and hot rolled. When the heating temperature of the cast flat steel slab is less than 1100 ° C, the homogenization of the cast flat steel slab and the inadequate melting of carbonitrides will lead to a decrease in strength or processability. Therefore, the heating temperature of the cast flat steel slab is set to 1100. Above ℃. It is preferably above 1150 ° C.

On the other hand, if the heating temperature of the cast flat steel slab exceeds 1300 ° C, the manufacturing cost will increase, and the productivity will decrease. In the initial stage, the particle size of Vostian iron will increase locally to become a mixed grain structure, and there is a concern that the ductility will decrease. Therefore, the heating temperature of the cast flat steel blank is preferably below 1300 ° C. The temperature is preferably 1250 ° C or lower.

The cast flat steel slab can also be hot rolled at the high temperature (above 1100 ° C, preferably below 1300 ° C) after casting.

The hot rolling is finished at a temperature of Ar3 or more. When the completion hot rolling temperature is lower than the Ar3 point, there is a concern that cracks may occur in the steel sheet during the subsequent cold rolling, which causes the material to fall. Therefore, the completion hot rolling temperature is set to be higher than the Ar3 point. It is preferably (Ar3 + 15) ° C or higher.

The hot-rolled completion temperature is within a temperature range above the Ar3 point corresponding to the composition and material of the hot-rolled steel sheet, which can be appropriately set, so the upper limit of the hot-rolled completion temperature is not specifically set.

The Ar3 point can be calculated by the following formula. Ar3 = 901-325 × [C] + 33 × [Si] + 287 × [P] + 40 × [Al] -92 ([Mn] + [Mo]) [Element]: mass% of element

The hot-rolled steel sheet after hot rolling is finished is coiled at a temperature of 680 ° C or lower. If the coiling temperature exceeds 680 ° C, the Xueming carbon iron will be coarsened and the annealing time will be longer. Moreover, the particle size of the ferrous iron in the de-C layer of the surface layer exceeds 30 mm, so the coiling temperature is set to 680 ° C or lower. The temperature is preferably 630 ° C or lower, and more preferably 580 ° C or lower.

The lower limit of the coiling temperature is not particularly limited. If the coiling temperature is lower than 400 ° C, the strength of the hot-rolled steel sheet is excessively increased, and the rolling load of cold rolling is increased. Therefore, the coiling temperature is preferably 400 ° C or higher.

(b) Rolling ratio: 30% or more and 70% or less After the scale layer is removed by pickling the hot-rolled steel sheet, the hot-rolled steel sheet is cold-rolled. When the rolling reduction is less than 30%, the formation of recrystallized nuclei is difficult to occur, and the grain growth begins due to the coarsening of the recovered grains, the recrystallization becomes insufficient, the ductility decreases, and the thickness of the ferrite grains in the thickness direction is less than 20mm. The ratio of the number of iron nuggets is reduced, so the rolling reduction is set to 30% or more.

In order to reduce the area ratio of unrecrystallized fertilizer and iron, and to further improve the ductility of the steel sheet, the higher the rolling rate, the better, but as the rolling rate increases, the rolling load also increases, so the rolling rate is set to 70 %the following. When the rolling load is high, there is a concern that the shape accuracy of the steel sheet is reduced, so the rolling reduction rate is preferably 65% or less.

In order to improve the uniformity of the structure in the width direction, the hot-rolled steel sheet is leveled before and / or after the pickling of the hot-rolled steel sheet. By this treatment, the ratio of the number of Asada scattered iron in which the aspect ratio of the Asada scattered iron in the de-C layer is 5 or more can be reduced.

By performing the leveling, the strain of the leveling and the strain of the cold rolling remain after the cold rolling. Due to the strain accumulated in the surface layer of the steel sheet, the ferrous iron recovers to recrystallize during annealing, and is close to being equiaxed. After that, the inverted state becomes a Wastfield iron with a small aspect ratio, and it cools into a Asada loose iron with a small aspect ratio. It is estimated that the distribution is also uniform in the width direction. Therefore, when leveling is not performed, the ratio of Asada loose iron with an aspect ratio of 5 or more becomes higher, and the ratio difference in the width direction becomes larger (for example, the ratio of Asa loose iron with an aspect ratio of 5 or more in the width direction exceeds 10 %), The fatigue ratio difference also becomes large, and the fatigue characteristics decrease.

In addition, when a plating treatment and an alloying treatment are performed on a uniform surface layer structure in the aforementioned width direction, it is easy to uniformly alloy, and the Fe concentration difference in the width direction in the alloyed hot-dip galvanized layer becomes small.

In addition, by performing leveling, a leveling strain inferior to that of the surface layer is applied in the vicinity of 1/4 of the plate thickness in the width direction. Compared with the case where no leveling is applied, the ferrous iron will recrystallize finely when the temperature is increased. In addition, during maintenance, Vostian iron is precipitated from the fine ferritic iron grain boundaries, thereby dispersing the large ferritic iron granules. As a result, the number of fertile iron nuggets with a thickness of 20 mm or less in the thickness direction is more than 50% of the total number of ferrous iron nuggets, which can ensure the required hole expandability. In addition, when leveling is performed, the rolling in the width direction becomes uniform in the subsequent cold rolling, and the leveling strain remains uniformly in the width direction. Therefore, the ferrous iron is in the vicinity of 1/4 thickness in the width direction. Organizations are also fragmented, improving organizational homogeneity. For example, if the maximum amount of strain applied to the surface layer of the steel sheet by roll leveling is 0.2% or more, it can be regarded as affecting the structure change of the surface layer.

(c) Step The annealing step is the most important step in producing the microstructure of the alloyed hot-dip galvanized steel sheet according to this embodiment. The conditions of each step are described below.

(c-1) Annealing ambient gas Ambient gas composition: 1 to 10% by volume of H ₂ , and N ₂ , H ₂ O, and O ₂ or more of the remainder are tropical water pressure and hydrogen content Pressure ratio: log (P _H2O / P _H2 ) is -1.7 or more, -0.2 or less

Annealing step, 1 to 10% by volume of H _2, and N _2, H ₂ O, and the remaining portion of one or _more kinds of O annealing atmosphere gas is formed, and the partial pressure of water and hydrogen are divided Tropical The pressure ratio is controlled to be -1.7 or more and -0.2 or less in terms of log (P _H2O / P _H2 ).

In the annealing ambient gas, when the steel sheet is annealed, the scale on the surface layer of the steel sheet disappears, and oxides are generated inside the steel sheet. As a result, the plateability of the steel sheet can be ensured, and a molten zinc plating layer can be formed on the surface of the steel sheet with good adhesion in the plating step described later.

When H _{2 is} less than 1% by volume, it is difficult to set log (P _H2O / P _H2 ) in the range of -1.7 to -0.2 in the soaking zone. Because the plating property of the steel sheet is reduced, H _{2 is} set to 1% by volume or more. It is preferably at least 3% by volume. When H ₂ exceeds 10% by volume, the cost of the ambient gas increases, so H _{2 is} set to 10% by volume or less. It is preferably 7 vol% or less.

When the log (P _H2O / P _H2 ) of the tropic zone is less than -1.7, the thickness of the de-C layer is less than 10 mm. Because of the decrease in plating properties, the log (P _H2O / P _H2 ) of the tropic zone is set to -1.7 or more. It is preferably -1.3 or more, and more preferably -1 or more. If the log (P _H2O / P _H2 ) of the soaking zone exceeds -0.2, the thickness of the de-C layer is more than 150 mm, and the fatigue characteristics are reduced. Therefore, the log (P _H2O / P _H2 ) of the soaking zone is set to -0.2 or less. It is preferably -0.5 or less, and more preferably -0.7.

Furthermore, as long as the thickness of the de-C layer can be controlled, the ratio of the water pressure to the hydrogen partial pressure can be replaced, for example, the ratio of the partial pressure of carbon dioxide to the partial pressure of carbon monoxide can be controlled.

The conditions of the annealing ambient gas described above are homogeneous tropical conditions, but it is also possible to control the pre-tropical zone to be -1.7 or more and -0.2 or less in terms of log (P _H2O / P _H2 ). In the pre-heat zone, adjusting the ratio of the partial pressure of water vapor P _H2O to the partial pressure of hydrogen P _H2 will affect the thickness of the de-C layer, the thickness of the refined layer, the aspect ratio of Asada loose iron, the uniformity of Fe concentration in the width direction, and plating. Surface properties of the front steel plate.

As described in the aforementioned cold rolling, the leveling strain and the cold rolling strain in the width direction remain. In addition, by adjusting the ratio of the partial pressure of water vapor P _H2O to the partial pressure of hydrogen P _{H2 in} the pre-tropic zone, the decrease in the concentration of C in the surface layer can be suppressed, and recrystallization can be prevented from proceeding excessively. Therefore, the ferrite grains recrystallized at the time of heating are fine Into. As a result, the Vostian iron was finely precipitated on the surface layer during the subsequent annealing in the uniform tropical zone, and the aspect ratio of the loose Asta iron after cooling was reduced, and the ferrous iron was also fine. In this way, the leveling is performed, and the log (P _H2O / P _H2 ) in the pre-heating zone is controlled to be -1.7 or more and -0.2 or less, thereby improving the aspect ratio of Asada loose iron in the de-C layer.

In addition, by controlling log (P _H2O / P _H2 ) in the preheat zone to be -1.7 or more and -0.2 or less, excessive decarburization on the surface of the steel plate can be suppressed, and the surface of the steel plate can be suppressed in the subsequent plating step and alloying step. Fe-Zn alloy with excess grain boundaries reacts. Thereby, the formation of a uniform Fe-Al alloy layer is promoted in the alloyed hot-dip galvanizing layer, and the Fe concentration in the width direction is uniformized, and excellent plating adhesion and uniform appearance can be obtained.

If the log (P _H2O / P _H2 ) exceeds -0.2 in the preheat zone, decarburization on the surface of the steel sheet becomes excessive, and the thickness of the C-removed layer exceeds 150 mm, and the fatigue characteristics decrease. Therefore, the log (P _H2O / P _H2 ) of the preheat zone is set to -0.2 or less. It is preferably -0.5 or less, and more preferably -0.7. On the other hand, when the log (P _H2O / P _H2 ) in the pre-tropic zone is less than -1.7, the surface of the steel sheet has a high carbon concentration and no fine layer is formed on the surface, so the Fe concentration in the width direction tends to become uneven. Even plating adhesion is reduced. Therefore, the log (P _H2O / P _H2 ) of the preheat zone is set to -1.7 or more. It is preferably -1.3 or more, and more preferably -1 or more.

(c-2) Heating and holding at an average heating rate of 500 ° C or higher and a maximum temperature of -50 ° C: 1 ° C / s or higher Maximum temperature: 720 ° C or higher and 900 ° C or lower maximum temperature: 30 seconds The average heating rate in the temperature range of 500 ° C or higher and the highest reaching temperature of -50 ° C in the annealing step above and below 30 minutes is important to form the ferritic iron system in a desired form.

When the steel sheet is heated, since the formation of ferrous iron starts at 500 ° C or higher, the lower limit of the temperature range in which the average heating rate is specified is set to 500 ° C. Finally, the steel plate is heated to a maximum reaching temperature of 720 ° C or higher and 900 ° C or lower, and maintained for 30 seconds or more and 30 minutes or less, and the temperature range heated at an average heating rate of 1 ° C / second or higher is set to a maximum reaching temperature of -50 ° C. .

From the perspective of controlling the iron morphology of the fertilizer, the average heating rate in the aforementioned temperature range is preferably fast. When the average heating rate is less than 1 ° C / sec, nuclear generation will be preferentially started from the nucleation position, the ferrite grains will become larger, and the proportion of the ferrite grains with a thickness of 20 mm or more in the thickness direction will exceed 50%, and the hole expandability will decrease. Therefore, the average heating rate in the aforementioned temperature range is set to 1 ° C / second or more. It is preferably 5 ° C / sec or more.

When the steel sheet contains carbides such as Ti, Nb, V, etc., when heating the steel sheet, it will stay in the temperature range of 550 ~ 760 ° C for 30 seconds, and then heat up to the maximum temperature of -50 ° C and the maximum temperature of 720 ~ 900. When annealing at ℃, carbides such as TiV, NbC, and VC can be finely precipitated in the steel sheet structure.

The maximum reaching temperature of the annealing step is set to 720 or more and 900 or less. When the maximum temperature reached is less than 720 ° C, the Vostian iron cannot be fully formed, and the Asada loose iron cannot be sufficiently ensured, and the residual cis carbon iron is melted, the tensile strength (TS) and the hole expandability (l) Since the temperature drops, the maximum temperature reached is 720 ° C or higher. In order to fully form Vosstian iron and fully dissolve citronite, to ensure the required tensile strength (TS) and hole expandability (l), the maximum reachable temperature is preferably 770 ° C or higher.

If the maximum reaching temperature exceeds 900 ° C, the iron particles in Vostian will be coarsened, and the formation of fertile iron will be delayed and the ductility will be reduced. Therefore, the maximum reaching temperature is set to 900 ° C or lower. In order to ensure the required ductility and increase the strength-ductility balance, the highest temperature reached is preferably below 850 ℃.

The holding time of the maximum reaching temperature is set to 30 seconds or more and 30 minutes or less. When the holding time is less than 30 seconds, the Vostian iron is not sufficiently formed, and the Asada loose iron is not sufficiently ensured, and the remaining skeletal carbon iron is melted. The decrease of the loose iron in Mata resulted in the decrease of the tensile strength (TS), and due to the presence of fused skeletal carbon iron, even if the strength decreased, the ductility or hole expandability (l) did not increase, so TS × l decreased. Set the hold time to 30 seconds or more. It is preferably 60 seconds or more.

If the holding time exceeds 30 minutes, the iron particles in Vostian are coarsened, and the thickness of the iron particles in the fertilizer particles is larger than the specified range. Therefore, the hole expandability is reduced, and the value of the strength × l becomes low. Therefore, the holding time is set to 30 minutes or less. A score of 20 or less is preferred.

It should be noted that the holding time is a holding time in a temperature range from the maximum arrival temperature to the maximum arrival temperature of -50 ° C.

(c-3) Cooling temperature range for cooling and bending: the highest temperature reached -50 ° C ~ the cooling stop temperature T (° C) which satisfies the following formula (B) average cooling rate: X (° C) which satisfies the following formula (A) / Sec) Bending with a radius of 800 mm or less during cooling: 1 or more times

Immediately following the above-mentioned holding, the steel plate was brought from the highest temperature reached to -50 ° C to satisfy the average cooling rate of X (° C / sec) or more in the following formula (A), and cooled to a cooling stop satisfying the following formula (B). The temperature is T (° C), and the steel sheet is subjected to one or more bending processes with a bending radius of 800 mm or less.

The following formula (A) is an empirical formula in which the average cooling rate (° C./second) that can suppress the generation of boron iron is defined in relation to the composition of the components. The following formula (B) is an empirical formula that specifies the lower limit of the temperature range of the Asada scattered iron that suppresses the generation of toughened iron and can ensure a sufficient amount of the iron composition and the composition of the iron.

If the cooling stop temperature T (° C) does not satisfy the following formula (B), a large amount of toughened iron will be generated, a sufficient amount of Asada loose iron will not be obtained, and the required strength will not be secured. Therefore, the cooling stop temperature T ( ° C) is set to a temperature satisfying the following formula (B).

When the average cooling rate to the cooling stop temperature T (° C) is slow, waved iron that inhibits elongation and hole expansion is generated during cooling. Therefore, in order to suppress the waved iron fraction to 5% or less, the cooling stop temperature T is reached. The average cooling rate X (° C / sec) of (° C) is an average cooling rate that satisfies the following formula (A).

X ≧ (Ar3-350) / 10 ^a ××× (A) a = 0.6 [C] +1.4 [Mn] +1.3 [Cr] +3.7 [Mo] -100 [B] -0.87 T ≧ 730-350 [ C] -90 [Mn] -70 [Cr] -83 [Mo] ××× (B) [Element]: mass% of element

The cooling steel plate is subjected to a bending process of a bending radius of 800 mm or more once. By this bending process, the particle diameter of the steel sheet surface layer can be made fine, and the particle size of the ferrous iron in the de-C layer can be 30 mm or less. The reason is not clear, but it can be considered as promoting the nucleation of crystal grains having different crystal orientations, and the crystal grain size of the surface layer of the steel sheet obtained after annealing becomes smaller.

If the bending radius exceeds 800 mm, the amount of strain introduced into the surface layer of the steel sheet is small, no nucleation of crystal grains occurs, and the effect of miniaturizing crystal grains cannot be obtained. Therefore, the bending radius is set to 800 mm or less. The larger the amount of bending (processing amount), the more the nucleation is promoted, and the effect of miniaturizing crystal grains is obtained. Therefore, the bending radius is preferably set to 730 mm or less. It is preferably 650 mm or less.

The bending radius may be appropriately set according to the thickness of the steel plate or the equipment load specification, so the lower limit of the bending radius is not specifically set.

(d) Step hot-dip galvanizing bath temperature: 440 ~ 480 ° C Steel plate temperature: 430 ~ 490 ° C The steel plate after the annealing step is immersed in the plating bath, and the hot-dip galvannealing is performed on the surface of the steel plate. Zinc.

The plating bath is a plating bath mainly composed of molten zinc, and may also include Al, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, Sc , I, Cs, REM 1 or more. The amount of Al may be appropriately adjusted depending on the ease of alloying.

The temperature of the plating bath is preferably 440 ~ 480 ° C. When the temperature of the plating bath is less than 440 ° C, the viscosity of the plating bath will increase excessively, and it is difficult to properly control the thickness of the plating layer, which will cause the appearance of the steel plate or the plating adhesion to decrease. Therefore, the temperature of the plating bath is 440 ° C or higher. good. The temperature is preferably 450 ° C or higher.

On the other hand, if the temperature of the plating bath exceeds 480 ° C, a large amount of smoke will be generated, the working environment will be deteriorated, and the safety of the operation will be hindered. Therefore, the temperature of the plating bath is preferably 480 ° C or lower. The temperature is preferably 470 ° C or lower.

When the temperature of the steel plate immersed in the plating bath is less than 430 ° C, it is difficult to stably maintain the temperature of the plating bath at 450 ° C or higher. Therefore, the temperature of the steel plate entering the plating bath is preferably 430 ° C or higher. The temperature is preferably 450 ° C or higher.

On the other hand, if the temperature of the steel plate immersed in the plating bath exceeds 490 ° C, it is difficult to stably maintain the temperature of the plating bath below 470 ° C. Therefore, the temperature of the steel plate immersed in the plating bath is preferably 490 ° C or lower. The temperature is preferably 470 ° C or lower.

It is also possible to perform cold rolling on a hot-dip galvanized steel sheet cooled to room temperature after plating. The cold rolling can correct the shape of the hot-dip galvanized steel sheet and adjust the endurance or ductility of the steel sheet. In addition, if the rolling reduction exceeds 3%, the ductility decreases, so the rolling reduction is preferably 3% or less.

Alloying of hot-dip galvanizing Heating temperature: 470 ~ 620 ℃ Heating time: 2 ~ 200 seconds The hot-dip galvanizing layer formed after immersing a steel plate in a plating bath is alloyed to form an alloyed hot-dip galvanizing layer on the surface of the steel plate.

When the alloying treatment temperature is less than 470 ° C, the alloying treatment is not sufficiently performed, so the alloying treatment temperature is preferably 470 ° C or higher. The temperature is preferably 490 ° C or higher. On the other hand, if the alloying treatment temperature exceeds 620 ° C, coarse citronite will be generated, and wave iron will be formed, and the strength will be reduced. Therefore, the alloying treatment temperature is preferably 620 ° C or lower. The temperature is preferably 600 ° C or lower.

When the alloying treatment time is less than 2 seconds, the alloying treatment time is preferably 2 seconds or more because the alloying of the hot-dip galvanized layer is not sufficiently performed. It is more preferably 5 seconds or more. On the other hand, if the alloying treatment time exceeds 200 seconds, the boron iron is generated and the plating layer is overalloyed. Therefore, the alloying treatment time is preferably 200 seconds or less. It is preferably 150 seconds or less.

Furthermore, the alloying treatment may be performed immediately after the steel sheet is pulled out of the plating bath, or the plated steel sheet may be temporarily cooled to room temperature and then reheated.

After the alloying treatment, a cold rolling reduction of 3% or less can be performed on the alloyed hot-dip galvanized steel sheet cooled to room temperature. The shape of the alloyed hot-dip galvanized steel sheet can be corrected by the cold rolling, and the endurance or ductility of the steel sheet can be adjusted. In addition, if the rolling reduction exceeds 3%, the ductility decreases, so the rolling reduction is preferably 3% or less. [Example]

Next, examples of the present invention will be described, but the conditions of the examples are an example of a condition used to confirm the feasibility and effect of the present invention, and the present invention is not limited by the one example of the condition. The present invention can use various conditions as long as the object of the present invention can be achieved without departing from the gist of the present invention.

(Example 1) A molten steel having a composition shown in Table 1 was continuously cast according to a general method to prepare a cast flat steel blank. In Table 1, the component compositions of symbols A to T satisfy the component composition of the present invention.

In the component composition of symbol a, C and Mo did not satisfy the composition of the present invention, in the component composition of symbol b, Mn and P did not satisfy the composition of the present invention, and in the composition of symbol c, Al and Nb did not satisfy the composition of the present invention. In the component composition of the symbol d, C and Mn do not satisfy the component composition of the present invention.

In the component composition of the symbol e, Si and S do not satisfy the composition of the present invention, in the component composition of the symbol f, N and Ti do not satisfy the composition of the present invention, and in the component composition of the symbol g, Si, N, and Ti do not satisfy the present invention. Among the component compositions, the component composition of the symbol h does not satisfy the component composition of the present invention, and the component composition of the symbol i does not satisfy the component composition of the present invention.

[Table 1]

The cast flat steel slabs with the composition shown in Table 1 are heated, hot-rolled, pickled, and leveled, and then cold-rolled to produce a steel plate with a thickness of 1.6 mm, and the steel plate is shown in Tables 2 to 6. Condition annealing, cooling, and plating after cooling.

[Table 2]

[table 3]

[Table 4]

In the processing numbers (letters + numbers) shown in Tables 2 to 4 and subsequent Tables 5 to 7, the letters represent steels with the composition shown in Table 1, and the numbers represent the numbers of the examples. For example, the process number "A1" indicates the first embodiment implemented using steel A having the composition shown in Table 1.

Tables 2 to 4 show the heating temperature of the cast flat steel slab, Ar3, the completion temperature of hot rolling, the coiling temperature, the treatment of the hot rolled steel sheet before pickling, the rolling reduction rate of cold rolling, and the environment in the furnace. Gas, heating rate of annealing step, reaching temperature (maximum temperature), holding time, average cooling rate of cooling step, cooling stop temperature. In addition, the values on the right of the formulas (A) and (B) are also displayed. In the invention examples and some comparative examples, a maximum strain of 0.2% or more was applied to the surface by leveling.

In addition, the bending radius and the number of bending times, the galvanizing bath temperature, and the entering plate temperature in the plating bath are shown in the annealing process. Moreover, the alloying person showed the alloying temperature and the alloying time.

After the steel plates were subjected to the conditions shown in Tables 2 to 4, the appearance and mechanical characteristics of the microstructure were measured and evaluated.

According to the method described above, the respective microstructure fractions, the thickness of the ferrite grains, and the thickness of the de-C layer were obtained. By observation with a scanning electron microscope as follows, the number density of the aspect ratio of the iron particle size of the ferrous grains in the de-C layer and the loose iron in the C-layer was calculated to be 5 or more.

In the area outside the half of the thickness of the de-C layer in the de-C layer, observe the area with an area of 40,000 mm ² or more, draw a line segment parallel to the rolling direction, and divide the total of the line length by the number of intersections between the line segment and the grain boundary. The average value is regarded as the iron particle size of the fertilizer.

Calculate the number of Asada scattered iron and the short and long axis lengths of each Asada scattered iron, divide the length of the long axis by the value of the short axis length as the aspect ratio, and divide the number of Asada scattered iron with an aspect ratio of 5 or more by The number density of the entire Asada scattered iron is calculated. In addition, as the difference in the widthwise structure, the difference in number density was calculated by dividing the number of Asada scattered iron with an aspect ratio of 5 or more of the de-C layer divided by the total number of Asada scattered iron.

The evaluation of the appearance of the plating is as follows, the difference in Fe concentration in the width direction of the plating layer, and the judgment of the occurrence of non-plating by visual judgment. "X" is a case where unplating with a diameter of 0.5 mm or more was observed, and it deviated from the allowable range in appearance. "○" is a case where non-plating having a diameter of 0.5 mm or more was not observed, but unevenness was caused by a difference in Fe concentration in the width direction of 1.0% or more. In addition, "◎" means other cases.

The peeling state after the 60 ° V bending test was used to evaluate the plating adhesion when the compressive stress was applied. "×" is a case where the peeling width is 7.0 mm or more and cannot be practically allowed, and "○" is a case other than.

The test was performed in accordance with JIS Z 2241 to evaluate mechanical properties (drop stress, tensile strength, elongation, and drop point elongation). The hole expandability was tested in accordance with JIS Z 2256. The fatigue characteristics were measured by a plane bending fatigue test. The test piece used JIS No. 1 test piece, and the stress ratio was -1. The repetition frequency was set to 25 Hz, and the maximum number of repetitions was set to 2 × 10 ⁶ times. Divide the fatigue limit strength by the maximum tensile strength value as the fatigue ratio. In addition, the difference in the fatigue ratio in the width direction was also calculated as an indicator of whether or not the characteristics in the width direction of the steel sheet were uniform.

The measurement results and evaluation results are shown in Tables 5 to 7.

[table 5]

[TABLE 6]

[TABLE 7]

In order to confirm the goodness of the steel type in the examples, the cases of TS × EL ≧ 16000MP%, TS × EL × l ≧ 480,000MP %%, fatigue ratio ≧ 0.40, and fatigue ratio difference ≦ 0.10 are shown as the invention steel.

In the steel plates of process numbers A1 and F7, the rolling reduction rate was low, the "ratio of the thickness of the ferrite grains below 20 mm" was low, and TS × EL × l became low. In the steel plate of process number A6, the coiling temperature was high, the grain size of the ferrous iron in the de-C layer was large, and the fatigue ratio became low. In the steel plate of treatment number A9, the bending radius of the bending process is large, the grain size of the ferrous iron in the de-C layer is large, and the fatigue ratio becomes low.

In the steel plates of treatment numbers A11 and B6, since no bending was performed, the grain size of the ferrous iron in the de-C layer was large, and the fatigue ratio became low. The log (P _H2O / P _H2 ) of the ambient gas in the preheat zone furnace in the process number A12 is high, the thickness of the de-C layer becomes thicker, and the fatigue ratio becomes lower. The log (P _H2O / P _H2 ) of the ambient gas in the preheating furnace in the process number A13 is low, unevenness is generated on the surface, and the plating adhesion is reduced. Moreover, in the de-C layer, since the proportion of Asada scattered iron having an aspect ratio of 5 or more exceeds 50%, the fatigue ratio decreases.

In the steel plate of process number B1, because the cooling rate is slow, the wrought iron fraction is high, and TS × EL and TS × EL × l become low. In the steel plate of process number C1, because the holding time during heating is short, the microstructure fraction is not within the scope of the present invention, and TS × EL and TS × EL × l become low.

In the steel plate of process number C3, since the holding time during heating is long, TS × EL × l becomes low. In the steel plate of process number C5, since the log (P _H2O / P _H2 ) of the ambient gas in the soaking zone furnace is low and the thickness of the de-C layer is less than 10 mm, the appearance and adhesion of the plating are reduced. In the steel plate of process number C6, the log (P _H2O / P _H2 ) of the ambient gas in the furnace of the soaking zone is high, the thickness of the de-C layer becomes thicker, and TS × EL × l and the fatigue ratio become lower.

In the steel plate of process number D1, the highest reaching temperature was low, the microstructure fraction was outside the scope of the present invention, and TS × EL and TS × EL × l became low. In the steel plate of treatment number D4, the alloying treatment temperature is high, the plating appearance is reduced, and there is a large amount of wave iron, so TS × EL and TS × EL × l become low. In the steel plate of treatment number D5, the alloying treatment time was short, and the plating appearance was deteriorated.

In the steel plate of treatment number D8, the alloying treatment time was long, and the appearance of plating was deteriorated. In addition, since there are many waves of iron, TS × EL and TS × EL × l become low. In the steel plates of process numbers G1 and E1, the heating rate was slow, "the ratio of the thickness of the ferrite grains to 20 mm or less" was low, and TS × EL × l became low.

In the steel plate of process number E5, the cooling stop temperature was low, the toughening iron metamorphosis progressed excessively, the Asada loose iron fraction became lower, and TS × EL and TS × EL × l became lower. In the steel plate of process number F1, the highest reaching temperature is high, the ferrous iron content is reduced, and TS × EL and TS × EL × l are reduced. In the steel plate of process number F5, the coiling temperature is high, the grain size of the ferrous iron in the de-C layer becomes larger, and the fatigue ratio becomes lower.

In the steel plate of process number F6, the bending radius of the bending process is large, the grain size of the ferrous iron in the de-C layer becomes larger, and the fatigue ratio becomes lower. In the steel plates of treatment numbers G5 and H2, the temperature of the galvanizing bath was low, and although the appearance of the plating was reduced, the elongation, hole expansion, and fatigue characteristics were excellent, and the characteristics in the width direction of the steel plates were also uniformized. In the steel plate of process number L2, the heating temperature was low, the fraction of loose iron in Asada exceeded the scope of the present invention, and TS × EL and TS × EL × l became low.

In the steel plates with treatment numbers B4, C9, G3, and G9, no leveling was performed before and / or after pickling. Therefore, the number of ferritic iron nuggets with a thickness of less than 20mm in the thickness direction is less than 50% of the total number of ferrous iron nuggets, and the proportion of Asada loose iron in the de-C layer with an aspect ratio of 5 or more exceeds 50%. As a result, the difference in the proportion of Asada loose iron in the width direction with an aspect ratio of 5 or more all exceeded 10%, and the difference in the structure in the width direction in the de-C layer became large, so the difference in the fatigue ratio also became large. In addition, TS × EL × l is low, and the fatigue ratio is small.

In the steel plates of process numbers a1, b1, c1, d1, e1, f1, g1, h1, and i1, since the component composition is outside the scope of the present invention, TS × EL, TS × EL × l, and the like become low. In other conditions, the structure is within the scope of the present invention, and the surface quality (appearance, plating adhesion), TS × EL, TS × EL × l, fatigue ratio, and the difference between the fatigue ratios are all good. Industrial availability

As described above, according to the present invention, it is possible to provide a high-strength alloyed hot-dip galvanized steel sheet having excellent elongation, hole expansion, and fatigue characteristics, and uniformizing the characteristics in the width direction of the steel sheet. Therefore, the present invention has high applicability in the steel plate manufacturing industry, the automobile manufacturing industry, and other machinery manufacturing industries.

Claims (5)

An alloyed hot-dip galvanized steel sheet has an alloyed hot-dip galvanized layer on the surface of the steel sheet, and is characterized in that the composition of the steel sheet is composed of the following: in mass%, C: 0.06% or more, 0.22% or less, Si: 0.50% or more, 2.00% or less, Mn: 1.50% or more, 2.80% or less, Al: 0.01% or more, 1.00% or less, P: 0.001% or more, 0.100% or less, S: 0.0005% or more, 0.0100% or less, N: 0.0005% or more, 0.0100% or less, Ti: 0% or more, 0.10% or less, Mo: 0% or more, 0.30% or less, Nb: 0% or more, 0.050% or less, Cr: 0% or more, 1.00% or less, B: 0% or more, 0.0050% or less, V: 0% or more, 0.300% or less, Ni: 0% or more, 2.00% or less, Cu: 0% or more, 2.00% or less, W: 0% or more, 2.00% or less, Ca: 0% or more, 0.0100% or less, Ce: 0% or more, 0.0100% or less, Mg: 0% or more, 0.0100% or less, Zr: 0% or more, 0.0100% or less, La: 0% or more, 0.0100% or less, REM: 0% or more, 0.0100% or less, Sn: 0% or more, 1.000% or less, Sb: 0% or more, 0.200% or less, the remainder: Fe and impurities; among them, The microstructure in the range of 1/8 plate thickness to 3/8 plate thickness centered on 1/4 plate thickness from the surface of the steel plate toward the plate thickness direction is composed of the following: in terms of area ratio, fertilizer iron: 15% or more, Less than 85%, residual Vostian iron: less than 5%, Asada loose iron: 15% to 75%, boron iron: 5% or less, and the rest (including 0%): toughened iron; the aforementioned plate thickness The number of ferrous iron nuggets with a thickness of 20 mm or less in the direction is 50% or more of the total number of ferrous iron nuggets; a de-C layer with a thickness of 10 mm to 150 mm is formed on the surface layer of the steel plate; Hereinafter, the proportion of Asada scattered iron with an aspect ratio of 5 or more in Asada scattered iron is 50% or less. The alloyed hot-dip galvanized steel sheet according to claim 1, further comprising a micronized layer having an average thickness of 0.1 mm to 5.0 mm between the alloyed hot-dip galvanized layer and the de-C layer. The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein in the aforementioned alloyed hot-dip galvanized layer, the Fe concentration difference in the width direction is less than 1.0% by mass%, and the aforementioned aspect ratio is 5 or more in the width direction. The difference in the proportion of loose iron in Asada is less than 10%. For example, the alloyed hot-dip galvanized steel sheet according to claim 1 or 2, wherein the foregoing component composition includes one or more of the following: in mass%, Ti: 0.01% or more, 0.10% or less, Mo: 0.01% or more, 0.30 % Or less, Nb: 0.005% or more, 0.050% or less, Cr: 0.01% or more, 1.00% or less, B: 0.0002% or more, 0.0050% or less, V: 0.001% or more, 0.300% or less, Ni: 0.01% or more, 2.00 % Or less, Cu: 0.01% or more, 2.00% or less, W: 0.01% or more, 2.00% or less, Ca: 0.0001% or more, 0.0100% or less, Ce: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100 % Or less, Zr: 0.0001% or more, 0.0100% or less, La: 0.0001% or more, 0.0100% or less, REM: 0.0001% or more, 0.0100% or less, Sn: 0.001% or more, 1.000% or less, Sb: 0.001% or more, 0.200 %the following. The alloyed hot-dip galvanized steel sheet according to claim 3, wherein the aforementioned component composition includes one or more of the following: in terms of mass%, Ti: 0.01% or more, 0.10% or less, Mo: 0.01% or more, 0.30% or less Nb: 0.005% or more, 0.050% or less, Cr: 0.01% or more, 1.00% or less, B: 0.0002% or more, 0.0050% or less, V: 0.001% or more, 0.300% or less, Ni: 0.01% or more, 2.00% or less , Cu: 0.01% or more, 2.00% or less, W: 0.01% or more, 2.00% or less, Ca: 0.0001% or more, 0.0100% or less, Ce: 0.0001% or more, 0.0100% or less, Mg: 0.0001% or more, 0.0100% or less , Zr: 0.0001% or more, 0.0100% or less, La: 0.0001% or more, 0.0100% or less, REM: 0.0001% or more, 0.0100% or less, Sn: 0.001% or more, 1.000% or less, Sb: 0.001% or more, 0.200% or less .