JP2010024525A - Galvannealed steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a galvannealed steel sheet in which the increase of production cost is suppressed, and which has surface appearance properties more excellent than those of the conventional one. <P>SOLUTION: Into a steel sheet having a chemical composition comprising, by mass, 0.02 to 0.2% C, 1.5 to 3.5% Mn, 0.03 to 0.5% Cr and 0.01 to 0.15% Al, also comprising ≤0.04% Si, ≤0.03% P, ≤0.03% S, and the balance Fe with inevitable impurities, further, one or more selected from Se, Sb, Zn and Ba are incorporated in the range of 0.001 to 0.2% in total, and hot dip galvanizing is performed, so as to produce a hot dip galvannealed steel sheet. By the action of Se, Sb, Zn and Ba, the concentration of easily oxidizable elements such as Si and Mn onto the surface of the steel sheet caused by selective oxidation upon annealing is suppressed, and the reduction of its plating properties and alloying treatability is prevented. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an alloyed hot-dip galvanized steel sheet having an excellent surface appearance, which is used for applications such as automobiles, home appliances, and building materials, and is particularly used as a rust-proof surface-treated steel sheet for automobile bodies. The present invention relates to an alloyed hot-dip galvanized steel sheet having good surface strength, high strength and high tension.

  Hot-dip galvanized steel sheets are used in a wide range of applications such as automobiles, home appliances, and building materials. In particular, alloyed galvanized steel sheets are excellent in corrosion resistance and spot weldability, and are widely used as automotive steel sheets in Japan. In recent years, in automobiles, from the viewpoint of increasing the weight of the vehicle body for improving fuel efficiency due to increased awareness of global environmental problems and improving collision safety, there is an increasing demand for hot-dip galvanized steel sheets with higher strength and thinner materials. In order to ensure a ductile balance, many of them are added with easily oxidizable elements such as Si and Mn. These additive elements are known to be selectively oxidized at the time of annealing before the plating treatment, and significantly inhibit the wettability of hot dip zinc and the alloying processability of the plating layer. Plating defects such as non-plating where plating does not adhere uniformly, and plating defects called “Sazanami”, which has a pattern that appears to be wavy on the surface of plating but has an excellent appearance, are likely to occur. Moreover, since it is difficult to control the alloying of the plating layer, it has been difficult to stably manufacture the galvannealed steel sheet. When the above-mentioned plating defects (non-plating), ripples, and alloying unevenness occur, the powdering (powder-like peeling) property is also inferior, resulting in a problem that the plating layer is peeled off in the subsequent component processing step.

In order to solve such a problem, for example, in Patent Document 1, in the annealing process of the steel sheet to be plated, a heating process in which, for example, an oxygen partial pressure or a hydrogen sulfide partial pressure in a nitrogen atmosphere is adjusted to the surface layer of the steel sheet. And alloying reactivity is greatly improved by forming a reaction product such as Si-Mn composite oxide or MnS between steel plate additive elements such as Si, Mn, Al, Ti and annealing atmosphere components. Also disclosed is a method for producing an galvannealed steel sheet having excellent plating adhesion, no alloying unevenness, and excellent powdering resistance. In Patent Document 2, an ammonium salt containing S is attached to the surface of a high-tensile steel plate containing Mn, and after annealing in a nitrogen gas atmosphere containing a small amount of hydrogen gas, hot dip galvanizing is performed. By applying it, the concentration of Mn on the steel sheet surface layer is suppressed, the wettability of the steel sheet and hot dip zinc is prevented, and hot dip galvanized steel sheets with excellent plating properties, plating adhesion and plating appearance are alloyed. A method of manufacturing with high productivity without delaying processing is disclosed. Furthermore, in Patent Document 3, after annealing a steel sheet containing an element that is easier to oxidize than Fe during annealing, such as Si, Mn, P, before passing a galvanizing bath and performing a hot dip galvanizing treatment, The surface layer of the steel sheet is removed by a dry etching method, and the steel sheet from which the surface layer has been removed is subjected to a hot dip galvanizing process, and then the alloying process is performed to improve the wettability between the steel sheet and the hot dip galvanizing bath. There has been disclosed a method for producing galvannealed galvanized iron that is excellent in uniformity and powdering resistance without causing non-plating or alloying unevenness in the galvanized layer. And in patent document 4, after annealing the high-tensile steel plate containing 1 type or 2 types or more in Si, Mn, and Al by a continuous annealing furnace which has a non-oxidizing furnace or a direct-fired furnace type heating zone, After removing 70% or more of the surface concentrated layer of the elements contained in the high-tensile steel plate by pickling, it was reheated and then hot-dip galvanized on the high-tensile steel plate to prevent unplating and excellent A method for producing a high-tensile hot-dip galvanized steel sheet having surface properties and excellent plating adhesion is disclosed.
Japanese Patent Laying-Open No. 2005-200711 JP 2001-279410 A JP-A-6-88193 JP 2004-263271 A

  However, in the method for producing an alloyed hot-dip galvanized steel sheet described in Patent Documents 1 to 4, the wettability between the steel sheet and hot-dip zinc and the alloying processability of the plating layer are reduced, the plating property and the plating adhesion, To suppress the concentration of steel additive elements such as Si and Mn on the steel sheet surface, which cause uneven alloying, an extra heat treatment pattern is added in the annealing process before hot dip galvanizing, or the steel sheet surface An alloyed hot-dip galvanized steel sheet manufacturing process, such as annealing after adhering an ammonium salt containing S to the surface, or removing the concentrated layer of the additive element on the steel sheet surface by dry etching or pickling There is a problem that becomes complicated.

  The present invention has been made in view of the above-described problems of the prior art, and the problem is that the manufacturing process is not complicated, the increase in manufacturing cost is suppressed, and the surface appearance is superior to the conventional one. It is to provide a galvannealed steel sheet.

  In order to solve the above problems, the present invention employs the following configuration.

  The alloyed galvanized steel sheet according to claim 1 is C: 0.02-0.2 mass%, Mn: 1.5-3.5 mass%, Cr: 0.03-0.5 mass%, Al: 0.01 to 0.15% by mass, Si: 0.04% by mass or less, P: 0.03% by mass or less, S: 0.03% by mass or less, and having the chemical composition of the balance Fe and inevitable impurities An alloyed hot-dip galvanized steel sheet obtained by subjecting a steel sheet to hot-dip galvanization and then alloying the plating layer, wherein the steel sheet having the chemical composition is further selected from Se, Sb, Zn, and Ba. 1 type or 2 types or more are contained in the range of 0.001-0.2 mass% in total.

  In the alloyed galvanized steel sheet according to claim 2, the steel sheet is further Cu: 0.003-0.5 mass%, Ni: 0.003-1.0 mass%, Ti: 0.003-1. 1 type or 2 types or more selected from 0 mass% is contained in 0.003-2.5 mass% in total.

  In the alloyed galvanized steel sheet according to claim 3, the steel sheet is further divided into V: 0.003-1.0 mass%, Nb: 0.003-1.0 mass%, and B: 0.0002-0. 1 mass%, Mo: 0.001-1.0 mass% of 1 type (s) or 2 or more types chosen from 0.003-1.0 mass% is contained.

  In the alloyed galvanized steel sheet according to claim 4, the steel sheet further comprises one or two of Ca: 0.0005 to 0.005 mass% and Mg: 0.0005 to 0.01 mass%. It is characterized by including.

  In the present invention, the concentration of easily oxidizable elements such as Si and Mn added to secure the strength ductility balance of the steel sheet to the steel sheet surface due to selective oxidation during annealing is suppressed, and plating properties and alloying are achieved. Since the chemical composition is defined so as to contain an alloying element that prevents deterioration of the processability, a high-strength galvannealed steel sheet with good plating properties, less unevenness in alloying, and excellent surface appearance is manufactured. be able to. Such a remarkable quality effect makes it possible to expand the application of alloyed galvanized steel sheets as automotive steel sheets having a high required level of plating quality, and an economic effect by expanding sales is also expected.

  Hereinafter, embodiments of the present invention will be described with reference to technical grounds and examples.

  The annealing in the manufacturing process of the alloyed hot-dip galvanized steel sheet is usually performed in an atmosphere in which a small amount of hydrogen gas is mixed with nitrogen gas. However, during annealing, there is a small amount of air intrusion. In such an atmosphere, Fe is not oxidized, but easily oxidizable elements such as Si and Mn are oxidized. As a result, diffusion to the surface occurs, and single oxides or composite oxides of these elements are generated, which causes plating defects such as non-plating and alloying defects such as uneven alloying. In particular, when Si is concentrated on the surface of a steel sheet, it is known that a thin oxide layer or grain boundary oxidation is formed on the outermost surface, and the plating property and alloying processability are significantly deteriorated. On the other hand, since Mn concentrates on the surface layer of the steel sheet and grows as a granular oxide (MnO), the barrier effect on the diffusion of Fe in the plating layer direction during alloying heat treatment of the plating layer performed after hot dip galvanizing is more than that of Si. If it is small and added in a small amount, there is little influence on the processing speed of alloying. However, since Mn is an element having a low strengthening ability, it is necessary to increase the amount of addition. When added in a large amount, MnO is likely to be formed on the surface, so that the alloying behavior becomes complicated and the alloying control becomes difficult.

  As a result of examining the state of the steel sheet surface, in particular, the relationship between the MnO generation form and the alloying of the plating layer, the demand for high strength and thinning has increased. From the standpoint of securing, it is possible to produce an alloyed hot-dip galvanized steel sheet with excellent surface appearance that does not cause plating defects or alloying irregularities even in steel sheets to which easily oxidizable elements such as Si, Mn, and P are added. I figured out that. That is, as a result of detailed analysis of the surface state of the steel sheet, if surface oxidation of Si and Mn could be suppressed, it was conceived that plating defects and alloying unevenness could be reduced and a good plating surface appearance could be obtained. As elements for suppressing the surface oxidation of Si and Mn, attention was focused on Se, Sb, Zn, and Ba, which are elements that are easily concentrated on the steel sheet surface, do not dissolve in the base iron, and are easily segregated at the grain boundaries. These elements tend to exist at grain boundaries. Therefore, if these elements are added, they will segregate in the vicinity of the steel plate surface (surface layer) or surface (surface layer) grain boundary and suppress the formation of Si complex oxide and Mn oxide formed through the grain boundary. It becomes possible to do. And since there are few oxides of Si or Mn in the surface layer, the wettability with molten zinc is improved and the progress of the alloy reaction is made uniform, so that non-plating and poor alloying (unevenness) are also reduced. In addition, such a good plating surface appearance also improves paint bake hardenability. The reasons for limiting the chemical composition in the present invention will be described below.

(1) C: 0.02 to 0.2% by mass
C greatly affects the strength of the steel sheet, and affects the ductility such as elongation and stretch flangeability by changing the amount and form of the low-temperature transformation product. If it is less than 0.02% by mass, the high strength needs for automobiles cannot be met, and if it exceeds 0.2% by mass, weldability is deteriorated. For this reason, the lower limit of the C amount is 0.02% by mass, preferably 0.04% by mass, and the upper limit is 0.2% by mass, preferably 0.15% by mass.
(2) Mn: 1.5 to 3.5% by mass
Mn is a strengthening element, and an addition amount of at least 1.5% by mass or more is necessary in order to obtain characteristics as a high-strength steel sheet having high strength and extremely excellent workability. Moreover, in order to avoid the bad influence on weldability by the fall of ductility, such as elongation, and the increase in a carbon equivalent, it is desirable that it is the addition amount of 3.5 mass% or less.
(3) Cr: 0.03-0.5 mass%
Cr is an element effective in enhancing hardenability and strengthening the structure. In addition to concentrating C in the austenite phase to increase phase stability and facilitating the formation of martensite, Cr oxide is used as a steel plate. Since it is formed on the surface, the plating property is also affected. If the addition amount is less than 0.03% by mass, the effect of improving hardenability cannot be expected. On the other hand, even if added over 0.5 mass%, the effect of improving hardenability is saturated, which is disadvantageous in terms of manufacturing cost. And the addition amount exceeding 0.5 mass% impairs plating property.
(4) Al: 0.01 to 0.15 mass%
Since Al is an effective element as a deoxidizer in the steelmaking stage, an addition amount of 0.01% by mass or more is necessary. However, an Al addition amount exceeding 0.15% by mass causes an increase in production cost and adversely affects the steel sheet surface properties.
(5) Si: 0.04% by mass or less Si is an element that improves ductility such as elongation, that is, workability, by reducing the amount of solid solution C in the α phase. However, since Si is an element that forms an oxide film on the steel sheet surface and extremely deteriorates the wettability with molten zinc, the present invention basically does not require addition, and when mixed as an unavoidable impurity, It is necessary to limit the upper limit to 0.04 mass% or less, preferably 0.02 mass% or less.
(6) P: 0.03% by mass or less P is an element effective for increasing the strength of a steel sheet. However, if it exceeds 0.03% by mass, uneven plating tends to occur and alloying treatment is difficult. Therefore, in the present invention, basically, it is not added, and when it is mixed as an unavoidable impurity, the upper limit must be limited to 0.03% by mass or less.
(7) S: 0.03 mass% or less S is an element that causes hot cracking during hot rolling and significantly impairs spot weldability. Although fixed as precipitates in the steel sheet, increasing the amount causes ductile deterioration such as elongation and stretch flangeability, so when mixed as an unavoidable impurity, the upper limit is set to 0.03% by mass or less. It is necessary to stop.
(8) Se, Sb, Zn, Ba: 0.001 to 0.2% by mass in total
As described above, it is necessary to suppress surface oxidation of Si and Mn in order to reduce plating defects and alloying unevenness and obtain an excellent surface appearance. For this purpose, it is necessary to add one or more of Se, Sb, Zn and Ba, which are easily concentrated on the steel sheet surface and segregated at the grain boundaries, in a total amount of 0.001% by mass or more. If the amount added is less than this amount, the effect of suppressing the surface oxidation is not exhibited. If the amount exceeds 0.2% by mass, the effect of suppressing the surface oxidation is saturated, and the mechanical properties of the steel sheet such as embrittlement are deteriorated. Invite. The action of these elements is basically the same, and a plurality of two or more elements may be added within the range of the total addition amount. As will be described later, when added in combination with elements such as Cu, Ni, and Ti, the synergistic effect reduces plating defects and alloying unevenness and increases the effect of providing an excellent surface appearance.

  The structure of the base steel sheet of the present invention, that is, the steel sheet before hot dip galvanizing, may be any structure as long as it is mainly composed of a mixed structure of ferrite and martensite, and each fraction (volume ratio) of ferrite and martensite in the metal structure. Is not particularly limited. Martensite is a structure that is easily strengthened, and is a structure that is suitable for manufacturing a high-strength steel sheet that has a high degree of demand in steel sheets for automobiles. In addition, by mixing the ferrite structure, it is possible to adjust ductility such as bendability and elongation, and workability such as deep drawability. What is necessary is just to determine the fraction itself of both structures according to the balance of the intensity | strength requested | required of a steel plate, and ductility. Generally, as the ferrite fraction increases, the strength decreases, but the ductility tends to improve. When the martensite fraction is increased, the strength is improved, but the ductility tends to be lowered. These fractions are preferably 5 to 90% by volume of ferrite from the viewpoint of ductility and 5 to 90% by volume of martensite from the viewpoint of strength. Further, since the workability deteriorates when the mixed amount of different structures such as pearlite and bainite (intermediate transformation structure) increases, the total amount of ferrite and martensite is preferably 70% by volume or more, and 95% by volume or more. It is more preferable that It should be noted that the fraction of each structure in the metal structure of the base steel sheet is the thickness (t) direction center (t / 2) and the sheet width (w) direction center (w / 2) in the cross section perpendicular to the rolling direction. The position can be observed with a partial scanning electron microscope (SEM) at least at a magnification of about 3000 times to determine the area ratio of each tissue, and this area ratio can be regarded as the volume ratio of each tissue. The base steel sheet used in the present invention satisfies the above-described structure conditions, and does not particularly limit the production conditions, and an example thereof will be described later.

Cu, Ni, and Ti have the following effects on the properties of the steel sheet, such as plating properties and mechanical properties, respectively.
(9) Cu: 0.003-0.5 mass%, Ni: 0.003-1.0 mass%
Cu and Ni are effective elements that can improve the strength of the steel sheet itself and improve the plating properties. In particular, Cu and Ni, which are more difficult to oxidize than Fe, are concentrated on the surface of the steel sheet, whereby the form of oxides of Si and Mn can be changed to prevent a decrease in plating performance. From the viewpoint of the effect of preventing such a decrease in plating property, an addition amount of 0.003% by mass or more is necessary, but an excessive addition amount causes an increase in cost and deterioration of workability. .5% by mass and 1.0% by mass.
(10) Ti: 0.003 to 1.0 mass%
Since Ti forms carbides, Ti is an element effective for increasing the strength of the steel sheet, and also has the effect of fixing C and N to the grain boundaries and increasing the r value of the steel sheet. In order to exert these effects, an addition amount of 0.003% by mass or more is necessary, and as in the case of Cu and Ni, an excessive addition amount causes an increase in cost and deterioration of workability. Is 1.0 mass%.

  By adding these Cu, Ni, and Ti in combination, the surface cleanliness of the steel sheet is improved, and when it is dissolved in the upper process, a complex oxide with Fe is formed to suppress the surface concentration of Si and Mn. And since it has the effect | action which improves metal-plating property, it is desirable to contain 1 type (s) or 2 or more types in total in 0.003-2.5 mass% among Cu, Ni, and Ti.

Furthermore, V, Nb, B, and Mo are elements that further improve characteristics such as plating properties and mechanical properties of the alloyed galvanized steel sheet, and have the following effects.
(11) V: 0.003 to 1.0 mass%, Nb: 0.003 to 1.0 mass%
V and Nb are elements that form carbides and are effective in increasing the strength of the steel sheet, as in the case of Ti, and an additive amount of 0.003% by mass or more is necessary to exert the effect. . An excessive addition amount causes an increase in cost and deterioration of workability, so the upper limit is made 1.0 mass%.
(12) Mo: 0.003 to 1.0 mass%
Mo is an effective element that can increase the strength by solid solution strengthening without impairing the plating property. In order to exert the effect, an addition amount of 0.003% by mass or more is necessary. An excessive addition amount causes an increase in cost, so the upper limit is made 1.0 mass%.
(13) B: 0.0002 to 0.1% by mass
B has the effect of improving the weldability and hardenability of the steel sheet. In order to effectively exhibit such an action, the addition amount is preferably 0.0002% by mass or more. However, an excessive addition amount not only saturates the above-mentioned improving effect, but also deteriorates ductility and lowers workability, so the upper limit of the addition amount is 0.1% by mass.

  These elements V, Nb, B, and Mo are slightly less capable of suppressing the surface oxidation of Si and Mn than Se and Sb, but are easily segregated at grain boundaries. It has the effect of uniformly controlling and reducing alloying irregularities and plating defects. In addition, since a synergistic effect between the element such as Se or Sb or an element such as Cu, Ni or Ti is also expected, a composite of an element such as Se or Sb or an element such as Cu, Ni or Ti. By the addition, effects such as improvement in plating properties, reduction in alloy unevenness, and increase in strength are increased.

And Ca and Mg exert the following actions.
(14) Ca: 0.0005 to 0.005 mass%, Mg: 0.0005 to 0.01 mass%
Ca has the effect | action which controls the form of the inclusion in steel, raises ductility, and improves workability. In order to effectively exhibit such an action, an addition amount of 0.0005% by mass or more is preferable. However, if added excessively, the amount of inclusions in the steel increases, ductility deteriorates, and workability decreases, so the upper limit is made 0.005% by mass. Also for Mg, the action and the preferable addition amount range are the same as in the case of Ca. Since Ca and Mg have a steel cleaning action, they also improve the cleanliness of the steel sheet surface and promote the application of a plating layer and the homogenization of the alloying treatment.

  Below, the suitable manufacturing method of this invention is described. A viewpoint of securing a finishing temperature of 800 to 950 ° C. and preventing coarsening of the austenite grain size so that a texture that inhibits workability is not formed in the steel slab having any chemical composition of the present invention. In order to suppress the formation of pearlite in the cooling zone after performing hot rolling after heating to 1000 to 1300 ° C and passing through the finish rolling mill, the cooling rate is in the range of 30 to 120 ° C / s. And cooled to 700 ° C. or lower. Here, the winding temperature is set to 700 ° C. or less because when the winding is performed at a temperature higher than 700 ° C., the scale on the surface of the steel sheet becomes thick and the pickling property is lowered. If the coiling temperature is too low, the hardness of the steel sheet increases and the cold rolling property decreases, so the lower limit temperature is preferably 250 ° C., and the lower limit temperature is preferably 400 ° C. or higher. After winding, the steel sheet is cold-rolled at a cold rolling rate (total rolling reduction) of 30% or more after the surface scale is removed by pickling. Here, if the cold rolling rate is 30% or more, if less than 30%, in order to obtain a cold rolled steel sheet having a desired thickness, the thickness of the hot rolled steel sheet used for cold rolling is reduced. This is because the steel plate becomes longer as the plate thickness becomes thinner, and inconveniences such as reduced productivity during pickling occur.

  The cold-rolled steel sheet is heated and held at a temperature of Ac1 point or higher before the plating process in a continuous hot dip galvanizing line and subjected to an annealing process. In order to reliably realize a desired structure by this annealing treatment and stabilize the workability after the plating treatment, the heating and holding temperature is preferably set to a heating and holding temperature that is about 50 ° C. higher than the Ac1 point. . This desirable heating and holding temperature is 780 ° C. or higher in the chemical composition range of the present invention. With respect to the upper limit of the heating and holding temperature, there is no particular problem as long as it is 900 ° C. or lower in the chemical composition range of the present invention from the viewpoint of preventing coarsening of austenite crystal grains. The heating and holding time is heating and holding in a high temperature range of 780 to 900 ° C., and so long as it is held for 10 seconds or more, the temperature is sufficiently soaked to obtain a ferrite + austenite structure. The steel plate soaked by this heating and holding is cooled at an average cooling rate of 1 ° C./s or more to the temperature of the hot dip galvanizing bath normally set to a temperature range of 440 to 470 ° C. The composition of the plating bath is not particularly limited, and a known hot dip galvanizing bath can be used. In addition, since Al is an element which acts on the control of the alloying speed of the hot dip galvanized layer, the Al content in the plating bath is preferably in the range of 0.05 to 0.2% by mass. If the Al content in the plating bath exceeds this range, alloying becomes insufficient. On the other hand, if the Al content is less than this range, alloying proceeds too much and the proportion of Fe in the galvanizing becomes too high. The alloying degree (Fe%) is not achieved, and the plating peel resistance is reduced. From the viewpoint of such an alloying degree and plating peel resistance, the Al content is more preferably in the range of 0.07 to 0.18% by mass.

  After the hot dip galvanizing treatment is performed, the austenite in the base steel sheet is transformed into martensite by cooling to room temperature at an average cooling rate of 1 ° C./s or more, and a mixture mainly composed of ferrite and martensite. You can get an organization. When the average cooling rate is less than 1 ° C./s, martensite is difficult to be generated, and there is a possibility that perlate and bainite (intermediate transformation structure) are generated. The average cooling rate is preferably 10 ° C./s or more. And when manufacturing an alloyed hot dip galvanized steel sheet, after performing the hot dip galvanizing process to the said steel plate, it is usually in the temperature range of 400-750 degreeC, Preferably it is in the range of 500-600 degreeC, 2- An alloying process is performed in which the plating layer is made into an alloy of Zn—Fe by heating and holding for 30 seconds. As a heating means for performing this alloying treatment, various commonly used heating means such as gas heating and induction heater heating can be used. After the alloying treatment, the steel sheet is cooled to an average cooling rate of 1 ° C./s or more and usually to room temperature, whereby a mixed structure mainly composed of ferrite and martensite can be obtained.

  In an galvannealed steel sheet using a steel sheet having a mixed structure mainly composed of ferrite and martensite as a base steel sheet, the tensile strength of the base steel sheet is 590 to 1470 MPa, and the balance between strength and ductility is balanced. Since it is good, this good characteristic is reflected, and the alloyed hot-dip galvanized steel sheet also has a good balance between strength and ductility. As a result, structural parts of automobiles are suitable for the use of this alloyed hot-dip galvanized steel sheet, including front and rear side members, crash boxes, bumper reinforcements, and impact absorbing parts such as door impact beams. It can be used as vehicle components such as pillars such as center pillar reinforcements, roof rail reinforcements, side sills, floor members, and kick parts.

  Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the following examples as well as the present invention, and appropriate modifications are made within a scope that fits the gist of the present invention. Of course, it is possible to implement, and any such modifications are included in the technical scope of the present invention.

  A steel having the chemical composition shown in Table 1 (the balance is Fe and inevitable impurities) is melted, and a slab obtained by continuously casting the molten steel is heated to 1150 ° C., and then hot-rolled, and the finishing temperature is After finishing a steel plate having a thickness of 2.6 mm at 870 to 900 ° C., the steel sheet was cooled at an average cooling rate of 40 ° C./s in a water cooling zone on the exit side of the finish rolling mill, and then wound at 480 ° C. The hot-rolled steel sheet was pickled and then cold-rolled to a sheet thickness of 1.4 mm at a cold rolling rate (total rolling reduction) of 46% to produce a cold-rolled steel sheet. From this cold-rolled steel sheet with a thickness of 1.4 mm, a sample having a sheet width of 100 mm and a length of 250 mm was cut out and the steel sheet surface was cleaned by pickling. → Alloying process ”was performed.

In the annealing treatment, heating was held for 120 seconds in a temperature range of 750 to 900 ° C. in a nitrogen atmosphere to which hydrogen gas was added at a flow rate ratio of 3%. At that time, the dew point of the atmosphere was adjusted to −30 ° C. After annealing, the steel sheet is cooled from the heating and holding temperature to an average cooling rate of 1 ° C./s or higher to a temperature of a hot dip galvanizing bath containing 0.13% by mass of Al, 460 ° C., an intruding steel plate temperature of 460 ° C., and an immersion time of 2 sec. The hot dip galvanizing process was performed. Immediately after the plating treatment, the alloying treatment was performed using an infrared heating furnace in the plating simulator at an alloying heating temperature of 550 ° C. and a heating time of 10 seconds. After this alloying treatment, it was cooled to room temperature at an average cooling rate of 1 ° C./s or more. The plating property and powdering resistance (plating peeling property) of the alloyed galvanized steel sheet thus prepared were evaluated by the following procedure.
(1) Evaluation of plating property The plating property was evaluated by visually observing the presence or absence of occurrence of these evaluation items, with the non-plated portion and the alloying unevenness as evaluation items, and the occurrence state was indicated by a code. The specific display contents of the symbols are as follows.
◎: No plating, no alloying unevenness ◎ ○: No plating, no alloying unevenness (less than 4% in area ratio) ○: No plating, no alloying unevenness (area) X: Occurrence of non-plated parts, occurrence of uneven alloying (10% or more in area ratio) (2) Evaluation of powdering resistance Powdering resistance has a bending angle of 60 °, Using a V-shaped punch with a bending radius of 1 mm, a V-bending test was performed, the amount of plating peeling inside the bent portion was measured, and the peeling state was indicated by a sign. The specific display contents of the symbols are as follows.
A: Plating peeling amount is 6 mg or less. ○: Plating peeling amount is 10 mg or less. X: Plating peeling amount exceeds 10 mg.

  Table 2 shows the evaluation results of the platability and powdering resistance of the alloyed galvanized steel sheets having the chemical compositions No. 1 to No. 22. As shown in Table 2, the alloyed galvanized steel sheets (No. 1 to No. 20) having the respective chemical compositions had no unplated portion in any of the alloyed galvanized steel sheets having any chemical composition. In addition, it has excellent powdering resistance. On the other hand, in the case of an alloyed galvanized steel sheet (No. 21, N0.22) that does not satisfy the chemical composition requirements of the present invention, both the plating property and the powdering resistance are inferior.

  Moreover, the structure evaluated in the center part of the plate width direction and the center part of a plate | board thickness direction of the alloyed galvanized steel plate (No. 1-No. 20) of each said chemical composition is ferrite, and the area ratio is 5-90. %, Martensite is in the range of 5 to 90%, and the total area ratio of ferrite and martensite is 95% or more, and is composed of a mixed structure mainly composed of ferrite and martensite.

Claims (4)

  C: 0.02-0.2 mass%, Mn: 1.5-3.5 mass%, Cr: 0.03-0.5 mass%, Al: 0.01-0.15 mass%, and Si : 0.04% by mass or less, P: 0.03% by mass or less, S: 0.03% by mass or less, and after performing hot dip galvanizing treatment on the steel sheet having the chemical composition of the remaining Fe and inevitable impurities An alloyed hot-dip galvanized steel sheet in which layers are alloyed, wherein the steel sheet having the chemical composition further contains one or more selected from Se, Sb, Zn, and Ba in a total of 0. An alloyed hot-dip galvanized steel sheet containing 0.001 to 0.2% by mass.   The steel plate is further selected from Cu: 0.003-0.5 mass%, Ni: 0.003-1.0 mass%, Ti: 0.003-1.0 mass%, The alloyed hot-dip galvanized steel sheet according to claim 1, wherein two or more kinds are contained in a total range of 0.003 to 2.5 mass%.   The steel plate is further provided with V: 0.003 to 1.0 mass%, Nb: 0.003 to 1.0 mass%, B: 0.0002 to 0.1 mass%, Mo: 0.003 to 1. The alloyed hot-dip galvanized steel sheet according to claim 1 or 2, comprising 0.001 to 1.0% by mass of one or more selected from 0% by mass.   The steel sheet further includes one or two of Ca: 0.0005 to 0.005 mass% and Mg: 0.0005 to 0.01 mass%. The alloyed hot-dip galvanized steel sheet according to any one of the above.