TW201414859A - Galvannealed steel sheet and producing method thereof - Google Patents

Abstract

A galvannealed steel sheet comprising: a scale-removed rolled steel sheet; and a galvannealed layer arranged on the scale-removed rolled steel sheet, wherein, when ten measurement points are selected in a transverse direction of the galvannealed steel sheet by equally dividing the line-segment with a reference length of 50 mm, minimum P content in the ten measurement points of the galvannealed layer is more than 50% as compared with maximum P content therein.

Description

Alloyed hot-dip galvanized steel sheet and manufacturing method thereof Field of invention

The present invention relates to an alloyed hot-dip galvanized steel sheet for press working mainly used in the automotive field, and a method for producing the same, that is, an alloyed hot-dip galvanized steel sheet for press working excellent in surface appearance and a method for producing the same.

Background of the invention

In recent years, from the viewpoint of preventing global warming, as a countermeasure against carbon dioxide emissions, it is required to upgrade fuels for automobiles, such as setting new targets for improving automobile fuels. For the improvement of automobile fuel, the weight reduction of the automobile body is an effective means, and from the viewpoint of the weight reduction, the steel plate for the automobile body is required to be thinned. On the other hand, from the viewpoint of ensuring the safety of the vehicle body, the steel sheet for an automobile body is required to have a high strength.

In addition to the demand for thinning and high strength of the above-mentioned steel sheet, the steel sheet for automobile body which is pressed into a complicated shape is also required to have excellent surface corrosion resistance and electrical coating property and excellent surface appearance.

In general, in order to increase the strength of the steel sheet in a high tensile steel sheet (high tensile steel), the steel contains solid solution strengthening elements such as Si (manganese), Mn (manganese), and P (phosphorus).

After the press working, the alloyed hot-dip galvanized steel sheet produced by the above-mentioned composition containing elements such as Si, Mn, and P often has surface defects such as lines or ribs on the surface, and remains traced after painting. It is not ideal in appearance, and it is a problem.

In response to the reduction of the surface defects, various countermeasures have been proposed so far, such as grinding a steel sheet (flat blank) before hot rolling, or grinding a hot-rolled steel sheet or a cold-rolled steel sheet before plating.

For example, as a method for producing an alloyed hot-dip galvanized steel sheet having a very low carbon steel sheet to which Ti (titanium) is added as a base material and having a small number of pattern defects on the plating surface, it is proposed to perform electromagnetic stirring in the mold during continuous casting. In order to prevent segregation of the slab component, the method of reducing the amount of slab casting and the amount of grinding of the steel sheet to prevent the ridge-like defect is greatly reduced (Patent Document 1). In addition, as a method for producing an alloyed hot-dip galvanized steel sheet having a high Si-based steel sheet or a high-P-based steel sheet as a base material and having excellent surface appearance, plating adhesion, and workability, it is proposed to grind a surface of a steel sheet to be plated. A method in which the surface roughness Ra: 0.3 to 0.6 is immersed in a hot-dip galvanizing bath and then subjected to a heating alloying treatment (Patent Document 2).

In general, in order to increase the strength of the steel sheet, P is added to the steel. However, P is an element which is very easy to segregate, and P which has been segregated on the surface of the flat embryo extends to the steel sheet by hot rolling and cold rolling. In the longitudinal direction, a P-concentrated layer is formed on the surface of the web. In the P-concentrated layer, the alloying during plating is delayed, which causes a linear flaw in the alloyed hot-dip galvanized steel sheet. In order to solve this problem, as a method for producing a alloyed hot-dip galvanized steel sheet having a steel sheet having a P content of 0.03% or more as a base material, there has been proposed a grinding amount in response to the unevenness of the surface of the steel sheet in response to the amount of P in the steel sheet. A method in which the surface of the steel sheet is ground and alloyed by an induction heating alloy furnace (Patent Document 3).

In addition, there is also a proposed steel plate to prevent pickling. A mountain-like pattern generated on the surface is produced by pickling a hot-rolled steel sheet by a normal method, followed by pickling, and dissolving the surface layer by 1.0 μm or more (Patent Document 4).

In the prior art, in order to prevent linear pattern defects of the alloyed hot-dip galvanized steel sheet, for example, when a Ti-added ultra-low carbon steel sheet having a P content of 0.03% or more is used, the surface is cut in the continuous casting casting stage. (melting) 3 mm or more was removed, and the surface was further ground to 5 μm or more in the steel sheet stage before plating. In order to prevent the occurrence of pattern-like defects after plating, the surface quality is ensured. When using a very low-carbon steel sheet with Ti added with a small P content, the surface is also cut (melted) by more than 3 mm at the slab stage, and after cold rolling, the surface of the steel sheet is ground to 2 μm or more by re-grinding and brushing, and acid In order to prevent the mountain pattern after washing, it is the status quo to melt to 1 μm or more. These are now the reasons for the reduced yield.

Prior technical literature Patent literature

Patent Document 1: Japanese Patent Laid-Open Publication No. 2004-149866

Patent Document 2: Japanese Patent Laid-Open Publication No. 2004-169160

Patent Document 3: Japanese Patent No. 2576329

Patent Document 4: Japanese Patent Laid-Open Publication No. 2005-281775

Summary of invention

An aspect of the present invention is an alloyed hot-dip galvanized steel sheet having a very low carbon steel as a basic component and a high tensile steel sheet containing a strengthening element P as a base material for improving workability, and the object of the present invention is to provide a press working. Rear An alloyed hot-dip galvanized steel sheet for press working which still exhibits a beautiful surface appearance and a method for producing the same. Further, an object of the present invention is to provide an alloyed hot-dip plating for press working which can reduce the amount of surface removal of a steel sheet to reduce the surface defects such as a linear pattern and obtain a beautiful surface appearance, and can optimize the amount of removal thereof. Zinc steel plate and its manufacturing method. That is, the steel sheet of one aspect of the present invention is also superior to the manufacturing cost.

The inventors of the present invention have intensively studied the cause of uneven P concentration which causes surface defects such as linear patterns on high-strength alloyed hot-dip galvanized steel sheets containing extremely low carbon steel as a basic component and containing a strengthening element P. The results obtained the following insights. When the hot-dip galvanized steel sheet is alloyed, the alloying speed in the alloying treatment is lowered at the P segregation at the surface portion of the steel sheet. Due to the difference in the alloying speed, the plating thickness may be staggered. The variation in the thickness of the plating layer may appear as a surface defect of a long-length pattern (line pattern) which appears to be white or appears to be ochre. Once the alloyed hot-dip galvanized steel sheet having the surface defect is subjected to press working, the convex portion forming the linear pattern on the surface of the steel sheet is removed, and the pattern is changed significantly. Further, in the hot-rolled steel sheet, P, Ni (nickel) and Cu (copper) are segregated at the same point at the interface between the scale (oxidized coating) and the steel, and the segregation portion is not acidified even if the pickling step is performed. Wash and survive. As a result, surface defects such as linear patterns also become remarkable.

Therefore, in order to prevent surface defects which are linear patterns from being formed during the alloying treatment after plating, the surface properties may be removed by removing the segregation portions of the elements segregated at the interface between the scale and the steel after the hot rolling. The harmful P segregation part makes it harmless.

The gist of the present invention is as follows.

(1) An alloyed hot-dip galvanized steel sheet having a descaling rolled steel sheet and an alloyed hot-dip galvanized layer disposed on the descaling rolled steel sheet, the descaling rolled steel sheet The chemical component is, in mass%, C: 0.0005% to 0.01%, Si: 0.001% to 1.0%, Mn: 0.01% to 2.0%, P: 0.005% to 0.1%, and Al: 0.01% to 0.10%, And limiting S: 0.02% or less, Ni: 0.1% or less, Cu: 0.1% or less, and N: 0.01% or less, and the residual portion is composed of Fe and unavoidable impurities; when by the aforementioned alloyed hot-dip galvanized steel sheet In the direction of the plate width, when the line segment of the reference length of 50 mm is divided into 10 equal parts and 10 measurement points are set, the minimum value of the P content of the plating layer is compared with the maximum value of the P content in the above 10 measurement points. above 50.

(2) The alloyed hot-dip galvanized steel sheet according to the above (1), wherein the chemical component of the scale-removed steel sheet may further contain at least one of the following elements: B: 0.0001% ~0.0050%, Nb: 0.001% to 0.1%, Ti: 0.001% to 0.1%, and Mo: 0.001% to 0.1%.

(3) In the alloyed hot-dip galvanized steel sheet according to the above (2), the line portion of the reference length of 50 mm is divided into 10 equal portions and set at 10 points in the direction of the sheet width of the rolled steel sheet. At each measurement point of the above-mentioned 10 measurement points of the above-mentioned descaling rolled steel sheet, the surface of the surface of the descaling rolled steel sheet having a depth of 0.1 μm from the surface in the thickness direction of the steel sheet is rolled. The content of Ni, Cu, and the content of P, Ni, and Cu in the base material of the descaling rolled steel sheet exceeding the depth of 2 μm from the surface in the direction of the sheet thickness, each component may be 105% or more and 150% or less. .

(4) In the alloyed hot-dip galvanized steel sheet according to the above (1), the line portion of the reference length of 50 mm is divided into 10 equal portions and set at 10 points in the direction of the sheet width of the rolled steel sheet. At each measurement point of the above-mentioned 10 measurement points of the above-mentioned descaling rolled steel sheet, the surface of the surface of the descaling rolled steel sheet having a depth of 0.1 μm from the surface in the thickness direction of the steel sheet is rolled. The content of Ni, Cu, and the content of P, Ni, and Cu in the base material of the descaling rolled steel sheet exceeding the depth of 2 μm from the surface in the direction of the sheet thickness, each component may be 105% or more and 150% or less. .

(5) A method for producing an alloyed hot-dip galvanized steel sheet according to one aspect of the present invention, wherein the chemical component is in a mass%, containing: C: 0.0005% to 0.01%, Si: 0.001% to 1.0%, Mn: 0.01% ~2.0%, P: 0.005%~0.1%, and Al: 0.01%~0.10%; and S: 0.02% or less, Ni: 0.1% or less, Cu: 0.1% or less, and N: 0.01% or less; a molten steel partially composed of Fe and unavoidable impurities, performing the following steps: casting step, casting molten steel to obtain a flat embryo; heating step, heating the aforementioned flat embryo at 1100 to 1300 ° C; hot rolling step, at the end The flattened embryo after the heating step is hot rolled to obtain a hot rolled steel sheet at a temperature of 800 ° C or higher and 1050 ° C or lower and a coiling temperature of 500 ° C or higher and 800 ° C or lower; In the plate width direction of the hot-rolled steel sheet, 10 line sections of the reference length of 50 mm are divided into 10 equal parts, and the measurement points are set, and the interface between the self-rust skin and the steel of the hot-rolled steel sheet is set to be in the thickness direction of the steel. The Ni and Cu contents of the steel surface portion having a depth of 2 μm on the side, and the maximum value among the above-mentioned 10 measurement points is N by mass%. In the case of i _max and Cu _max , the heat is applied to the steel side in the thickness direction in the range of the thickness of the GL expressed by the following formula 1 and not more than the GU represented by the following formula 2 in the thickness direction. a surface of the rolled steel sheet is removed to obtain a rust-removed rolled steel sheet; and a plating step is performed to obtain a hot-dip galvanized steel sheet by performing hot-dip galvanizing on the rust-removed rolled steel sheet after the surface removing step; and an alloying step, The alloyed hot-dip galvanized steel sheet is obtained by subjecting the hot-dip galvanized steel sheet after the plating step to an alloying heat treatment.

GL = (Ni _max + 0.8 × Cu _max ) × 0.2 (...)

GU=(Ni _max +0.8×Cu _max )×4.........(Equation 2)

(6) The method for producing an alloyed hot-dip galvanized steel sheet according to the above (5), wherein the chemical component of the molten steel is at least one of the following elements in terms of % by mass: B: 0.0001% ~0.0050%, Nb: 0.001% to 0.1%, Ti: 0.001% to 0.1%, and Mo: 0.001% to 0.1%.

(7) The method for producing an alloyed hot-dip galvanized steel sheet according to the above (6), wherein at least one of the surface removal step or the surface removal step may be subjected to pickling to remove the scale The pickling step of the surface of the steel sheet.

(8) The method for producing an alloyed hot-dip galvanized steel sheet according to the above (5), which may have at least one of the surface removal step or the surface removal step, and may have pickling and removing the scale The pickling step of the surface of the steel sheet.

(9) The method for producing an alloyed hot-dip galvanized steel sheet according to any one of the above (5) to (8), further comprising the step of: cold rolling step of rolling the descaling surface before the plating step The steel sheet is further subjected to cold rolling at a cold rolling ratio of 50% or more and 95% or less; and an annealing step before the aforementioned cold rolling step The rust-rolled rolled steel sheet is annealed at a temperature equal to or higher than the recrystallization temperature.

The alloyed hot-dip galvanized steel sheet according to the above aspect of the present invention satisfies the mechanical properties such as tensile strength, and has excellent workability and a plating layer having few surface defects such as a line pattern, and can maintain a beautiful surface even after press working. Exterior. Further, it is possible to reduce the amount of surface removal of the hot-rolled steel sheet to reduce surface defects such as linear patterns, and to optimize the amount of removal to produce an alloyed hot-dip galvanized steel sheet, thereby reducing steel loss. , play a significant effect better than cost.

Form for implementing the invention

Hereinafter, preferred embodiments of the present invention will be described in detail.

An alloyed hot-dip galvanized steel sheet according to an embodiment of the present invention has an alloyed hot-dip galvanized layer disposed on a scale-removed steel sheet. Here, the scale-removed steel sheet is defined as a rolled steel sheet which is subjected to surface removal by a surface removal step which will be described later. In order to produce an alloyed hot-dip galvanized steel sheet having excellent surface properties (surface appearance), the P content in the above-mentioned plating layer is small. Specifically, when the line segment of the reference length of 50 mm is divided into 10 equal parts in the plate width direction of the alloyed hot-dip galvanized steel sheet to provide 10 measurement points at equal intervals, the plating is performed at the 10 measurement points. The minimum value of the P content of the layer must be above 50% (and below 100%) as compared to the maximum value of the P content.

When the minimum value of the P content of the plating layer and the maximum value of the P content of the plating layer are less than 50% in the above-mentioned 10 measurement points, the difference in alloying speed during the alloying heat treatment after the plating step Will become significant. the result, The surface defects of the linear pattern of the alloyed hot-dip galvanized steel sheet also become remarkable. Therefore, the P content in the plating layer must satisfy the above conditions. Preferably, in the above-mentioned 10 measurement points, the minimum value of the P content of the plating layer is 60% or more as compared with the maximum value of the P content of the plating layer.

The measurement of the P content in the plating layer can be carried out by using a Glow Discharge Spectroscopy (GDS) or the like. In the plate width direction of the alloyed hot-dip galvanized steel sheet, a line segment having a reference length of 50 mm is divided into 10 equal portions, and 10 measurement points at equal intervals are provided, and the P content can be measured by GDS at each measurement point.

Further, in order to produce an alloyed hot-dip galvanized steel sheet having excellent surface properties (surface appearance), the thickness of the alloyed hot-dip galvanized layer is preferably small. Specifically, in the above-mentioned 10 measurement points, the minimum value of the thickness of the plating layer is preferably 50% or more (and 100% or less) as compared with the maximum value of the thickness.

When the minimum value of the thickness of the plating layer and the maximum thickness of the plating layer are less than 50% in the above-mentioned 10 measurement points, it is feared that the plating layer is removed during the press working of the alloyed hot-dip galvanized steel sheet. The thick surface of the middle makes the surface defects of the linear pattern also significant. Therefore, it is preferable that the thickness of the plating layer satisfies the above conditions. Preferably, in the above-mentioned 10 measurement points, the minimum value of the thickness of the plating layer is 60% or more as compared with the maximum value of the thickness of the plating layer.

It is considered that the surface defects are linearly formed parallel to the rolling direction, and the plane width direction orthogonal to the rolling direction of the alloyed hot-dip galvanized steel sheet is the observation surface, and the plane cutting is performed along the thickness direction. The thickness of the plating layer may be measured on the cut surface. And by the plated steel sheet In the direction of the plate width, 10 line portions having a reference length of 50 mm are divided into 10 equal-part measurement points at equal intervals, and the cut surface is observed at each measurement point to measure the thickness of the plating layer. The cut surface observation is performed, for example, at a magnification in which the observation field is the plate width direction and about 1000 μm.

In order to obtain the technical configuration of the alloyed hot-dip galvanized steel sheet according to the present embodiment, the above-described descaling rolled steel sheet of the alloyed hot-dip galvanized steel sheet base material preferably has no segregation of P, Ni and Cu on the surface of the steel sheet. Specifically, when 10 line portions having a reference length of 50 mm are divided into 10 equal portions and 10 measurement points are equally spaced in the plate width direction of the rolled steel sheet, the measurement points at the 10 measurement points are determined. The content of P, Ni, and Cu on the surface of the steel sheet of the rust-removed steel sheet having a depth of 0.1 μm from the surface in the direction of the thickness of the rust-removed steel sheet, and the rust removal exceeding 2 μm from the surface in the direction of the sheet thickness The content of P, Ni and Cu in the base material of the rolled rolled steel sheet is preferably 105% or more and 150% or less.

The content of P, Ni and Cu in the surface portion of the above-mentioned descaling rolled steel sheet is compared with the contents of P, Ni and Cu in the base material portion of the above-mentioned descaling rolled steel sheet, and each component exceeds 150% even if descaling is performed. The pickling of the rolled steel sheet still leaves the segregation portions of P, Ni and Cu on the surface of the steel sheet, and the surface defects of the linear pattern of the alloyed molten zinc steel sheet are remarkable. Further, when the value is less than 105%, the amount of surface removal of the scale-removed steel sheet is excessive, which causes a burden on the time or equipment for surface removal, and further reduces the yield of the steel material. Therefore, the content of P, Ni, and Cu on the surface portion of the steel sheet for rolling removal must satisfy the above conditions. More preferably, the above range is 110% or more and 130% or less.

The measurement of the P, Ni and Cu contents of the above-mentioned descaled rolled steel sheet can be carried out by GDS. In this case, the average value of the steel sheet rolled from the surface to 0.1 μm in the thickness direction of the steel sheet is taken as the measurement result of the surface portion of the rolled steel sheet, and the average value of the measurement is more than 2 μm from the surface. As a result of measurement of the base material part of the scale-removed steel sheet. On the other hand, the GDS of the scale-removed steel sheet base portion has a measurement depth of more than 2 μm and preferably 4 μm or less.

The following is a detailed description of the development process to arrive at the above-mentioned technical composition.

In order to improve the fuel consumption of automobiles, it is required to reduce the thickness of the steel sheet, and to ensure the high strength of the steel sheet for the safety of the automobile body. Moreover, the steel sheet for automobile bodies also requires excellent surface appearance and good press formability.

In order to increase the strength of the steel sheet, a steel sheet containing P is used as the steel sheet to be plated. However, P is an element which is very easy to segregate, and P which has been segregated on the surface of the flat embryo extends in the longitudinal direction of the steel sheet by hot rolling and cold rolling, and forms a linear P segregation portion on the surface of the steel sheet. When alloying hot-dip galvanizing is applied to the steel sheet, unevenness occurs in the plating rate of the plating in the P segregation portion, and irregularities are formed on the surface of the alloyed hot-dip galvanized steel sheet. As a result, surface defects of the line pattern are produced. Further, once the plated steel sheet is subjected to press working, the linear pattern is more remarkable in order to remove the convex portion.

The inventors of the present invention have intensively studied the cause of surface defects such as linear patterns by using alloyed hot-dip galvanized steel sheets having extremely low carbon steel as a basic component and using a high tensile hot-rolled steel sheet containing a strengthening element P. As a result, it was learned that at the interface between the scale of the hot-rolled steel sheet and the steel, once the P, Ni, and Cu segregate at the same place, the segregation portion remains even after the pickling step, and during the alloying treatment after the plating, The plating thickness in the segregation portion is generated by the variation Surface defects in linear patterns.

The segregation mechanism of P, Ni, and Cu which is a cause of surface defects causing a linear pattern on the surface of the alloyed hot-dip galvanized steel sheet is considered to be a mechanism as described below.

In general, the alloyed hot-dip galvanized steel sheet is heated by a heating furnace to continuously cast the flat embryo, and after being derusted, hot rolled and coiled to form a hot rolled coil, and the hot rolled steel sheet is required according to the demand. It is produced by cold rolling and annealing, and subjected to alloying hot-dip galvanizing treatment.

In the heating step of the slab, the continuous casting flat embryo containing P, Ni and Cu is heated in a heating furnace at 1100 to 1300 ° C to oxidize Fe (iron) on the surface of the flat embryo to a primary scale. However, since Ni and Cu of the steel component are more difficult to oxidize than Fe, Ni and Cu are not oxidized and segregate at the interface between the primary scale and the steel.

Next, the scale is removed once by the descaling (rust removal) according to the demand, and Ni and Cu which are segregated on the steel surface are not removed and remain. When the flat embryo is hot rolled in the hot rolling step, the Ni and Cu segregation portions extend in the longitudinal direction of the steel sheet, and the thickness of the Ni and Cu segregation portions becomes thin. On the other hand, by the oxidation of the surface of the steel sheet which is subjected to hot rolling, the scale is generated twice, and Ni and Cu are more segregated on the steel surface.

Moreover, at the time of coiling after hot rolling, P is segregated at the interface or grain boundary between the scale and the steel. When P segregates into the same region as Ni and Cu, even if the pickling step is performed, the P cannot be removed and remains on the steel surface portion.

Once the hot-rolled steel sheet is cold-rolled and annealed according to the demand, and then alloyed by hot-dip galvanizing, a linear pattern is produced. Face defects. The portion where the surface defect is generated is a portion where P, Ni, and Cu are mixed in segregation. From this, it can be judged that the surface defects of the linear pattern are caused not only by the segregation of P but also by the segregation of Ni, Cu, and P on the surface portion.

Accordingly, a statistical review was conducted on various steel sheets in order to reduce the P segregation portion which has been segregated on the surface of the steel sheet. The residue of P is also a part of segregation of Ni and Cu, and focuses on the Ni and Cu segregation portions at the interface between the scale of the hot-rolled steel sheet and the steel. This result clearly shows that the amount of surface removal required is proportionally increased in order to make P harmless in response to the Ni and Cu contents of the steel surface portion on the steel side from the above interface. Specifically, it is understood that, by the direction of the width direction of the hot-rolled steel sheet, 10 line portions having a reference length of 50 mm are divided into 10 equal-part measurement points at equal intervals, and the interface between the self-rust skin and the steel of the hot-rolled steel sheet is obtained. The content of Ni and Cu in the surface portion of the steel having a depth of 2 μm from the steel side in the thickness direction of the steel sheet is expressed by the following formula A in terms of unit μm when the maximum value of the above-mentioned 10 measurement points is Ni _max and Cu _max by mass %. In the range of GL or more and represented by the following formula B, the surface of the hot-rolled steel sheet is removed from the steel side in the thickness direction in the range of the thickness of the above-mentioned interface to form a descaled steel sheet. In addition to the segregation portion of Ni and Cu, the P segregation portion can be removed to make it harmless. In other words, since the segregation portions of P, Ni, and Cu cannot be removed by the pickling step, it is important to remove the surface having the removal amount within the above range in order to remove the segregation portion. Further, the surface removal amount has been optimized, and the P segregation portion can be reliably removed even though the amount of removal is small.

GL=(Ni _max +0.8×Cu _max )×0.2......(Formula A)

GU=(Ni _max +0.8×Cu _max )×4.........(Formula B)

Descaling rolled steel sheet which has been subjected to the surface removal as a substrate By alloying hot-dip galvanizing, an alloyed hot-dip galvanized steel sheet having a plating layer having surface defects such as a random pattern and maintaining a beautiful surface appearance even after press working can be obtained. On the other hand, even if the rust-removed steel sheet which has been subjected to the surface removal is subjected to a cold rolling step or an annealing step, and then alloyed hot-dip galvanizing is performed, the same effect as described above can be obtained.

Next, the component elements of the descaling rolled steel sheet which becomes the alloyed hot-dip galvanized steel sheet base material in this embodiment are demonstrated. Here, the symbol % described is % by mass.

As a steel sheet for automobiles, it is necessary to satisfy press formability such as high tensile strength and deep drawability. In order to improve the workability of the rust-removed rolled steel sheet which is a base material of the alloyed hot-dip galvanized steel sheet in the present embodiment, a high tensile force such as Si, Mn, and P containing a reinforcing element is added as an essential component of extremely low carbon steel. Steel plate. The reason for adding the basic component elements and the reasons for their limitation will be described below.

C: 0.0005~0.01%

C (carbon) is an element which reduces the elongation of the press workability and the r value (Lankford value). The C content is preferably small, but in order to reduce it to less than 0.0005%, the cost of the steelmaking process is consumed, which is not practical enough. On the other hand, once it exceeds 0.01%, the r value of the processability index is impaired, so the upper limit is made 0.01%. The upper limit of the ideal system is 0.008%.

Si: 0.001~1.0%

Si (矽) is an element that increases the strength of steel and is used in combination with other strengthening elements. When the Si content is less than 0.001%, the above effects cannot be obtained. On the other hand, when the Si content exceeds 1.0%, Si oxide is formed on the surface of the steel sheet, and unplating or the plating adhesion is lowered during hot-dip galvanizing, and the processability is referred to. The elongation or r value of the standard is lowered. Further, in order to further increase the tensile strength, it is preferably 0.1% or more.

Mn: 0.01~2.0%

Mn (manganese) is an element that increases the strength of steel and is used in combination with other strengthening elements. When the Mn content is less than 0.01%, the above effects cannot be obtained, and the refining cost is increased, so that the lower limit is made 0.01%. On the other hand, when the content exceeds 2.0%, the steel sheet is hardened to lower the r value of the workability index, and Mn oxide is formed on the surface of the steel sheet to impair the hot-dip galvanizing property. Therefore, the upper limit of the Mn content is 2.0%. Further, in order to further increase the tensile strength, it is preferably 0.15% or more.

P: 0.005~0.1%

P (phosphorus) is an element that has a high ability to increase the strength of steel. Compared with Si and Mn, it has less adverse effects on workability and contributes to the strengthening of steel. When the P content is less than 0.005%, the effect cannot be obtained. In order to further increase the tensile strength, it is preferably 0.01% or more. On the other hand, the P-based element which causes the alloying reaction of the hot-dip galvanization to be delayed is also an element which causes a linear pattern on the plating surface to deteriorate the surface properties and also adversely affects the spot weldability. Therefore, the upper limit of the P content is made 0.1%.

Al: 0.01~0.10%

Al (aluminum) is an element that contains a deoxidizing element of steel and which increases the strength of the steel. When the Al content is less than 0.01%, the above effects are not obtained, and the deoxidation is insufficient to retain the oxide, and the processability of the steel cannot be obtained. Further, when the Al content exceeds 0.10%, the r value of the workability index is lowered, so the upper limit is made 0.10%.

In addition to the above basic elements, at least one of B, Nb, Ti, and Mo may be further included as a selection element. The reason for adding the selection element and the reason for its limitation are explained below. Here, the symbol % described is % by mass.

B: 0.0001~0.0050%

B (boron) has a strong affinity with N (nitrogen), can form nitride at the time of solidification or hot rolling, reduces the N dissolved in steel, improves the workability and improves the strength of the steel. In order to obtain such effects, the B content is preferably 0.0001% or more. However, when the B content exceeds 0.0050%, the welded portion and the heat-affected portion thereof are hardened at the time of welding, and the toughness is deteriorated. In addition, the strength of the hot-rolled steel sheet will also become high, increasing the load of the cold rolling delay. Further, as the recrystallization temperature is increased, the in-plane anisotropy of the r value of the workability index is increased to deteriorate the press formability. Therefore, the B content is preferably 0.0001 to 0.0050%. On the other hand, if the B content is from 0% to 0.0050%, it will not adversely affect the properties of the alloyed hot-dip galvanized steel sheet.

Nb: 0.001~0.1%

Nb (铌) has strong affinity with C and N, and can form carbonitrides during solidification or hot rolling, reducing C and N which are solid-solubilized in steel, improving workability and increasing steel strength. In order to obtain such effects, the Nb content is preferably 0.001% or more. However, when the Nb content exceeds 0.1%, the recrystallization temperature becomes high, and the in-plane anisotropy of the r value of the workability index is increased to deteriorate the press formability. Moreover, the toughness of the welded portion is also deteriorated. Therefore, the Nb content is preferably 0.001 to 0.1%. On the other hand, if the Nb content is 0% to 0.1%, it will not adversely affect the characteristic values of the alloyed hot-dip galvanized steel sheet.

Ti: 0.001~0.1%

Ti is an element which improves the workability and increases the strength of steel by fixing N in the steel with TiN and reducing the amount of solid solution N. In order to obtain such effects, the Ti content is preferably 0.001% or more. However, even if the Ti content is more than 0.1%, the effect is saturated, and TiC is formed to deteriorate the r value of the workability index. Therefore, the Ti content is preferably 0.001 to 0.1%. It is more desirable to be 0.015% to 0.09%. On the other hand, if the Ti content is 0% to 0.1%, it will not adversely affect the respective characteristic values of the alloyed hot-dip galvanized steel sheet.

Mo: 0.001~0.1%

Mo is added in a small amount to suppress aging, and an element which delays aging is obtained. In order to obtain this effect, the Mo content is preferably 0.001% or more. However, even if the Mo content is more than 0.1%, the effect is not only saturated, but the steel sheet is hardened and the workability is lowered. Therefore, it is preferred that the Mo content is 0.001 to 0.1%. On the other hand, if the Mo content is 0% to 0.1%, it will not adversely affect the respective characteristic values of the alloyed hot-dip galvanized steel sheet.

In the descaling rolled steel sheet which is a base material of the alloyed hot-dip galvanized steel sheet according to the present embodiment, in addition to the above components, unavoidable impurities may be contained. Here, the unavoidable impurities are such as an auxiliary material such as scrap or an element such as S, Ni, Cu, N, Mg, Pb, Sb, Sn, or Cd which is inevitably mixed in from the plating step. In order to fully exert the effects of the present invention, S, Ni, Cu, and N are preferably limited as follows. The content of such impurities is preferably a small amount, and thus the content of the impurities is limited to 0%. Here, the symbol % described is % by mass.

S: 0.02% or less

Impurities contained in S (sulphur) steels cannot be avoided. Once the S content exceeds 0.02% will cause the r value of the deep pull index to decrease. As long as the S content is limited to 0% or more and 0.02% or less, there is no substantial adverse effect, and it is an allowable range.

Ni: 0.1% or less

Ni is an element which is difficult to remove when adjusting the steel composition in steel making, and may be contained in a small amount (for example, 0.001% or more). However, if it exceeds 0.1%, it is easy to form a pattern on the hot-dip galvanized steel sheet, so it is limited to 0% or more and 0.1%. the following. In addition, when a large amount is added, it is necessary to intentionally add a high-priced Ni to incur an increase in cost, so the upper limit is made 0.1%.

Cu: 0.1% or less

Similarly to Ni, Cu is an element which is difficult to remove when adjusting the steel composition in steelmaking, and may be contained in a small amount (for example, 0.001% or more). However, if it exceeds 0.1%, it is easy to produce a pattern on the hot-dip galvanized steel sheet, and it also causes grain. The boundary is embrittled or the cost is increased, so it is limited to 0% or more and 0.1% or less.

N: 0.01% or less

Impurities contained in N-series steel cannot be avoided. Once the N content exceeds 0.01%, the deep drawability index, ie, the r value, is lowered. As long as the N content is limited to 0% or more and 0.01% or less, there is no substantial adverse effect and it is an allowable range.

Next, a method of producing the alloyed hot-dip galvanized steel sheet according to the embodiment will be described.

As a casting step, a flat embryo can be obtained by casting a molten steel satisfying the above chemical composition. The casting method is not particularly limited, and a vacuum casting method, a continuous casting method, or the like may be used.

As a heating step, the flat embryo is heated at 1100 to 1300 °C. The reason why the flat embryo is heated at 1100 to 1300 ° C is that the load at the hot rolling is high and the desired hot rolling final temperature cannot be ensured because it is lower than 1100 ° C. On the other hand, heating exceeding 1300 ° C causes excessive use of energy and incurs an increase in cost. Further, after the heating step, a descaling (rust removal) for removing the primary scale of the surface of the flat embryo may be performed as a primary scale removing step.

As the hot rolling step, the heated flattened billet is hot rolled at a final temperature of 800 ° C or more and 1050 ° C or less and a coiling temperature of 500 ° C or more and 800 ° C or less to obtain a hot rolled steel sheet. In the hot rolling, when the temperature is lower than 800 ° C, the mixed structure becomes a cause of material variation, and the rolling temperature is too low, so that the strength is increased, and the r value of the workability index is deteriorated. On the other hand, in order to achieve a final temperature of 1050 ° C or higher, it is necessary to make the heating temperature high, which causes an increase in cost and also causes a decrease in strength. Therefore, the final temperature of the hot rolling is made 800 ° C or more and 1050 ° C or less.

The coiling temperature is lower than the cause of the shape of the 500° CC system. On the other hand, if the coiling is performed at more than 800 ° C, it is easy to cause scale scars. Therefore, the coiling temperature is made 500 ° C or more and 800 ° C or less. Further, depending on the demand, pickling may be performed by pickling to remove the scale of the hot-rolled steel sheet as a pickling step after the hot rolling step and before the surface removing step described later.

The surface removal of the above-mentioned hot-rolled steel sheet is performed as a surface removal step. Specifically, in the plate width direction of the hot-rolled steel sheet, 10 line portions having a reference length of 50 mm are divided into 10 equal-part measurement points at equal intervals, and the interface between the self-rust skin and the steel of the hot-rolled steel sheet is increased. The Ni and Cu contents in the steel surface portion having a depth of 2 μm from the steel side in the thickness direction are expressed by the following formula C in terms of unit μm when the maximum value among the above 10 measurement points is Ni _max and Cu _max by mass % In the range of GL or more and less than or equal to GU represented by the following formula D, the surface of the hot-rolled steel sheet is removed from the steel side in the thickness direction on the basis of the above-described interface to obtain a rust-removed rolled steel sheet.

GL=(Ni _max +0.8×Cu _max )×0.2......(Formula C)

GU=(Ni _max +0.8×Cu _max )×4.........(Formula D)

The surface removal method is simple for mechanical processing, and for example, a wire brush roll, an abrasive grain belt, a sand blast method, or the like can be used, and any means can be used as long as the above amount can be removed.

When the surface removal amount is lower than GL in units of μm, the segregation portions of P, Ni, and Cu remain in the steel surface portion. When the removal exceeds GU in units of μm, the time required for removal and the burden on the equipment are incurred, which incurs an increase in cost, which in turn leads to a decrease in the yield of the steel.

The Ni and Cu contents of the steel surface portion can be measured by a glow discharge luminescence spectrometer (GDS) and an electron probe microanalysis (EPMA: Electron Probe Micro Analyzer). In the plate width direction of the hot-rolled steel sheet, 10 line portions having a reference length of 50 mm are divided into 10 equal-part measurement points at equal intervals, and the Ni and Cu contents can be measured by GDS or EPMA at each measurement point. . When the analysis is performed by GDS, it is preferable to remove the surface scale of the sample for GDS measurement in advance from the viewpoint of shortening the measurement time, and then perform GDS measurement. Moreover, when the analysis is performed by EPMA, the cut surface which is planarly cut along the thickness direction is polished so that the plate width direction orthogonal to the rolling direction of the hot-rolled steel sheet is the observation surface, and the cutting is performed. The EPMA measurement of the surface can be performed.

Further, depending on the demand, the surface of the steel sheet after the surface removal step is rolled and pickled to remove the pickling step. The pickling method is not particularly limited, and a pickling method using a usual method such as sulfuric acid or hydrochloric acid may be used. Further, as described above, it is preferred to carry out a pickling step of pickling and descaling the surface of the steel sheet after the hot rolling step and before the surface removing step, or after the surface removing step and before the plating step described later. Since the segregation portions of P, Ni, and Cu cannot be removed in the pickling step, in order to remove the segregation portion, it is necessary to remove the surface in such a range that the removal amount is within the above range. However, it is preferable to carry out the above-described pickling step to improve the adhesion between the descaled rolled steel sheet and the alloyed hot-dip galvanized layer.

In general, in the heating step and the hot rolling step, P, Ni, and Cu are segregated at the interface between the steel sheet and the steel. Further, the segregation portion is extended in the longitudinal direction of the steel sheet by hot rolling to form linear segregation portions of P, Ni, and Cu. Further, in the coiling of the hot rolling step, P is further segregated at the interface between the scale and the steel. Since Ni and Cu cannot be removed by pickling, once the Ni and Cu segregation portions remain in the steel sheet after the surface removal step, even if the pickling step is performed, the Ni and Cu segregation portions remain on the surface of the steel sheet. The P present in the Ni and Cu segregation sections delays the alloying reaction during the alloying step, resulting in surface defects of the linear pattern. When P is segregated separately, P can be removed in the pickling step to be harmless, and no surface defects of the linear pattern are produced.

In the method for producing an alloyed hot-dip galvanized steel sheet according to the present embodiment, the surface of the hot-rolled steel sheet is most suitably removed, and P existing in the Ni and Cu segregation portions can be removed and detoxified. The surface removal amount is Those who have been optimized have been able to surely remove the P segregation portion even though they are smaller than the conventional removal amount.

Further, depending on the demand, the descaling rolled steel sheet after the surface removing step or the descaling rolled steel sheet after the pickling step may be cold rolled at a cold rolling ratio of 50% or more and 95% or less. Cold rolling step. By performing cold rolling of 50% or more and 95% or less, it is preferable to ensure the r value and ensure workability, and to control the rolled steel sheet to a desired thickness. Once the cold rolling rate is less than 50%, it is necessary to increase the length of the coil of the hot-rolled steel sheet in the hot rolling step, which may cause an increase in cost on the equipment. On the other hand, if the cold rolling ratio is more than 95%, a cold rolling mill capable of withstanding a high load is required, which may cause an increase in cost. On the other hand, in the two steps of the pickling step and the cold rolling step after the surface removing step, the order of the steps may be the surface removing step, the pickling step, and the cold rolling step.

Further, depending on the demand, the steel sheet after the cold rolling step is annealed at a temperature higher than the recrystallization temperature as an annealing step. By annealing at a temperature equal to or higher than the recrystallization temperature, strain generated by rolling can be removed, and softening can be performed to improve workability. Further, as described above, even if the cold rolling step or the annealing step is applied to the descaling rolled steel sheet in accordance with the demand, the effect of the present invention can be obtained invariably.

Next, as a plating step, the rust-rolled steel sheet is subjected to hot-dip galvanizing after the surface removing step, the pickling step, the cold rolling step, or the annealing step, to obtain a hot-dip galvanized steel sheet. On the other hand, in the annealing step, it is preferred that the descaling rolled steel sheet is cooled to room temperature between the annealing step and the plating step, and continuously treated in a continuous annealing furnace.

As the alloying step, the alloyed hot-dip galvanized steel sheet is produced by subjecting the hot-dip galvanized steel sheet after the plating step to alloying heat treatment. At this time, it is preferred that the molten galvanized steel sheet is not cooled and continuously treated between the plating step and the alloying step.

Example 1

Hereinafter, the effects of one aspect of the present invention will be further specifically described by way of examples, and the conditions in the examples are examples of conditions for confirming the practicability and effects of the present invention, and the present invention is not limited by the conditions. The present invention can adopt various conditions without departing from the gist of the present invention and up to the object of the invention.

Alloyed hot-dip galvanized steel sheets were produced under the steel compositions shown in Table 1 and the manufacturing conditions shown in Tables 2 and 3. Specifically, a test piece slab (flat embryo) composed of the steel shown in Table 1 was produced by continuous casting. After the slab is heated and held in a heating furnace (heating step), it is taken out for descaling (rust removal), and is subjected to hot rolling at the final hot rolling temperature and coiling temperature shown in Table 2 ( Hot rolling step). The surface of the hot-rolled steel sheet which has been subjected to the hot rolling is pickled according to the demand (the pickling step), and the surface of the hot-rolled steel sheet is removed to form a scale-removed steel sheet (surface removal step). Thereafter, the surface is cleaned by pickling in response to demand (pickling step). The descaled rolled steel sheet is cold rolled and expanded to a predetermined thickness according to the demand (cold rolling step), and annealed in a continuous annealing furnace (annealing step). Then, it is immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing (plating step), and alloying treatment (alloying step) is performed to obtain a alloyed hot-dip galvanized steel sheet. In Tables 2 and 3, for example, the case where pickling is performed is expressed as C.O. (Carrying Out), and the case where no pickling is performed is indicated. Is Not C.O. (Not Carrying Out). The other steps are also expressed in the same way.

However, the residual portion of the components in the steel composition shown in Table 1 is Fe and unavoidable impurities. Also, the bottom line in the table is outside the scope of the present invention.

Further, Table 2 shows the processing conditions in the surface removal (surface removal step) of the hot-rolled steel sheet. Here, the interface between the scale of the hot-rolled steel sheet before the surface removal and the steel is in the thickness direction, and the Ni _max and Cu _max contents of the steel surface portion having a depth of 2 μm are measured on the steel side, and are calculated in units of μm. For the appropriate range of GL and GU for surface removal. Further, the amount of surface removal actually performed is also shown in Table 2. On the other hand, the measurement of the Ni _max and Cu _max contents was measured using an EPMA (Electron Probe Micro Analyzer). In the plate width direction of the hot-rolled steel sheet, 10 measurement points were divided into 10 equal portions in a line segment having a reference length of 50 mm, and Ni and Cu contents were measured by EPMA at each measurement point. In this case, the average value of the measurement in the thickness direction from the surface to 2 μm in the thickness direction of the steel sheet is taken as the measurement result of the steel surface portion. Further, the maximum value of the Ni and Cu contents at the above-mentioned 10 measurement points is, by mass%, Ni _max and Cu _max .

In the same manner, the contents of P, Ni, and Cu in the surface portion and the base material portion of the steel sheet after the surface removal were measured, and the segregation state was confirmed. The measurement of the content of P, Ni, and Cu is performed by dividing the line segment of the reference length of 50 mm into 10 equal parts in the direction of the plate width of the rolled steel sheet, and measuring points at the measurement points of the 10 measurement points. It was measured using GDS. At this time, the average value of the rolled steel sheet rolled in the thickness direction from the surface to 0.1 μm is taken as the measurement result of the surface portion of the scale-removed steel sheet. The measurement result from the surface exceeding 2 μm to 4 μm was measured as a measurement result of the base material portion of the scale-removed steel sheet. Further, the P, Ni, and Cu contents in the surface portion of the steel sheet which was rolled by the descaling of each component were compared with P, Ni, and Cu in the base material portion of the steel sheet which was rolled and rolled, and the segregation state was expressed in percentage. This result is considered to be acceptable when each component is 105% or more and 150% or less. Table 5 shows the measurement results of the segregation states of P, Ni, and Cu expressed by the ratio of the surface portion of the descaling rolled steel sheet to the base portion of the descaling rolled steel sheet. On the other hand, in Table 5, only the measurement results of the P, Ni, and Cu segregation states in the above-mentioned 10 measurement points were measured, and the value was the distance of 127.5% (the intermediate value between 105% and 150%).

Next, the alloyed hot-dip galvanized steel sheets of the examples and the comparative examples produced by the above-described methods were evaluated for tensile properties and deep drawing indexes, that is, r values (Lankford values) and surface properties. Hereinafter, the evaluation method will be described.

The tensile properties are measured, for example, according to JIS Z 2241:2011 or ISO 6892-1:2009, using JIS No. 5 test piece taken out from each alloyed hot-dip galvanized steel sheet in a straight direction in the tensile direction and the rolling direction and the thickness direction. Tensile test, tensile strength (TS) was evaluated in units of MPa and elongation (E1) was evaluated in unit %. Further, the case where the tensile strength is 320 MPa or more and the elongation is 25% or more is considered to be acceptable.

The deep drawing processing index, that is, the r value evaluation is based on, for example, JIS Z 2254:2008 or ISO 10113-1:2006, from the direction of the rolling direction, the direction of the rolling direction, the direction of the 45° direction and the direction of the right angle of each alloyed hot-dip galvanized steel sheet. A JIS No. 5 tensile test piece was taken and the r value of each test piece was measured. For example, the r value measurement system measures the amount of change in the thickness of the sheet and the amount of change in the sheet width at the time point of the tensile deformation of about 10% in the tensile test, and the ratio of the change in the sheet width with respect to the sheet thickness can be determined. . Further, when the r value parallel to the rolling direction is r ₀ , the r value in the 45° direction is r ₄₅ , and the r value in the orthogonal direction is r ₉₀ , the r directions obtained by the following E formula are obtained. The average value of the values r _ave is evaluated. However, in the present embodiment, the case where r _ave is 1.2 or more is regarded as pass.

r _ave =(r ₀ +2×r ₄₅ +r ₉₀ )/4.........(E)

The evaluation of the surface properties was carried out by investigation of the P content in the alloyed hot-dip galvanized layer, the investigation of the thickness of the plating, and the presence or absence of the surface pattern.

The P content in the plating layer was measured using GDS. In the plate width direction of the alloyed hot-dip galvanized steel sheet, 10 measurement points were set by dividing the line segment of the reference length of 50 mm into 10 equal parts, and the P content in the plating layer was measured by GDS at each measurement point. The minimum value of the P content of the plating layer of the alloyed hot-dip galvanized steel sheet at the 10 measurement points was compared with the maximum value of the P content, and was considered to be 50% or more.

The thickness measurement of the plating layer is performed on the cut surface which is planarly cut along the thickness direction so that the plate width direction orthogonal to the rolling direction of the alloyed hot-dip galvanized steel sheet is the observation surface. In the plate width direction of the plated steel sheet, 10 measurement points are set by dividing the line segment portion having the reference length of 50 mm into 10 equal parts, and the metal structure of the cut surface is observed at each measurement point, and the thickness of the plating layer is measured. . The observation of the metal structure was carried out at a magnification of about 1000 μm in the sheet width direction. The minimum value of the thickness of the plating layer of the alloyed hot-dip galvanized steel sheet of the 10 measurement points was compared with the maximum value of the thickness, and was considered to be 50% or more.

Whether the surface pattern is judged or not on the alloyed hot-dip galvanized steel sheet The surface was ground with a grinding stone and visually observed. The grinding stone grinding system assumes friction under press processing, by which it is possible to determine whether or not a pattern is generated in actual pressing processing. In Table 5, by this method, the alloyed hot-dip galvanized steel sheet which does not generate a pattern is represented as Good, and the alloyed hot-dip galvanized steel sheet which produces a pattern is shown as Bad.

The above results are shown in the table below. The tensile strength, elongation and r _ave values of the mechanical properties are shown in Table 4. Table 5 shows the segregation state of P, Ni, and Cu in the rolled steel sheet, the P content in the alloyed hot-dip galvanized layer, the difference in the thickness of the plating, and the presence or absence of the surface pattern.

As shown in Tables 4 and 5, the alloyed hot-dip galvanized steel sheets of the examples satisfy the mechanical properties and have excellent workability, and the P content in the plating layer is uneven or the plating thickness is small, and no surface pattern is generated.

On the other hand, the alloyed hot-dip galvanized steel sheet other than the above is out of the comparative example of the scope of the present invention.

Since steel No. C and steel No. M have a surface removal amount of GL or less below the lower limit, P, Ni, and Cu are still segregated on the steel surface portion after the surface removal step. Therefore, the P content in the plating layer and the thickness of the plating layer are less than 50% in terms of the ratio of the minimum value to the maximum value, and there is a defect that a line pattern is generated.

The surface removal amount of steel No. G and steel No. J exceeded the upper limit. Therefore, the amount of surface removal is not optimal and is excessive, which is time consuming to remove and causes an increase in cost.

A comparative example in which the steel No. Q system P content exceeds the upper limit. As a result of the retardation of the alloying speed of the plated steel sheet, the surface properties were uneven, and a part of the pattern was confirmed.

A comparative example in which the steel No. R system Mn content exceeds the upper limit. The plated steel sheet has an r value as low as 1.1. In addition, since the hot-dip galvanizing property has deteriorated, an unplated portion has been confirmed in some cases.

A comparative example in which the steel No. S system C content exceeds the upper limit and the surface removal amount exceeds the upper limit. The plated steel sheet had an r value of 0.9, and the workability was poor, and the amount of surface removal was not optimal and was excessive and incurred an increase in cost.

A steel comparative example in which the Ti content exceeds the upper limit. The plated steel sheet had an r value of 0.9 and was inferior in workability.

A comparative example in which the steel No. U-based Ni content exceeded the upper limit. Further, the steel No. V is a comparative example in which the Cu content exceeds the upper limit. Further, since the amount of surface removal of these steels is less than or equal to the lower limit, it is confirmed that the plated steel sheets have streaks and patterns on the surface plating properties.

Steel No. W is a comparative example in which the Nb content exceeds the upper limit. The plated steel sheet had an r value of 1.1 and was inferior in workability.

Steel No. KK is a comparative example in which the C content is at most the lower limit. In order to reduce the amount of C, the plated steel sheet has a large load in the steel and causes an increase in cost.

Steel No. LL is a comparative example in which the C content exceeds the upper limit. The plated steel sheet has poor workability.

Steel No. MM is a comparative example in which the Si content is at most the lower limit. The plated steel sheet has poor tensile strength.

Steel No. NN is a comparative example in which the Si content exceeds the upper limit. The plated steel sheet has poor workability.

Steel No. OO is a comparative example in which the Mn content is at most the lower limit. The plated steel sheet has poor tensile strength.

Steel No. PP is a comparative example in which the P content is at most the lower limit. The plated steel sheet has poor tensile strength.

Steel No. QQ is a comparative example in which the Al content is at most the lower limit. Since the plated steel sheet is insufficiently deoxidized and has residual oxides, the workability is poor.

Steel No. RR is a comparative example in which the Al content exceeds the upper limit. The plated steel sheet has poor workability.

A comparative example in which the steel No. SS system S content exceeds the upper limit. The plated steel sheet has poor workability.

A comparative example in which the steel No. TT system B content exceeds the upper limit. The plated steel sheet has poor workability.

In the steel No. UU, the heating temperature in the heating step is lower than the lower limit, and the final temperature in the hot rolling step is lower than the lower limit. The plated steel sheet has poor workability.

The steel No. VV is a comparative example in which the final temperature in the hot rolling step is lower than the lower limit. The plated steel sheet has poor workability.

The steel No. WW is a comparative example in which the coiling temperature in the hot rolling step is equal to or lower than the lower limit. The plated steel sheet has a poor shape and cannot be used as a finished product.

A comparative example in which the coiling temperature in the steel No. XX hot rolling step exceeds the upper limit. The plated steel sheet has too many scales and cannot be used as a finished product.

A comparative example in which the steel No. AB Mo content exceeded the upper limit. The plated steel sheet has poor workability.

Steel No. AC is a comparative example in which the N content exceeds the upper limit. The plated steel sheet has poor workability.

Industrial availability

According to the above aspect of the invention, it is possible to provide a plating layer having mechanical properties satisfying tensile strength and the like, and having excellent workability, and having few surface defects such as linear patterns, and exhibiting beautifulness even when subjected to press working. The alloyed hot-dip galvanized steel sheet for press working of the surface appearance and the method for producing the same have high industrial applicability.

Claims (9)

An alloyed hot-dip galvanized steel sheet characterized in that it has a descaling rolled steel sheet and an alloyed hot-dip galvanized layer disposed on the descaling rolled steel sheet, and the chemical composition of the descaling rolled steel sheet is % by mass, including: C: 0.0005% to 0.01%, Si: 0.001% to 1.0%, Mn: 0.01% to 2.0%, P: 0.005% to 0.1%, and Al: 0.01% to 0.10%, and limit S : 0.02% or less, Ni: 0.1% or less, Cu: 0.1% or less, and N: 0.01% or less, and the residual portion is composed of Fe and unavoidable impurities; when the sheet width of the molten zinc-plated steel sheet is used by the foregoing alloying In the direction, when the line segment having a reference length of 50 mm is divided into 10 equal parts and 10 measurement points are set, the minimum value of the P content of the plating layer in the 10 measurement points is 50% or more compared with the maximum value of the P content. . The alloyed hot-dip galvanized steel sheet according to the first aspect of the invention, wherein the chemical component of the steel-rolled rolled steel sheet is at least one of the following elements: B: 0.0001% to 0.0050%, Nb: 0.001% to 0.1%, Ti: 0.001% to 0.1%, and Mo: 0.001% to 0.1%. The alloyed hot-dip galvanized steel sheet according to the second aspect of the patent application, wherein, in the direction of the sheet width of the rolled steel sheet by the descaling, a line portion of the reference length of 50 mm is divided into 10 equal portions and 10 measurement points are set. At each measurement point of the above-mentioned 10 measurement points of the rust-removed steel sheet, the surface of the rust-removed steel sheet has a depth of 0.1 μm from the surface, and the surface portion of the rust-removed steel sheet is P and Ni. The content of Cu and the content of P, Ni and Cu in the base material of the rust-removed steel sheet which is more than 2 μm in depth from the surface in the thickness direction are 105% or more and 150% or less. The alloyed hot-dip galvanized steel sheet according to the first aspect of the invention, wherein, in the direction of the sheet width of the rolled steel sheet by the descaling, a line portion of the reference length of 50 mm is divided into 10 equal portions and 10 measurement points are set. At each measurement point of the above-mentioned 10 measurement points of the rust-removed steel sheet, the surface of the rust-removed steel sheet has a depth of 0.1 μm from the surface, and the surface portion of the rust-removed steel sheet is P and Ni. The content of Cu and the content of P, Ni and Cu in the base material of the rust-removed steel sheet which is more than 2 μm in depth from the surface in the thickness direction are 105% or more and 150% or less. A method for producing an alloyed hot-dip galvanized steel sheet, characterized in that the chemical composition is in mass%, comprising: C: 0.0005% to 0.01%, Si: 0.001% to 1.0%, and Mn: 0.01% to 2.0% P: 0.005% to 0.1%, and Al: 0.01% to 0.10%, and S: 0.02% or less, Ni: 0.1% or less, Cu: 0.1% or less, and N: 0.01% or less, and the residual portion is Fe. And the molten steel formed by the unavoidable impurities, performing the following steps; casting step, casting the molten steel to obtain a flat embryo; heating step, heating the flat embryo at 1100 to 1300 ° C; hot rolling step, at a final temperature of 800 The above-mentioned flat embryo after the heating step is hot rolled to obtain a hot-rolled steel sheet under the conditions of ° C or more and 1050 ° C or less and a coiling temperature of 500 ° C or more and 800 ° C or less; the surface removing step is performed by the heat In the plate width direction of the rolled steel sheet, 10 line sections of the reference length of 50 mm are divided into 10 equal parts, and 10 measurement points are set, and the interface between the self-rust skin and the steel of the hot-rolled steel sheet is 2 μm from the steel side in the thickness direction. The Ni and Cu contents of the deep steel surface portion are the highest at the above 10 measurement points. % When calculated as Ni _max and Cu _max, in unit of μm meter GL represented by the following formula in the range of 1 or more and represented by the following formula GU 2 or less, to a reference to the interface of the steel plate thickness direction side Performing the surface removal of the hot-rolled steel sheet to obtain a rust-removed steel sheet; a plating step of performing hot-dip galvanizing on the rust-rolled rolled steel sheet after the surface removing step to obtain a hot-dip galvanized steel sheet; and an alloy a step of subjecting the hot-dip galvanized steel sheet after the plating step to an alloying hot-dip galvanized steel sheet; GL=(Ni _max +0.8×Cu _max )×0.2 (...) GU = (Ni _max + 0.8 × Cu _max ) × 4 (... 2). The method for producing an alloyed hot-dip galvanized steel sheet according to the fifth aspect of the invention, wherein the chemical component of the molten steel further contains at least one of the following elements in a mass%: B: 0.0001% to 0.0050%, Nb: 0.001% to 0.1%, Ti: 0.001% to 0.1%, and Mo: 0.001 to 0.1%. The method for producing an alloyed hot-dip galvanized steel sheet according to claim 6, wherein at least one of the surface removal step or the surface removal step has an acid which pickles the surface of the descaling rolled steel sheet. Wash the steps. The method for producing an alloyed hot-dip galvanized steel sheet according to the fifth aspect of the invention, wherein at least one of the surface removal step or the surface removal step has an acid which pickles the surface of the descaling rolled steel sheet. Wash the steps. Alloyed hot-dip galvanizing as claimed in any one of claims 5 to 8. a method for producing a steel sheet, comprising: a cold rolling step of further performing cold rolling on the steel strip rolling steel sheet before the plating step by a cold rolling ratio of 50% or more and 95% or less; and annealing In the step, the steel sheet rolled after the cold rolling step is annealed at a temperature equal to or higher than the recrystallization temperature.