JP4555738B2 - Alloy hot-dip galvanized steel sheet - Google Patents

Description

The present invention relates to a steel plate for alloying hot dip galvanizing having bake hardenability (hereinafter referred to as BH (Bake Hardenability)), and more specifically, as a P-containing high strength alloying hot dip galvanizing steel plate, The present invention relates to an alloyed hot-dip galvanized steel sheet that can be used as an inner and outer plate for automobiles.

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

  In recent years, in the automobile field, it has been necessary to increase the strength of plated steel sheets in order to ensure the function of protecting passengers in the event of a collision and to reduce the weight for the purpose of improving fuel efficiency.

  In order to increase the strength of the steel sheet without deteriorating workability, it is effective to add elements such as Si, Mn, and P. However, since addition of Si tends to cause non-plating, currently Mn, Steel plates that have been strengthened by the addition of P are most often used.

  Furthermore, a technique for improving dent resistance while securing workability by imparting BH properties that increase the strength in the paint baking process after pressing is also used.

  However, the steel sheet to which P is added is known to have a problem that streak defects are likely to occur. There are no problems with streak defects when machining, welding, or painting, but many consumers do not like the appearance defect. Moreover, in the low carbon steel to which P is added, it becomes easy to generate alumina inclusions, and the amount of alumina clusters becomes very large. The presence of such clusters causes surface defects after press working, and therefore, measures for reducing alumina inclusions are a major issue.

  As a method for producing a good surface appearance of a P-containing alloyed hot-dip galvanized steel sheet, a technique for attaching sulfur or a sulfur compound to the surface has been proposed (for example, see Patent Document 1). Since a facility for applying the sulfur compound is required, it is not possible to use it when there is no space. In addition, there is a problem that the production cost increases due to the installation of the coating equipment.

  Further, a technique has been proposed in which ferrite grains having a crystal grain size of 15 μm or less are defined to be 70 area% or less in a surface layer portion within 50 μm in the thickness direction from the surface of a hot-rolled sheet (for example, Patent Document 2). This only describes the phenomenon that generally occurs when the hot rolling finishing temperature is maintained at Ar3 + 20 ° C. or higher, in other words, a proposal of a manufacturing method for manufacturing at a hot rolling finishing temperature of Ar3 transformation point + 20 ° C. or higher. It is. This increase in the hot rolling finishing temperature causes a problem that the production cost increases because it is necessary to use a large amount of energy of the heating furnace.

  Similarly, a manufacturing method has been proposed in which the hot rolling finishing temperature is manufactured at an Ar3 transformation point + 30 ° C. or higher (see, for example, Patent Document 3), but increasing the hot rolling finishing temperature uses a large amount of energy from the heating furnace. As a result, the production cost increases.

Japanese Patent Laid-Open No. 11-50220 JP 2001-316663 A Japanese Patent No. 3339615

In view of the above situation, the present invention is excellent in bake hardenability that can suppress generation of surface defects such as streak patterns without installing new equipment and without increasing production costs in the hot rolling process. Another object of the present invention is to provide an alloyed hot-dip galvanized steel sheet in which an alloyed hot-dip galvanized layer is formed on a P-containing high-strength hot-dip galvanized steel sheet .

  As a result of examining various means for suppressing the occurrence of streak pattern defects without installing new equipment and reducing the productivity of the hot rolling process, the present inventor has reduced C, N, Al, etc. By defining the amount of C, N, Al, Ti, and Nb added to the steel sheet to which P and Mn are added, and adding one or more of Ce, La, Nd, Pr, and Sm, streaks, etc. It was found that the occurrence of surface defects can be suppressed, and the present invention has been achieved.

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

(1) In mass%,
C: 0.0001 to 0.015%,
Si: 0.001 to 0.45%,
Mn: 0.2 to 2.8%,
P: 0.02-0.2%
S: 0.015% or less,
Al: 0.008% or less,
Ti: 0.002 to 0.10%,
N: 0.0005 to 0.004%,
Further, the content of Ce, La, Nd, Pr, Sm or a combination of two or more of 0.0001 to 0.01% in total, and the acid-soluble Ti content of the steel sheet comprising the balance Fe and inevitable impurities However, it satisfies the condition given by the following formula (1) ([% X] is the content of alloy element X expressed in mass%), and the non-metallic inclusions having an equivalent circle diameter of 10 μm or more observed with a microscope 70% or more is within the composition range of the following formulas (3) to (5), on the surface of the steel sheet, Al: 0.05 to 0.5 mass%, Fe: 7 to 15 mass%, The balance consists of Zn and inevitable impurities, and the ratio Iζ / x of the X-ray diffraction intensity Iζ and dΓ of d = 1.26 and d = 1.222 and the d = 3.13 X-ray diffraction intensity ISi of the Si standard plate ISi and IΓ / ISi satisfy Iζ / ISi ≦ 0.004 and IΓ / ISi ≦ 0.004. Alloyed molten galvanized steel plate, characterized in that the formation of the galvannealed layer.
[% C] × 4 + [% N] × 48 / 14−0.006% ≦ [% Ti] ≦ [% C] × 4 + [% N] × 48 / 14−0.002% (1)
10% ≦ TiO ₂ / (TiO ₂ + Al ₂ O ₃ + REM oxide) <95% (3)
0 ≦ Al ₂ O ₃ <50% (4)
5% ≦ REM oxide <90% (5)
TiO ₂ : concentration of Ti oxide in non-metallic inclusions in the steel sheet. Ti oxides in the form of Ti ₂ O ₃ and Ti ₃ O ₅ exist, but the Ti oxide concentration converted as TiO ₂ .
Al ₂ O ₃ : Concentration of Al oxide in non-metallic inclusions in the steel plate.
REM oxide: The total concentration of Ce, La, Nd, Pr, and Sm oxides in the non-metallic inclusions of the steel sheet. Amount of oxide converted as Ce ₂ O ₃ , La ₂ O ₃ , Nd ₂ O ₃ , Pr ₂ O ₃ , Sm ₂ O ₃ .

(2) By mass%
C: 0.0001 to 0.015%,
Si: 0.001 to 0.45%,
Mn: 0.2 to 2.8%,
P: 0.02-0.2%
S: 0.015% or less,
Al: 0.008% or less,
Ti: 0.002 to 0.10%,
Nb: 0.002 to 0.10%,
N: 0.0005 to 0.004%,
Further, the content of Ce, La, Nd, Pr, Sm or a combination of two or more of 0.0001 to 0.01% in total, and the acid-soluble Ti content of the steel sheet comprising the balance Fe and inevitable impurities However, it satisfies the condition given by the following formula (2) ([% X] is the content of alloy element X expressed in mass%), and the non-metallic inclusions having an equivalent circle diameter of 10 μm or more observed with a microscope 70% or more is within the composition range of the following formulas (3) to (5), on the surface of the steel sheet, Al: 0.05 to 0.5 mass%, Fe: 7 to 15 mass%, The balance consists of Zn and inevitable impurities, and the ratio Iζ / x of the X-ray diffraction intensity Iζ and dΓ of d = 1.26 and d = 1.222 and the d = 3.13 X-ray diffraction intensity ISi of the Si standard plate ISi and IΓ / ISi satisfy Iζ / ISi ≦ 0.004 and IΓ / ISi ≦ 0.004. Alloyed molten galvanized steel plate, characterized in that the formation of the galvannealed layer.
[% C] × 4 + [% N] × 48/14 − [% Nb] × 48 / 93−0.006% ≦ [% Ti] ≦ [% C] × 4 + [% N] × 48 / 14− [ % Nb] × 48 / 93-0.002% (2)
10% ≦ TiO ₂ / (TiO ₂ + Al ₂ O ₃ + REM oxide) <95% (3)
0 ≦ Al ₂ O ₃ <50% (4)
5% ≦ REM oxide <90% (5)
TiO ₂ : concentration of Ti oxide in non-metallic inclusions in the steel sheet. Ti oxides in the form of Ti ₂ O ₃ and Ti ₃ O ₅ exist, but the Ti oxide concentration converted as TiO ₂ .
Al ₂ O ₃ : Concentration of Al oxide in non-metallic inclusions in the steel plate.
REM oxide: The total concentration of Ce, La, Nd, Pr, and Sm oxides in the non-metallic inclusions of the steel sheet. Amount of oxide converted as Ce ₂ O ₃ , La ₂ O ₃ , Nd ₂ O ₃ , Pr ₂ O ₃ , Sm ₂ O ₃ .

(3) steel sheet further as an additional component, in mass%, B: above, wherein the containing 0.0002 to 0.003% (1) or-out galvannealed plating according to (2) steel plate.

  The present invention makes it possible to provide a steel sheet to be plated that can produce a P-containing high-strength galvannealed steel sheet that can suppress the occurrence of surface defects such as workability and streaks, and contributes to industrial development. The place to do is extremely large.

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

  C: C is an element that increases the strength of the steel, and it is effective to contain 0.0001% or more. However, if it is excessively contained, the strength is excessively increased and the workability is lowered, so the upper limit content is 0.8. 015%. In particular, when high workability is required, the C content is preferably 0.010% or less.

  Si: Si is also an element that improves the strength of the steel and contains 0.001% or more. However, if excessively contained, workability and hot dip galvanizing properties are impaired, so the upper limit is made 0.45%. In particular, when high workability is required, the Si content is 0.10% or less.

  Mn: Mn is added in an amount of 0.2% or more as a solid solution strengthening element. The reason why the Mn content is 0.2% or more is that if Mn is less than 0.2%, it is difficult to ensure the required tensile strength. The upper limit is not particularly limited, but if the amount added is excessive, cracking of the slab is likely to occur and spot weldability is also deteriorated, so 2.8% is desirable. More preferably, it is 0.2 to 1.5% from the balance of strength, workability and cost.

  P: P is added in an amount of 0.02% or more as an element that increases the strength without significantly impairing the workability of the steel sheet, particularly the elongation. The reason why the P content is 0.02% or more is that it is difficult to ensure the required tensile strength when P is less than 0.02%. The upper limit is not particularly limited, but if added excessively, grain boundary embrittlement due to grain boundary segregation becomes significant, so 0.2% or less is desirable. More preferably, it is 0.02 to 0.1% from the balance of strength, workability and cost.

  S: Since S is an element that lowers the hot workability and corrosion resistance of steel, it is preferably as small as possible, and the upper limit content is 0.015%. However, it is costly to reduce the amount of S of the ultra-low carbon steel as in the present invention, and surface defects such as streaks are liable to occur if S is excessively reduced, so hot workability and corrosion resistance. It is sufficient to reduce S to a necessary level. Desirably, it is 0.008 to 0.015%.

  Al: Al is generally added as a deoxidizing element for steel. However, Al produces alumina inclusions by deoxidation, which aggregate and coalesce into coarse alumina clusters. In particular, in the low carbon steel to which P is added, the amount of alumina clusters is very large, and when the Al addition amount exceeds 0.008%, the generation rate of surface flaws becomes extremely high, so the addition amount of Al is 0.008%. The following. Desirably, it is less than 0.005%.

  Ti: In order to fix C and N in steel as carbides and nitrides, 0.002% or more of addition is necessary, and it is more preferable to contain 0.010% or more. On the other hand, even if added over 0.10%, the effect is no longer saturated, but the alloy addition cost only increases unnecessarily, so the upper limit content is 0.10%. Since excessive solute Ti may impair the workability and surface quality of the steel sheet, it is more preferably 0.050% or less.

In the present invention, for the purpose of further imparting BH properties, the acid-soluble Ti content is within the range satisfying the following formula (1) ([% X] is the content of alloy element X expressed in mass%): To do.
[% C] × 4 + [% N] × 48 / 14−0.006% ≦ [% Ti] ≦ [% C] × 4 + [% N] × 48 / 14−0.002% (1)
This is because if the acid-soluble Ti content is in the above range, BH properties can be imparted while securing non-aging at room temperature.

When the content of acid-soluble Ti is less than {[% C] × 4 + [% N] × 48 / 14−0.006%}, YP-El after artificial aging exceeds 0.2%, so that room temperature non-aging Sex cannot be secured.
Further, when the acid-soluble Ti content exceeds {[% C] × 4 + [% N] × 48 / 14−0.002%}, it is difficult to impart BH of 29 MPa or more.

  Here, the acid-soluble Ti is Ti that can be dissolved when the steel sheet is dissolved in an acid. Specifically, it refers to Ti dissolved in 20% by mass sulfuric acid when steel is dissolved in heated 20% by mass sulfuric acid. This includes Ti dissolved in steel and Ti existing as intermetallic compounds such as carbides, nitrides and sulfides. On the other hand, since Ti oxide does not dissolve in acid, Ti present as oxide is not included in acid-soluble Ti.

  N: N increases the strength of the steel while lowering the workability, so the upper limit is made 0.004%, and when high workability is particularly required, it is more preferably 0.003% or less. More preferably, the content is 0.002% or less. N is preferably as little as possible, but reducing it to less than 0.0005% requires excessive cost, so the lower limit content is made 0.0005%.

  Ce, La, Nd, Pr, Sm: Generation of surface defects such as streaks by adding one or more of Ce, La, Nd, Pr, and Sm in a total of 0.0001 to 0.01% It becomes possible to obtain an alloyed hot-dip galvanized steel sheet having a good appearance. The addition amount of one or more of Ce, La, Nd, Pr and Sm is required to be 0.0001% or more for the purpose of suppressing the generation of streak defects. Further, in order to perform deoxidation of steel with the addition amount of Al being 0.008% or less, the addition amount of one or more of Ce, La, Nd, Pr, and Sm needs to be 0.0001% or more. . However, if it exceeds 0.01%, not only will the cost be increased, but the oxides of these metals become inclusions in the steel sheet, which tends to cause surface defects after press working, so the amount added is 0.01% in total. % Or less.

  The reason why it becomes possible to suppress the generation of streak defects by adding one or more of Ce, La, Nd, Pr, and Sm is that these elements exist in the crystal grains and in the crystal grain boundaries. This is considered to be due to the effect of promoting recrystallization in the annealing process and reducing the difference in alloying reaction due to the difference in crystal orientation. For this reason, even when hot-rolling at a hot rolling finish temperature at which a streak defect occurs in a steel sheet to which one or more of Ce, La, Nd, Pr, and Sm are not added, Ce, La, Nd, Pr, Since the steel plate to which one or more of Sm is added becomes a uniform surface by recrystallization in the annealing process, it is considered that generation of pattern defects can be suppressed.

  From the balance of plating adhesion and cost, the addition amount of one or more of Ce, La, Nd, Pr, and Sm is more preferably 0.001 to 0.01%.

  Ce, La, Nd, Pr, and Sm can be added using a single metal, but it can also be added using an alloy containing Ce, La, Nd, Pr, and Sm such as misch metal.

Further, when the addition amount of Ce, La, Nd, Pr and Sm is small with respect to the amount of Ti oxide, the inclusion composition is mainly TiO ₂ -Al ₂ O ₃ inclusions, which aggregate and coalesce and become coarse. A cluster. On the other hand, if the amount of Ce, La, Nd, Pr, and Sm added is too large relative to the amount of Ti oxide, the inclusions are mainly oxides with a REM oxide concentration of 90% or more, which aggregate and coalesce. It becomes a coarse cluster. Since such a cluster causes surface defects after press working, the steel sheet is examined with a microscope, and 70% or more of non-metallic inclusions having an equivalent circle diameter of 10 μm or more are within the composition range of the following formula. More preferably.
10% ≦ TiO ₂ / (TiO ₂ + Al ₂ O ₃ + REM oxide) <95% (3)
0 ≦ Al ₂ O ₃ <50% (4)
5% ≦ REM oxide <90% ・ ・ ・ (5)
TiO ₂ : concentration of Ti oxide in non-metallic inclusions on the steel sheet. Ti oxides in the form of Ti ₂ O ₃ and Ti ₃ O ₅ exist, but the Ti oxide concentration converted as TiO ₂ .
Al ₂ O ₃ : Concentration of Al oxide in non-metallic inclusions in the steel plate.
REM oxide: The total concentration of Ce, La, Nd, Pr, and Sm oxides in the non-metallic inclusions of the steel sheet. Amount of oxide converted as Ce ₂ O ₃ , La ₂ O ₃ , Nd ₂ O ₃ , Pr ₂ O ₃ , Sm ₂ O ₃ .

  Here, “equivalent to a circle observed with a microscope” is an index indicating the size of non-metallic inclusions in a steel sheet and is defined as follows. In other words, “equivalent to a circle observed with a microscope” means that in a sample obtained by mirror polishing an arbitrary cross section of a steel plate, the inclusions in the steel plate are observed with an optical microscope at a magnification of 200 to 1000 times. The length (L) and width (D) are measured, the area of the nonmetallic inclusion is assumed to be a rectangle (LxD), and the area is obtained and defined as the diameter of a circle having the same area as the rectangular area. In addition, when the observed nonmetallic inclusions were in a form close to a circle, the diameter was taken as the equivalent circle diameter.

  In the present invention, in addition to the above, as an additional component, Nb can be added under the above Ti addition in order to fix C and N in the steel as carbides and nitrides. In order to fully exhibit the C and N fixing effect, addition of 0.002% or more is necessary, and more preferably 0.005% or more. Even if Nb is added in excess of 0.10%, the effect is no longer saturated, but the cost is increased unnecessarily, so the upper limit content is 0.10%. Excessive Nb addition raises the recrystallization temperature of the steel sheet and lowers the productivity of the hot dip galvanizing line.

Further, in the present invention, for the purpose of imparting BH property to the steel added with Nb, the acid-soluble Ti content is expressed by the following formula (2) ([% X] is the content of the alloy element X expressed by mass%). ).
[% C] × 4 + [% N] × 48/14 − [% Nb] × 48 / 93−0.006% ≦ [% Ti] ≦ [% C] × 4 + [% N] × 48 / 14− [ % Nb] × 48 / 93-0.002%
(2)
This is because if the acid-soluble Ti content is in the above range, BH properties can be imparted while securing non-aging at room temperature.

  When the acid-soluble Ti content is less than {[% C] × 4 + [% N] × 48/14 − [% Nb] × 48 / 93−0.006%}, YP-El after artificial aging is 0.00. Since it exceeds 2%, non-aging property at room temperature cannot be ensured.

  Further, when the acid-soluble Ti content exceeds {[% C] × 4 + [% N] × 48/14 − [% Nb] × 48 / 93−0.002%}, BH of 49 MPa or more is imparted. Is difficult.

  In the present invention, the steel sheet may further contain 0.0002 to 0.003% of B as an additional component, and this is intended to improve secondary workability. If the B content is less than 0.0002%, the secondary workability improvement effect is not sufficient, and even if added over 0.003%, the effect is no longer saturated, and the moldability is reduced. Therefore, when adding B, the range is made 0.0002 to 0.003%. In particular, when high deep drawability is required, the amount of B added is more preferably 0.0015% or less.

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

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

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

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

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

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

  As a manufacturing process of the steel sheet of the present invention, a manufacturing process of a normal hot-rolled steel sheet (hot strip) or a cold-rolled steel sheet (cold strip) may be applied.

  In the present invention, O in the steel sheet is not particularly limited, but O generates oxide inclusions and impairs the workability, corrosion resistance, and appearance of the steel. .

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

  Since the steel sheet of the present invention can be applied to a normal hot dip galvanized steel sheet production line to obtain an alloyed hot dip galvanized steel sheet having excellent workability, formability and plating adhesion, there is no particular restriction on the manufacturing process. . A process may be selected as appropriate in consideration of cost and productivity.

The steel sheet of the present invention is one of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and rare earth elements during hot dip galvanizing bath or galvanizing. Alternatively, even if two or more kinds are contained or mixed, the effects of the present invention are not impaired, and depending on the amount, the corrosion resistance may be improved. There are no particular restrictions on the amount of galvannealed coating, but it is preferably 20 g / m ² or more from the viewpoint of corrosion resistance and 150 g / m ² or less from the viewpoint of economy.

  In the present invention, the method for producing a plated steel sheet is not particularly limited, and a normal non-oxidizing furnace type hot dipping method can be applied. Although the alloying treatment conditions are not particularly defined, a treatment temperature range of 460 to 550 ° C. and a treatment time range of 10 to 40 seconds is appropriate for actual operation.

  In the present invention, the thickness of the steel sheet does not impose any restrictions on the present invention, and the present invention can be applied as long as it is a commonly used sheet thickness. Furthermore, the steel sheet of the present invention is sufficiently effective whether it is a cold-rolled steel sheet or a hot-rolled steel sheet manufactured by a normal process, and the effect does not change greatly depending on the history of the steel sheet. . Moreover, the hot rolling conditions, the cold rolling conditions, the annealing conditions, etc. may be selected according to the dimensions of the steel sheet and the required strength, depending on the hot rolling conditions, the cold rolling conditions, the annealing conditions, etc. The effect of the steel sheet of the present invention is not impaired.

  Of course, it is of course possible to apply various types of upper plating, especially electroplating, on the galvannealed steel sheet obtained by using the steel sheet of the present invention in order to improve the paintability and weldability. And does not depart from the present invention. Further, it is of course possible to add various treatments to the alloyed hot-dip galvanized steel sheet obtained by the method of the present invention. For example, chromate treatment, phosphate treatment, and phosphate treatment properties can be achieved. Even if the treatment for improving, the lubricity improving treatment, the weldability improving treatment, the resin coating treatment, etc. are performed, it does not depart from the scope of the present invention, and various types are added depending on the required additional characteristics. Can be applied.

The steel sheet of the present invention is a high-strength steel sheet (300, 340, 400, 440 N / mm ² class) having a performance satisfying a tensile strength of 300 N / mm ² or more.

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

  First, test materials shown in Table 1 were prepared, and an alloyed hot-dip galvanized steel sheet was manufactured using an in-line annealing method of continuous hot-dip galvanizing equipment. Table 1 shows the chemical component values of the test material, that is, the steel plate before plating, and the chemical component values of the nonmetallic inclusions in the steel plate. Here, the chemical component value of the nonmetallic inclusion was determined as follows. Specimen sample is embedded in resin so that the longitudinal section (cross section parallel to the rolling direction) of the steel sheet becomes the spectroscopic observation surface, and mirror polishing is performed, and non-metallic inclusions with a circle equivalent diameter of 10 μm or more are arbitrarily selected 20 were identified, and the composition was determined by EPMA. Table 1 shows the average chemical composition of 20 non-metallic inclusions.

In plating, the annealing atmosphere was 5% hydrogen + 95% nitrogen mixed gas, the annealing temperature was 800 to 840 ° C., and the annealing time was 90 seconds. The molten zinc bath was molten zinc containing 0.12% Al, and the basis weight of zinc was adjusted to 50 g / m ² with a gas wiper. The alloying was heated using induction heating type heating equipment so that the Fe content in the alloyed hot dip galvanizing was 10.5 to 11.5%. However, the hot-rolled steel sheet was preheated to 90 ° C. (90 seconds) instead of annealing and cooling. The Al content during plating of the galvannealed steel sheet thus obtained was 0.15 to 0.25%.

  After plating, temper rolling was performed at a rolling reduction of 0.5%, and a JIS No. 5 tensile test piece was collected and subjected to a tensile test. The tensile test results are shown in Table 2.

  As an index of workability, a tensile test of each alloyed hot-dip galvanized steel sheet was conducted, and tensile strength, elongation, and Rankford value (r values; average r values of 0 °, 45 °, 90 °, but cold rolling) Steel sheet only) was measured.

  BH properties are as follows. First, 2% tensile pre-strain is applied in the rolling direction, the load is once unloaded, a heat treatment equivalent to coating baking at 170 ° C. for 20 minutes is performed, and then a tensile test is performed again. It evaluated by calculating | requiring a raise, and 29 Mpa or more was set as the pass.

  The room temperature non-aging property was measured by measuring YP-El after artificial aging at 100 ° C. × 1 hr, and 0.2% or less was accepted.

Surface defects after press working were evaluated by performing a ball head overhang test. Test conditions are shown below.
・ Sample drawing width: 200 × 200mm
・ Mold: Ball head punch with radius 60mm, Dies with beads ・ Pressing load: 60t
-Overhang speed: 30 mm / min
・ Oiling: Antirust oil applied

For the evaluation of surface defects, 1000 ball head overhang tests were conducted, evaluated according to the following classification, and ◯ was set as acceptable.
○: Crack occurrence rate due to non-metallic inclusions is less than 0.5% Δ: Crack occurrence rate due to non-metallic inclusions is 0.5% or more and less than 3% ×: Caused by non-metallic inclusions Crack occurrence rate is 3% or more

The generation of streaks was evaluated by visually observing the entire length of the coil and evaluated according to the following classification.
○: The length of the portion mixed with the streak pattern is less than 0.5% of the total length Δ: The length of the portion mixed with the streak pattern is 0.5% or more and less than 10% of the total length X: The length of the portion where the streaks are mixed is 10% or more of the total length

  The results are shown in Table 2. In No. 24, since the addition amount of Ce, La, Nd, Pr, and Sm was outside the range of the present invention, the REM oxide became a cluster and the surface defect after press working was rejected. Nos. 25, 26, 27, and 32 have Al, Ce, La, Nd, Pr, and Sm addition amounts outside the scope of the present invention, so that alumina clusters increase and surface defects after press working are rejected. The streak pattern was also rejected. In No. 28, since the amount of Al added was outside the range of the present invention, alumina clusters increased, surface defects after press working were rejected, and streak patterns were also rejected. In No. 29, since the addition amount of P was outside the range of the present invention of the steel sheet, the tensile strength was insufficient. No. 30 was insufficient in tensile strength because the addition amount of P and Mn was outside the range of the present invention of the steel sheet. No. 31 is the addition amount of Al, Ce, La, Nd, Pr, Sm is outside the range of the present invention of the steel plate, but because the addition amount of P is small, the streak pattern after pressing was acceptable, but P, Mn Since the added amount is outside the range of the present invention of the steel sheet, the tensile strength was insufficient.

  The products of the present invention other than these were P-containing high-strength galvannealed steel sheets in which the occurrence of surface defects and streaks after pressing was suppressed.

First, specimen materials shown in Table 3 were prepared, and an alloyed hot-dip galvanized steel sheet was manufactured using a vertical hot-dip plating apparatus capable of simulating the thermal cycle and atmosphere of CGL. In plating, the annealing atmosphere was 5% hydrogen + 95% nitrogen mixed gas, the annealing temperature was 800 to 840 ° C., and the annealing time was 90 seconds. The molten zinc bath was molten zinc containing Al, and the basis weight of zinc was adjusted to 50 g / m ² by gas wiping. Heating for alloying was performed using induction heating type heating equipment so that the Fe content in the alloyed hot dip galvanizing was the value shown in Table 4. The Al concentration in the plating bath was variously changed so that the Al content in the alloyed hot dip galvanizing was the value shown in Table 4.

  The plated sample was subjected to temper rolling at a rolling reduction of 0.5%, and a JIS No. 5 tensile test piece was collected and measured for tensile strength, BH property, and room temperature non-aging property.

  All samples have a tensile strength of 300 MPa or more, an increase in yield stress after heat treatment equivalent to baking at 170 ° C. for 20 minutes is 29 MPa or more, and YP-El after artificial aging at 100 ° C. × 1 hr is 0.2% or less. Met.

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

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

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

As for press formability, a bead pull-out test was conducted in order to investigate the sliding property of plating in press working. Test conditions are shown below.
・ Sample drawing width: 30mm
-Mold: Square bead with one side shoulder R1mmR (convex part is 4x4mm) convex, the other side is concave shape with shoulder R1mmR-Pressing load: 800, 1000kg
・ Pullout speed: 200mm / min
・ Oiling: Antirust oil applied

Evaluation of press formability was evaluated according to the following classification, and ◎ and ○ were accepted.
◎: Pulled out with a pressing load of 1000 kg ○: Pulled out with a pressing load of 800 kg, but broken with a load of 1000 kg ×: Broken with a pressing load of 800 kg

For plating adhesion, the test piece with adhesive tape (cellophane tape) on the compression side is bent in a V shape so that the bending angle is 60 °, and after bending back, the adhesive tape is peeled off. The degree of peeling was visually observed and evaluated according to the following classification, and ◎ and ○ were accepted.
◎: Plating layer does not peel at all ○: Peeling of plating is slight △: Plating is peeled off to some extent ×: Plating is almost peeled off

  The evaluation results are as shown in Table 4. Nos. 1 and 26 failed in press formability because Fe% and Iζ / ISi in plating exceeded 0.004. In Nos. 5 and 30, since the Fe% during plating and IΓ / ISi exceeded 0.004, the plating adhesion was unacceptable. In Nos. 6 and 31, Al% during plating was less than 0.05 and IΓ / ISi exceeded 0.004.

  The products of the present invention other than these were P-containing high-strength galvannealed steel sheets excellent in press formability and plating adhesion.

  First, sample materials shown in Table 5 were prepared, and an alloyed hot-dip galvanized steel sheet was manufactured using an in-line annealing method of continuous hot-dip galvanizing equipment. Table 5 shows the chemical component values of the test materials, that is, the steel plates before plating, and the chemical component values of the non-metallic inclusions in the steel plates. Here, the chemical component value of the nonmetallic inclusion was determined as follows. Specimen sample is embedded in resin so that the longitudinal section (cross section parallel to the rolling direction) of the steel sheet becomes the spectroscopic observation surface, and mirror polishing is performed, and non-metallic inclusions with a circle equivalent diameter of 10 μm or more are arbitrarily selected 20 were identified, and the composition was determined by EPMA. Table 5 shows the average chemical composition of 20 non-metallic inclusions.

In plating, the annealing atmosphere was 5% hydrogen + 95% nitrogen mixed gas, the annealing temperature was 800 to 840 ° C., and the annealing time was 90 seconds. The molten zinc bath was molten zinc containing 0.12% Al, and the basis weight of zinc was adjusted to 50 g / m ² with a gas wiper. The alloying was heated using induction heating type heating equipment so that the Fe content in the alloyed hot dip galvanizing was 10.5 to 11.5%. However, the hot-rolled steel sheet was preheated to 90 ° C. (90 seconds) instead of annealing and cooling. The Al content during plating of the galvannealed steel sheet thus obtained was 0.15 to 0.25%.

  After plating, temper rolling was performed at a rolling reduction of 0.5%, and a JIS No. 5 tensile test piece was collected and subjected to a tensile test. Table 6 shows the results of the tensile test.

  As an index of workability, a tensile test of each alloyed hot-dip galvanized steel sheet was conducted, and tensile strength, elongation, and Rankford value (r values; average r values of 0 °, 45 °, 90 °, but cold rolling) Steel sheet only) was measured.

BH properties are as follows. First, 2% tensile pre-strain is applied in the rolling direction, the load is once unloaded, a heat treatment equivalent to coating baking at 170 ° C. for 20 minutes is performed, and then a tensile test is performed again. It evaluated by calculating | requiring a raise, and 29 MPa or more was set as the pass.
The room temperature non-aging property was measured by measuring YP-El after artificial aging at 100 ° C. × 1 hr, and 0.2% or less was accepted.

Surface defects after press working were evaluated by performing a ball head overhang test. Test conditions are shown below.
・ Sample drawing width: 200 × 200mm
・ Mold: Ball head punch with radius 60mm, Dies with beads ・ Pressing load: 60t
-Overhang speed: 30 mm / min
・ Oiling: Antirust oil applied

For the evaluation of surface defects, 1000 ball head overhang tests were conducted, evaluated according to the following classification, and ◯ was set as acceptable.
○: Crack occurrence rate due to non-metallic inclusions is less than 0.5% Δ: Crack occurrence rate due to non-metallic inclusions is 0.5% or more and less than 3% ×: Caused by non-metallic inclusions Crack occurrence rate is 3% or more

The generation of streaks was evaluated by visually observing the entire length of the coil and evaluated according to the following classification.
○: The length of the portion mixed with the streak pattern is less than 0.5% of the total length Δ: The length of the portion mixed with the streak pattern is 0.5% or more and less than 10% of the total length X: The length of the portion where the streaks are mixed is 10% or more of the total length

  The results are shown in Table 6. Nos. 3, 6, 9, 12, 15, 18, and 22 had a non-aging property at room temperature because the acid-soluble Ti content was outside the scope of the present invention. Nos. 4, 7, 10, 13, 16, 19, and 23 failed in BH property because the acid-soluble Ti content was outside the scope of the present invention.

  The products of the present invention other than these were P-containing high-strength galvannealed steel sheets imparted with BH properties while ensuring non-aging at room temperature.

  As described above, the present invention makes it possible to provide a steel sheet to be plated that can produce a P-containing high-strength galvannealed steel sheet that suppresses the occurrence of surface defects such as streaks. The place that contributes to the development of

Claims (3)

% By mass
C: 0.0001 to 0.015%,
Si: 0.001 to 0.45%,
Mn: 0.2 to 2.8%,
P: 0.02-0.2%
S: 0.015% or less,
Al: 0.008% or less,
Ti: 0.002 to 0.10%,
N: 0.0005 to 0.004%,
Further, the content of Ce, La, Nd, Pr, Sm or a combination of two or more of 0.0001 to 0.01% in total, and the acid-soluble Ti content of the steel sheet comprising the balance Fe and inevitable impurities However, it satisfies the condition given by the following formula (1) ([% X] is the content of alloy element X expressed in mass%), and the non-metallic inclusions having an equivalent circle diameter of 10 μm or more observed with a microscope 70% or more is within the composition range of the following formulas (3) to (5), on the surface of the steel sheet, Al: 0.05 to 0.5 mass%, Fe: 7 to 15 mass%, The balance consists of Zn and inevitable impurities, and the ratio Iζ / x of the X-ray diffraction intensity Iζ and dΓ of d = 1.26 and d = 1.222 and the d = 3.13 X-ray diffraction intensity ISi of the Si standard plate ISi and IΓ / ISi satisfy Iζ / ISi ≦ 0.004 and IΓ / ISi ≦ 0.004. Alloyed molten galvanized steel plate, characterized in that the formation of the galvannealed layer.
[% C] × 4 + [% N] × 48 / 14−0.006% ≦ [% Ti] ≦ [% C] × 4 + [% N] × 48 / 14−0.002% (1)
10% ≦ TiO ₂ / (TiO ₂ + Al ₂ O ₃ + REM oxide) <95% (3)
0 ≦ Al ₂ O ₃ <50% (4)
5% ≦ REM oxide <90% (5)
TiO ₂ : concentration of Ti oxide in non-metallic inclusions in the steel sheet. Ti oxides in the form of Ti ₂ O ₃ and Ti ₃ O ₅ exist, but the Ti oxide concentration converted as TiO ₂ .
Al ₂ O ₃ : Concentration of Al oxide in non-metallic inclusions in the steel plate.
REM oxide: The total concentration of Ce, La, Nd, Pr, and Sm oxides in the non-metallic inclusions of the steel sheet. Amount of oxide converted as Ce ₂ O ₃ , La ₂ O ₃ , Nd ₂ O ₃ , Pr ₂ O ₃ , Sm ₂ O ₃ . % By mass
C: 0.0001 to 0.015%,
Si: 0.001 to 0.45%,
Mn: 0.2 to 2.8%,
P: 0.02-0.2%
S: 0.015% or less,
Al: 0.008% or less,
Ti: 0.002 to 0.10%,
Nb: 0.002 to 0.10%,
N: 0.0005 to 0.004%,
Further, the content of Ce, La, Nd, Pr, Sm or a combination of two or more of 0.0001 to 0.01% in total, and the acid-soluble Ti content of the steel sheet comprising the balance Fe and inevitable impurities However, it satisfies the condition given by the following formula (2) ([% X] is the content of alloy element X expressed in mass%), and the non-metallic inclusions having an equivalent circle diameter of 10 μm or more observed with a microscope 70% or more is within the composition range of the following formulas (3) to (5), on the surface of the steel sheet, Al: 0.05 to 0.5 mass%, Fe: 7 to 15 mass%, The balance consists of Zn and inevitable impurities, and the ratio Iζ / x of the X-ray diffraction intensity Iζ and dΓ of d = 1.26 and d = 1.222 and the d = 3.13 X-ray diffraction intensity ISi of the Si standard plate ISi and IΓ / ISi satisfy Iζ / ISi ≦ 0.004 and IΓ / ISi ≦ 0.004. Alloyed molten galvanized steel plate, characterized in that the formation of the galvannealed layer.
[% C] × 4 + [% N] × 48/14 − [% Nb] × 48 / 93−0.006% ≦ [% Ti] ≦ [% C] × 4 + [% N] × 48 / 14− [ % Nb] × 48 / 93-0.002% (2)
10% ≦ TiO ₂ / (TiO ₂ + Al ₂ O ₃ + REM oxide) <95% (3)
0 ≦ Al ₂ O ₃ <50% (4)
5% ≦ REM oxide <90% (5)
TiO ₂ : concentration of Ti oxide in non-metallic inclusions in the steel sheet. Ti oxides in the form of Ti ₂ O ₃ and Ti ₃ O ₅ exist, but the Ti oxide concentration converted as TiO ₂ .
Al ₂ O ₃ : Concentration of Al oxide in non-metallic inclusions in the steel plate.
REM oxide: The total concentration of Ce, La, Nd, Pr, and Sm oxides in the non-metallic inclusions of the steel sheet. Amount of oxide converted as Ce ₂ O ₃ , La ₂ O ₃ , Nd ₂ O ₃ , Pr ₂ O ₃ , Sm ₂ O ₃ . Steel sheet further as an additional component, in mass%, B: alloyed hot-dip galvanized steel plate according to claim 1 or 2, characterized in that it contains 0.0002 to 0.003%.