JP3752898B2 - Method for producing high-strength hot-dip galvanized steel sheet and high-strength galvannealed steel sheet - Google Patents

Description

【０１２５】
【表２】

【０１２６】
【表３】

【０１２７】
【表４】

【０１２８】
【表５】

【０１２９】
【発明の効果】
本発明によれば、不めっき欠陥の発生を防止し、加工性およびめっき密着性に優れた高強度溶融亜鉛めっき鋼板および高強度合金化溶融亜鉛めっき鋼板を得ることが可能となった。
さらに、本発明によれば、プレス成形性に優れた高強度合金化溶融亜鉛めっき鋼板を得ることが可能となった。
【０１３０】
この結果、本発明のめっき鋼板を適用することにより、自動車の軽量化、低燃費化が可能となり、ひいては地球環境の改善に大きく貢献することができる。
【図面の簡単な説明】
【図１】焼鈍炉における加熱時の鋼板板温：T₁および溶融亜鉛めっき前の加熱還元時の鋼板板温：T₂とめっき性および加工性との関係を示すグラフである。
【図２】溶融亜鉛めっき前の加熱還元時の鋼板板温：T₂および溶融亜鉛めっき前の加熱時の雰囲気ガス中H₂O 濃度とH₂濃度との比：H₂O/H₂とめっき性との関係を示すグラフである。
【図３】溶融亜鉛めっき前の加熱還元時の鋼板板温：T₂および鋼中Ｐ含有量とめっき性との関係を示すグラフである。
【図４】合金化めっき層の相構造および鋼中Ｐ含有量とめっき密着性との関係を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to high-strength steel sheets for use in such automotive vehicle body manufacture how the hot-dip galvanized steel sheets and galvannealed steel sheet was material.
[0002]
[Prior art]
In recent years, the application of high-strength steel sheets and high-strength hot-dip galvanized steel sheets that are excellent in corrosion resistance for automobiles is increasing from the viewpoint of safety, weight reduction, fuel consumption reduction, and improvement of the global environment.
Among them, in order to produce a high-strength hot-dip galvanized steel sheet, the desired strength and workability are obtained after having good plating properties, passing through a hot-dip galvanizing bath, and further subjected to alloying treatment. It is necessary to manufacture the original plate to be manufactured in advance.
[0003]
Moreover, it is necessary that the plating adhesion of the plated steel sheet is excellent so that the plating peeling is prevented when the plated steel sheet is pressed and the mold need not be maintained.
In general, in order to increase the strength of the steel sheet, it is generally performed to add easily oxidizable elements such as Mn, but these elements become oxides during the reduction annealing before plating, and are concentrated on the steel sheet surface. A so-called non-plating defect is generated on the surface of the steel sheet, resulting in a decrease in wettability with the molten zinc and, as a result, almost no plating adheres to the surface of the steel sheet.
[0004]
This is because the recrystallization annealing atmosphere is a reducing atmosphere for Fe and does not produce Fe oxide, but it becomes an oxidizing atmosphere for oxidizable elements such as Mn, and these elements are concentrated on the steel sheet surface to form an oxide film. It is for forming and reducing the contact area of molten zinc and a steel plate.
As a method for producing a high-strength hot-dip galvanized steel sheet, Japanese Patent Application Laid-Open No. 55-50455 discloses a method for defining the cooling rate after annealing at the time of plating. No mention is made at all. Particularly, when the Mn content exceeds 1%, it is difficult to prevent non-plating, and there is no mention of a method for improving plating adhesion.
[0005]
For this reason, the high-strength steel sheet with excellent workability that is attractive as a high-strength material for automobiles is also hot-dip galvanized and used as a surface-treated steel sheet with excellent workability and excellent plating adhesion. The reality is that it lacks practical means.
Japanese Patent Publication No. 7-9055 discloses a method for improving the alloying rate of the P-added steel by performing galvanizing after pickling after annealing. It is intended and is not a method for preventing unplating.
[0006]
Moreover, the above-mentioned method does not mention the dew point of the atmospheric gas, the hydrogen concentration, and the temperature at the time of annealing immediately before plating, and it is considered that non-plating frequently occurs depending on the combination conditions of the steel type and the annealing atmosphere.
Japanese Patent Application Laid-Open No. 7-268584 discloses a method of secondary annealing at a temperature determined by the P content in steel. This is because the temperature range for preventing steel plate embrittlement is P in steel. This is based on the technical idea that it depends on the content, and is not a disclosure about the temperature for improving the plating property.
[0007]
As described later in the present invention, in the method of re-annealing a steel plate that has been once annealed, an important matter for ensuring plating properties is the atmosphere during reductive annealing.
This is because if the P-type pickling residue generated on the steel plate surface is not sufficiently reduced when pickling the steel plate once annealed, an oxide film having poor wettability with molten zinc is plated on the steel plate immediately after annealing. It is because it inhibits sex.
[0008]
Therefore, the basis for prescribing the secondary annealing temperature according to the P content in the steel is completely different between the present invention described later and the technique disclosed in Japanese Patent Application Laid-Open No. 7-268584. The optimal secondary annealing temperature range is not the same.
In addition to the above-described plating properties, the alloyed hot-dip galvanized steel sheet is required to have excellent press formability.
[0009]
That is, alloyed hot-dip galvanized steel sheets are originally poor in slidability during press working, and are often subjected to Fe-based upper layer plating to improve slidability.
On the other hand, from the economical aspect, an alloyed hot-dip galvanized steel sheet excellent in press formability that does not require upper plating is required, and the slidability of the alloyed hot-dip galvanized steel sheet must be improved.
[0010]
[Problems to be solved by the invention]
The present invention provides a high-strength hot-dip galvanized steel sheet and high-strength alloyed hot-dip galvanized steel that are excellent in workability and plating adhesion that can solve the problems of the prior art and prevent the occurrence of non-plating defects. an object of the present invention is to provide a manufacturing how of steel plate.
[0011]
The present invention further aims to provide a manufacturing how excellent galvannealed steel sheet press formability.
[0012]
[Means for Solving the Problems]
1st invention hot-rolls the slab of the steel containing P: 0.10 wt% or less, and after hot-rolling the obtained hot-rolled steel plate as it is or after performing cold rolling, heating temperature: T ₁ is 750 ° C. or higher, then heated at 950 ° C. or less, after cooling, the concentrated layer of steel in components of the steel sheet surface was removed by pickling, the resultant steel sheet, a reducing atmosphere, the heating temperature: T ₂₎ 650 ° C or higher and 900 ° C or lower, and heat reduction under conditions satisfying both of the following formulas (1) and (2), followed by hot dip galvanizing. This is a method for producing an excellent high-strength hot-dip galvanized steel sheet.
[0013]
T ₂ ≦ T ₁ + K ………… (1)
T ₂ ≧ −T ₁ + L ……… (2)
However, in the above formulas (1) and (2),
K = 50log (H ₂ O / H ₂ ) +220 ……… (3)
L = 1000P + 1440 ………………… (4)
Indicate
In the above formulas (1) to (4),
T ₁ : heating temperature before cooling (° C.)
T ₂ : Heating temperature (° C.) at the time of heat reduction after pickling removal of the concentrated layer of the steel components
H ₂ O: H ₂ O concentration (vol%) in the atmospheric gas at the time of heating reduction after pickling removal of the concentrated layer of the above-mentioned components in steel
H ₂ : H ₂ concentration (vol%) in the atmospheric gas at the time of heating reduction after pickling removal of the concentrated layer of the above-mentioned components in steel
P: P content in steel (wt%)
Indicates.
[0014]
In the first invention described above, the dew point of the atmospheric gas at the time of the heat reduction after pickling removal of the concentrated layer of the above-described steel component is -50 ° C to 0 ° C, and the hydrogen concentration in the atmospheric gas is 1 to 100 vol. % (First preferred embodiment of the first invention).
In the first preferred embodiment of the first invention and the first preferred embodiment of the first invention, the pickling method for the concentrated layer of the steel component described above is an acid having a pH of 1 and a liquid temperature of 40 to 90 ° C. The pickling method is preferably pickling in the washing solution for 1 to 20 seconds (second preferred embodiment of the first invention, third preferred embodiment).
[0015]
In the first invention and the first preferred embodiment to the third preferred embodiment of the first invention, the coating amount of the high-strength hot-dip galvanized steel sheet is 20 to 120 g / m per one surface of the steel sheet. ² is preferable (fourth to seventh preferred embodiments of the first invention).
In the first preferred embodiment and the third preferred embodiment of the first invention described above, when the hydrogen concentration in the atmospheric gas is 1 vol% or more and less than 100 vol%, the remaining gas is an inert gas. The inert gas is preferably nitrogen gas.
[0016]
According to a second aspect of the present invention, there is provided a hot dip galvanized steel sheet obtained by the manufacturing method of the high-strength hot dip galvanized steel sheet according to the first aspect of the first invention and the first aspect of the first aspect. A method for producing a high-strength alloyed hot-dip galvanized steel sheet excellent in workability and plating adhesion, characterized by performing a heat alloying treatment.
In the above-described second invention, it is preferable that the maximum reached plate temperature of the hot-dip galvanized steel sheet during the above-described heat alloying treatment is 465 to 510 ° C, and further, the maximum reached plate temperature is 470 to 505 ° C. Is more preferable (first preferred embodiment of the second invention).
[0017]
In the second preferred embodiment and the first preferred embodiment of the second invention, the Fe content in the galvannealed layer is preferably 7 wt% or more and 13 wt% or less (second embodiment) Second preferred embodiment and third preferred embodiment of the invention).
In the second preferred embodiment and the first preferred embodiment to the third preferred embodiment of the second invention, the surface area of the galvannealed layer is 4 to 4 in the surface region: 100 μm × 100 μm. It is preferable to have 100 recesses (fourth to seventh preferred embodiments of the second invention).
[0018]
Further, in the above-described second invention, the first preferred embodiment to the seventh preferred embodiment of the second invention, the amount of plating adhesion of the above-described high-strength alloyed hot-dip galvanized steel sheet, It is preferably ²⁰ to 120 g / m ² per one side of the steel plate (eighth preferred embodiment to fifteenth preferred embodiment of the second invention).
In the third invention, in the X-ray diffraction of the zinc-iron alloy phase in the galvannealed layer, the ratio of the diffraction intensity of the Γ phase or the diffraction intensity of the ζ phase and the diffraction intensity of the δ phase is A high-strength galvannealed steel sheet excellent in plating adhesion, characterized by satisfying the following formulas (5) and (6). However, this third invention is outside the scope of the present invention.
[0019]
Γ (2.59mm) / δ (2.13mm) ≤ 0.008 × [{P content in steel (wt%)} ^-0.8 ] …………… (5)
ζ (1.26Å) / δ (2.13Å) ≦ 0.03 ……… (6)
In the above formulas (5) and (6),
Γ (2.59Å): Diffraction intensity ζ phase of crystal lattice plane spacing d = 2.59Å ζ (1.26Å): Diffraction intensity ζ phase of crystal lattice spacing d = 1.26Å δ (2.13Å): Crystal lattice plane spacing The diffraction intensity of the δ phase at d = 2.13% is shown.
[0020]
In the third invention described above, the Fe content in the galvannealed layer is preferably 7 wt% or more and 13 wt% or less (first preferred embodiment of the third invention).
In the above-described third and third preferred embodiments of the present invention, the P content in the galvannealed layer is preferably 0.0011 wt% or more and 0.03 wt% or less ( 2nd suitable aspect of 3rd invention, 3rd suitable aspect).
[0021]
In the above-described third invention and the first to third preferred embodiments of the third invention, the surface of the galvannealed alloy layer is 4 to 4 in the surface region: 100 μm × 100 μm. It is preferable to have 100 recesses (fourth to seventh preferred embodiments of the third invention).
Further, in the above-described third invention, the first preferred embodiment to the seventh preferred embodiment of the third invention, the plating adhesion amount of the alloyed hot-dip galvanized steel sheet of the high-strength alloyed hot-dip galvanized steel sheet, It is preferably ²⁰ to 120 g / m ² per one side of the steel sheet (eighth preferred embodiment to fifteenth preferred embodiment of the third invention).
[0022]
Further, the first invention, the second invention, the third invention, the first preferred embodiment to the seventh preferred embodiment of the first invention, and the first preferred embodiment to the fifteenth preferred embodiment of the second invention. In the preferred embodiment of the present invention, the first preferred embodiment to the fifteenth preferred embodiment of the third invention, the C content in the steel is more preferably 0.01 to 0.2 wt%, and Mo is contained in the steel at 1.00 wt% or less. It is more preferable to contain.
[0023]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail.
In the first invention described above, the hot-rolled steel sheet containing P of 0.10 wt% or less is pickled, cold-rolled as necessary, heated in a heating furnace (annealing furnace), cooled, and then the steel sheet surface The concentrated steel layer is removed by pickling, and the obtained steel sheet is preferably heated again at a predetermined temperature and atmosphere in a hot dip galvanizing line, and then subjected to hot dip galvanization, thereby improving workability and A high-strength hot-dip galvanized steel sheet excellent in both plating adhesion is produced.
[0024]
Further, the second invention is excellent in both workability and plating adhesion by heat-alloying the hot-dip galvanized steel sheet obtained by the method for producing the high-strength hot-dip galvanized steel sheet of the first invention described above. High strength alloyed hot-dip galvanized steel sheet.
The third invention is a high-strength galvannealed steel sheet obtained by the method for producing a high-strength galvannealed steel sheet according to the second invention, and the zinc-iron in the galvannealed alloyed layer. In the X-ray diffraction of the alloy phase, the ratio of the diffraction intensity of the Γ phase or the diffraction intensity of the ζ phase and the diffraction intensity of the δ phase is limited to an appropriate range, and more preferably, the amount of Fe diffusion in the alloyed hot dip galvanized layer Furthermore, it is a high-strength galvannealed steel sheet excellent in plating adhesion that limits the amount of P diffusion to an appropriate range.
[0025]
Next, an experiment that is the basis of the present invention will be described.
0.09% C-0.01% Si-2.0% Mn-0.005 to 0.1% P-0.003% S-0.041% Al-0.0026% N-0.02% Cr chemical composition (where each% indicates wt%), thickness A 30 mm thick sheet bar was heated to 1200 ° C. to obtain a hot rolled steel sheet having a thickness of 2.5 mm in 5 passes.
Next, the obtained hot-rolled steel sheet was processed in the following order (1) → (10).
[0026]
(1); heat treatment at 540 ° C. for 30 minutes to perform winding equivalent processing.
(2); Liquid temperature: Pickling in a 5 wt% HCl solution at 80 ° C for 40 seconds.
(3); Hold in an annealing furnace at 700 to 1000 ° C (steel plate temperature) for 1 minute in a reducing atmosphere containing hydrogen.
(4); cooled to room temperature at a cooling rate of 10 ° C / sec.
[0027]
(5); Liquid temperature: Pickling in a 5 wt% HCl solution at 60 ° C. for 10 seconds.
(6); Maintained at 600-950 ° C (steel plate temperature) for 20 seconds in a reducing atmosphere containing hydrogen.
(7); Cooled to 480 ° C at a cooling rate of 10 ° C / sec.
(8); Bath temperature containing 0.15 wt% of Al: Hot dip galvanizing is performed by dipping in a hot dip galvanizing bath at 480 ° C. for 1 second.
[0028]
(9); Adjust the coating amount of the plated steel sheet pulled from the hot dip galvanizing bath to 40 g / m ² by gas wiping.
(10); Hot dip galvanization obtained by applying hot dip galvanization immediately after heat reduction under the conditions of H ₂ = 1 to 100 vol, dew point (: dp): −50 ° C. to 0 ° C. in the above (6) The steel sheet is heat-alloyed at 400 to 600 ° C.
[0029]
In addition, the steel plate sample was extract | collected also about the hot dip galvanized steel plate which does not give the above-mentioned heat alloying process.
Next, the performance of the obtained plated steel sheet was evaluated by the following evaluation methods and evaluation criteria.
[Plating properties:]
Visual evaluation of appearance of galvanized steel sheet after galvanizing (unalloyed galvanized steel sheet) ○: No non-plating defects (good plating properties)
X: Non-plating defect occurs [plating adhesion:]
The plated steel sheet was bent back 90 degrees, the compression-side plating layer was peeled off with cellophane tape, and the amount of plating film adhered to the cellophane tape was evaluated.
[0030]
(Unalloyed plated steel sheet)
○: No peeling of plating layer (good plating adhesion)
×: Plating layer peeled off (Poor plating adhesion)
(Alloyed plated steel sheet)
○: Less plating peeling (good plating adhesion)
×: Large amount of plating peeling (Poor plating adhesion)
[Appearance after alloying:]
Visually evaluated.
[0031]
○: Uniform appearance was obtained with no alloying unevenness ×: Alloying unevenness occurred [Workability:]
A sample satisfying El ≧ 30% and satisfying TS ≧ 590 MPa was judged good, and one or both of them satisfying the above criteria was judged defective.
[0032]
FIG. 1 shows the evaluation results of the plateability and workability when the P content in steel is 0.01 wt%.
In FIG. 1, T ₁ (° C.) indicates the steel plate temperature during heating in the annealing furnace [: step (3) described above], and T ₂ (° C.) is during heat reduction before hot dip galvanizing [: above The temperature of the steel sheet in step (6)] is shown.
[0033]
Moreover, (circle) shows favorable conditions for both metal-plating property and workability, and x shows the conditions from which either metal-plating property, workability, or both become defect.
That is, in order to ensure both good plating properties and good workability, T ₁ and T ₂ described above satisfy any of the following formulas (1), (2), (7), and (8): There is a need to.
[0034]
T ₂ ≦ T ₁ + K ……………… (1)
T ₂ ≧ −T ₁ + L ……… (2)
750 ℃ ≦ T ₁ ≦ 950 ℃ …… (7)
650 ℃ ≦ T ₂ ≦ 900 ℃ …… (8)
However,
K = 50log (H ₂ O / H ₂ ) +220 ……… (3)
L = 1000P + 1440 ………………… (4)
In the above formula (3),
H ₂ O: H ₂ O concentration (vol%) in the atmospheric gas during the heating reduction before the hot dip galvanizing [: step (6) described above]
H ₂ : H ₂ concentration (vol%) in the atmospheric gas during the heating reduction before hot dip galvanization [: step (6) described above]
In the above formula (4),
P: P content in steel (wt%)
Indicates.
[0035]
In FIG. 1, when both the above formulas (7) and (8) are satisfied, the workability is good.
Further, when both of the above formulas (1) and (2) were satisfied, the plating property was good.
Next, FIG. 2 shows the evaluation of the plating properties when the steel plate temperature T ₁ = 800 ° C. and the P content in the steel = 0.01 wt% during the heating in the annealing furnace described above (step (3) described above). Results are shown.
[0036]
In FIG. 2, T ₂ , H ₂ O, and H ₂ indicate the following contents, as described above, ◯ indicates a condition with good plating properties, and × indicates a condition with poor plating properties.
T ₂ : Steel plate temperature (° C.) during heat reduction before hot dip galvanizing [: step (6) described above]
H ₂ O: H ₂ O concentration (vol%) in the atmospheric gas during the heating reduction before the hot dip galvanizing [: step (6) described above]
H ₂ : H ₂ concentration (vol%) in the atmospheric gas during the heating reduction before hot dip galvanization [: step (6) described above]
That is, as shown in FIG. 2, when the above formula (1) is satisfied, the plating property was good.
[0037]
Next, FIG. 3 shows the plating properties when the steel plate temperature T ₁ = 800 ° C. and the P content in the steel = 0.02 to 0.095 wt% during the heating in the annealing furnace described above (step (3) described above). The evaluation result of is shown.
In FIG. 3, like the T ₂ are described above, shows the following contents, ○ is good conditions plating properties, × indicates the conditions for the plating resistance is poor.
[0038]
T ₂ : Steel plate temperature (° C.) during heat reduction before hot dip galvanizing [: step (6) described above]
That is, as shown in FIG. 3, when the above-described formula (2) is satisfied, the plating property was good.
As mentioned above, although the experimental result used as the basis of this invention was described, the above-mentioned experimental result is considered to be based on the following reason.
[0039]
In the present invention, by heating in a continuous annealing line (hereinafter also referred to as first stage heating), an easily oxidizable alloy element such as Mn is once sufficiently concentrated on the steel plate surface, and the resulting concentrated layer is pickled and removed. Thereafter, heating is performed again in a reducing atmosphere (hereinafter also referred to as second stage heating).
As a result, during the second stage heating, the alloy elements in the surface layer of the base iron are deficient, so that the concentration of the easily oxidizable alloy element on the steel plate surface during the second stage heating and the formation of the oxide film are suppressed. It becomes possible to prevent the occurrence of defects.
[0040]
However, when the heating temperature of the steel plate in the first stage heating is low, the deficiency of the alloy elements in the surface layer of the iron base becomes insufficient, and when the heating temperature of the steel plate in the second stage heating is increased, the oxidation is easily performed in the second stage heating. Concentration of the ferritic alloy element on the surface of the steel sheet and formation of an oxide film occur, resulting in non-plating defects.
For this reason, the lowering of the allowable lower limit of the heating temperature of the steel plate in the second stage heating occurs with respect to the reduction of the heating temperature of the steel plate in the first stage heating.
[0041]
Further, when the H ₂ O / H ₂ ratio is the ratio of water concentration and hydrogen concentration in the atmospheric gas in the second stage heating is reduced, since the surface segregation mentioned above by a decrease in oxygen potential increases, H ₂ O / H _{As the 2} ratio decreases, the allowable lower limit of the heating temperature of the steel sheet in the second stage heating decreases.
As a result, the condition under which the occurrence of non-plating defects can be prevented and excellent plating adhesion can be obtained is represented by the following formula (1).
[0042]
T ₂ ≦ T ₁ + K ………… (1)
However,
K = 50log (H ₂ O / H ₂ ) +220 ……… (3)
In the above formulas (1) and (3),
T ₁ : Heating temperature of the steel plate in the first stage heating (: heating before removing the pickled layer of the concentrated component of steel)
T ₂ : Heating temperature of the steel plate in the second stage heating (heat reduction after pickling removal of the concentrated layer of steel components)
H ₂ O: H ₂ O concentration (vol%) in the atmosphere gas during the second stage heating (: during heating reduction after pickling removal of the concentrated layer of steel components)
H ₂ : H ₂ concentration (vol%) in the atmosphere gas during the second stage heating (: during heating reduction after pickling removal of the concentrated layer of steel components)
In addition, as a method for improving the plating property when the content of an easily oxidizable element such as Mn contained in the high-tensile steel plate is large, the present inventors once annealed (first stage heating) in an annealing furnace. After precipitating the surface concentrate of easily oxidizable elements such as, after removing the concentrate by pickling, the P-based oxide determined from the dew point of atmospheric gas, hydrogen concentration, and heating temperature is reduced thermodynamically. By plating immediately after heat reduction (second stage heating) under appropriate heating conditions, high-strength hot-dip galvanized steel sheets with no plating defects and excellent plating adhesion can be produced. I found out.
[0043]
Appropriate heating conditions in the second stage heating described above are expressed by the following formula (2).
T ₂ ≧ −T ₁ + L ……… (2)
However,
L = 1000P + 1440 ………………… (4)
In the above formulas (2) and (4),
T ₁ : Heating temperature (° C) of the steel sheet in the first stage heating (heating before removal of pickling of the concentrated layer of steel components)
T ₂ : Heating temperature (° C) of the steel plate in the second stage heating (: Heat reduction after pickling removal of the concentrated layer of steel components)
P: P content in steel (wt%)
Indicates.
[0044]
In addition, when the hot-dip galvanized steel sheet obtained by the above-described production method of the present invention is further heat-alloyed, the degree of alloying, that is, the Fe content in the hot-dip galvanized layer is 7 to 13 wt%, more preferably It was found that a high-strength galvannealed steel sheet excellent in plating adhesion after alloying was obtained when the content was 8 to 11 wt%.
In addition, by performing the two-stage heating in the present invention, the grain boundary P of the surface iron surface layer portion is cleaned, an outburst is generated during the subsequent hot galvanizing heating alloying, and not only the alloying is promoted, A crater is formed on the surface of the alloyed hot-dip galvanized layer, the surface shape is formed into a ring pattern, the slidability is improved by the oil retained in the recess during pressing, and a high-strength alloy with excellent formability It was found that a galvannealed steel sheet was obtained.
[0045]
Furthermore, as shown in FIG. 4, in the X-ray diffraction of the zinc-iron alloy phase of the galvannealed steel sheet obtained by the production method of the present invention described above, the diffraction intensity of the Γ phase and the diffraction intensity of the δ phase When the ratio of P satisfies the following formula (5) with respect to the P content in the steel, and the ratio of the diffraction intensity of the ζ phase and the diffraction strength of the δ phase satisfies the following formula (6): It was found that a high-strength galvannealed steel sheet with excellent plating adhesion can be obtained.
[0046]
Γ (2.59mm) / δ (2.13mm) ≤ 0.008 × [{P content in steel (wt%)} ^-0.8 ] …………… (5)
ζ (1.26Å) / δ (2.13Å) ≦ 0.03 ……… (6)
In the above formulas (5) and (6),
Γ (2.59Å): Diffraction intensity of Γ phase with crystal lattice spacing d = 2.59Å ζ (1.26Å): Diffraction strength of ζ phase with crystal lattice spacing d = 1.26Å δ (2.13Å): Crystal lattice spacing The diffraction intensity of the δ phase at d = 2.13% is shown.
[0047]
Further, the degree of alloying of the high-strength galvannealed steel sheet, that is, the Fe content in the galvannealed layer is 7 wt% or more and 13 wt% or less, more preferably 8 wt% or more and 11 wt% or less. It was found that a high-strength galvannealed steel sheet excellent in plating adhesion after alloying can be obtained.
Furthermore, when the P content in the galvannealed layer in the high-strength galvannealed steel sheet is 0.0011 wt% or more and 0.03 wt% or less, the plating adhesion after alloying is excellent, It was found that a high-strength galvannealed steel sheet excellent in spot weldability can be obtained.
[0048]
Hereinafter, I. Composition of base steel sheet, II. Production conditions and III. Fe content, P content, phase structure, surface shape (number of recesses) of alloyed hot-dip galvanized alloy layer will be described.
[I. Composition of base steel sheet:]
(P: 0.10wt% or less :)
P is an effective and inexpensive element for obtaining a high-strength steel plate. However, if the content exceeds 0.10 wt%, the spot weldability is significantly impaired, so the P content of the base steel plate is 0.10 wt% or less. Stipulated.
[0049]
In the present invention, the P content of the base steel sheet is more preferably 0.005 to 0.05 wt%.
(Mo :)
In the present invention, the base steel sheet preferably contains 1.00 wt% or less of Mo.
Mo is an element that does not impair the plating property and is effective in strengthening the solid solution.
[0050]
Furthermore, when the base material steel plate which added Mo is used, the tendency for the corrosion resistance of the plated steel plate obtained to become favorable is seen.
This is presumably because Mo is an element that is less likely to be oxidized than Fe, and slight diffusion and addition of Mo into the plating layer improves the corrosion resistance.
In the present invention, the Mo content in the base steel sheet is preferably 0.04 wt% or more.
[0051]
However, addition of more than 1.00 wt% significantly increases the manufacturing cost, so 1.00 wt% or less is preferable.
In the present invention, the Mo content of the base steel plate is more preferably 0.04 to 0.5 wt%.
(C :)
C is an element to be contained as a strengthening element. If the content is 0.01 wt% or more, a strengthening effect appears. If the content exceeds 0.2 wt%, the elongation decreases significantly, and in addition, the carbon equivalent increases. Damage the weldability.
[0052]
Therefore, the content of C is preferably 0.01 to 0.2 wt%, more preferably 0.03 to 0.15 wt%.
(Si :)
Si is an element that improves workability such as elongation by reducing the amount of solid solution C in the α-phase, but inclusion of Si in excess of 1.0 wt% impairs the plateability.
[0053]
The Si content of the base steel sheet in the present invention is preferably 1.0 wt% or less, more preferably 0.5 wt% or less, but in the present invention, the Si content is not particularly limited.
(Mn :)
The Mn content of the base steel sheet in the present invention is preferably 1.0 wt% or more in order to obtain high strength, and is preferably 3.0 wt% or less in order to avoid a decrease in elongation or an increase in carbon equivalent.
[0054]
(S :)
In addition to causing hot cracking during hot rolling, S is an element that induces fracture in the nugget of the spot weld, so it is desirable to reduce the S content as much as possible.
Therefore, in the present invention, the S content is preferably 0.05 wt% or less, and more preferably 0.010 wt% or less. However, in the present invention, the S content is not particularly limited.
[0055]
(Al :)
Al is an element effective as a deoxidizer in the steelmaking stage and to fix N which causes aging deterioration as AlN.
However, when the Al content exceeds 0.10 wt%, the manufacturing cost increases.
For this reason, the Al content is preferably 0.10 wt% or less, more preferably 0.05 wt% or less. However, in the present invention, the Al content is not particularly limited.
[0056]
(N :)
N causes aging deterioration and causes an increase in yield point (yield ratio) and yield elongation, so the N content must be suppressed to 0.010 wt% or less, and more preferably 0.005 wt% or less. In the present invention, the N content is not particularly limited.
[0057]
(Cr :)
Cr is an effective element for strengthening the structure. However, the addition of more than 1.0 wt% impairs the plateability. Therefore, the Cr content of the base steel sheet in the present invention is preferably 1.0 wt% or less. More preferably, it is 0.5 wt% or less, but the present invention is not particularly limited to the Cr content.
[0058]
(Ti, Nb, V :)
Ti, Nb, and V are elements that are effective in forming carbides and increasing the strength of the steel. If necessary, the base steel plate is made of one or more selected from Ti, Nb, and V. You may contain 0.0010 wt% or more in a total amount.
However, addition of more than 1.0 wt% increases the cost and raises the yield point (yield ratio) and decreases the workability.
[0059]
For this reason, the total amount of the base steel plate is preferably 0.0010 to 1.0 wt%, more preferably 0.010 to 0.20 wt%. However, in the present invention, the total amount is particularly limited to the contents of Ti, Nb, and V. is not.
[II. Manufacturing conditions:]
The manufacturing conditions in the present invention are as follows: [1] manufacturing process, [2] conditions in annealing (first stage heating), [3] pickling conditions after annealing (first stage heating), [4] reheating ( The conditions in the second stage heating) (heat reduction), [5] hot dip galvanizing, and the conditions in the heat alloying treatment will be described in this order.
[0060]
[1] Manufacturing process:
As described above, the manufacturing process in the present invention is as follows.
〔Manufacturing process:〕
Hot rolled steel plate → Pickling → (Cold rolling) → Annealing (first stage heating) → Cooling → Pickling → Reheating under reducing atmosphere (second stage heating) (Heat reduction) → Hot galvanizing → ( Heating alloying)
That is, in the present invention, a steel slab containing 0.10 wt% or less of P is hot-rolled and then pickled to remove the black scale.
[0061]
The steel sheet thus obtained is directly annealed in the next step (first stage heating), pickling, reheating in a reducing atmosphere (second stage heating) (heat reduction), hot dip galvanizing, and The steel sheet may be passed through a heating alloying treatment process, or after hot rolling, pickled to remove the black skin scale, and then subjected to cold rolling, followed by annealing in the next process (first Step heating), pickling, reheating in a reducing atmosphere (second stage heating) (heating reduction), hot dip galvanizing, and further heating alloying step may be used.
[0062]
That is, the base steel plate of the plated steel plate according to the present invention may be either a hot rolled plate or a cold rolled plate.
The functions of annealing (first stage heating), pickling, and reheating (second stage heating) (heating reduction) in a reducing atmosphere in the above steps of the present invention are as follows.
Annealing (first stage heating):
By heating the base steel plate in an annealing furnace, an easily oxidizable element such as Mn is concentrated on the steel plate surface.
[0063]
In addition, by heating the base steel plate in the annealing furnace before the second stage heating, the grain boundary P of the surface layer of the base metal is cleaned, and an outburst is likely to occur at the time of hot alloying of the hot dip galvanization in the subsequent process, As a result, a recess (crater) is formed on the surface of the galvannealed layer.
Pickling:
The concentrated layer of the steel component (easily oxidizable element) on the surface of the steel sheet formed in the annealing furnace is removed by pickling.
[0064]
Reheating in a reducing atmosphere (second stage heating) (heating reduction):
The P-based oxide on the steel sheet surface, which is the Fe-P-based pickling residue generated by the pickling, is reduced.
[2] Conditions in annealing (first stage heating):
750 ° C ≤ Heating temperature in annealing (first stage heating): T ₁ ≤ 950 ° C
In the present invention, the heating temperature in the annealing furnace of the base material steel plate: T ₁ is, 750 ° C. or higher, is defined as 950 ° C. or less.
[0065]
Heating temperature in annealing furnace: When T ₁ is less than 750 ° C, the surface concentration of easily oxidizable elements such as Mn generally contained in high-tensile steel is small, and Mn is reduced during reheating immediately after plating. Again, it thickens on the steel plate surface.
Further, when the heating temperature T ₁ in the annealing furnace is less than 750 ° C., the grain boundary P on the surface layer of the ground iron is not sufficiently cleaned.
[0066]
Therefore, in the annealing furnace, which is the first stage of heating, heating is performed at 750 ° C. or more to sufficiently enrich the surface of oxidizable elements such as Mn in the steel plate surface layer, and the grain boundary P on the surface layer of the steel plate. It is necessary to sufficiently clean.
Moreover, when the heating temperature in the annealing furnace: T ₁ is less than 750 ° C., the concentrated Mn cannot be dispersed in the band-like second phase (in the cementite precipitation phase in the steel) in the base material, Since non-plating defects occur, the heating temperature in the annealing furnace: T ₁ is 750 ° C. or higher.
[0067]
Conversely, the heating temperature in the annealing furnace: T ₁ may exceed 950 ° C., alpha-.gamma.2 not obtained the desired tissue and material far off for the phase region.
[3] Pickling conditions after annealing (first stage heating):
In this invention, after heating a base material steel plate in an annealing furnace, it cools, Then, the concentrated layer of the component in steel of the steel plate surface is removed by pickling.
[0068]
The acid of the acid solution in the pickling is not limited to hydrochloric acid, and sulfuric acid, nitric acid and the like can be used, and the acid type is not particularly limited.
The pickling solution is preferably operated at a pH of 1 or less.
When pH exceeds 1, the removal effect of the surface concentrate by pickling becomes inadequate.
When hydrochloric acid is used, the HCL concentration is preferably 1 to 10 wt%.
[0069]
If the HCL concentration is less than 1 wt%, the effect of removing the surface concentrate by pickling will be insufficient. Conversely, if it exceeds 10 wt%, the steel plate surface will be roughened by over pickling, and the basic unit of acid used will be It is expensive and inappropriate.
The temperature of the pickling solution is preferably 40 to 90 ° C. When the temperature is lower than 40 ° C, the effect of removing the surface concentrate by pickling is insufficient. The surface of the steel sheet is rough, which is inappropriate.
[0070]
The temperature of the pickling solution is more preferably in the range of 50 ° C to 70 ° C.
The pickling time is preferably 1 to 20 seconds, and if it is less than 1 second, the effect of removing the surface concentrate by pickling is insufficient. Roughness occurs, and the manufacturing time becomes longer and the productivity is lowered.
The pickling time is more preferably in the range of 5 to 10 seconds.
[0071]
[4] Conditions for reheating (second stage heating):
(1) Atmosphere: Reducing atmosphere (2) 650 ℃ ≦ T ₂ ≦ 900 ℃
(3) T ₂ ≦ T ₁ + K …… (1)
(4) T ₂ ≧ −T ₁ + L (2)
However, in the above formulas (1) and (2),
K = 50log (H ₂ O / H ₂ ) +220 ……… (3)
L = 1000P + 1440 ………………… (4)
Indicate
In the above formulas (1) to (4),
T ₁ : Heating temperature (° C) during annealing (first stage heating)
T ₂ : Heating temperature (° C) during reheating (second stage heating)
H ₂ O: H ₂ O concentration (vol%) in the atmospheric gas during reheating
H ₂ : H ₂ concentration in the atmospheric gas during reheating (vol%)
P: P content in steel (wt%)
Indicates.
[0072]
In the present invention, after the above pickling, the obtained steel sheet is preferably heated again in a reducing atmosphere in a heating furnace provided in a hot dip galvanizing line, and then hot dip galvanized.
The heating temperature at the time of reheating which is the above-mentioned second stage heating: T ₂ is defined as 650 ° C. or more and 900 ° C. or less.
[0073]
When the heating temperature T ₂ at the time of re-heating is lower than 650 ° C., there is a possibility that the iron oxide of the steel sheet surface is not reduced.
Furthermore, in the present invention, for the following reasons, the heating temperature T ₂ at the time of reheating needs to satisfy the following formula (1).
T ₂ ≦ T ₁ + K ………… (1)
However, in the above formula (1),
K = 50log (H ₂ O / H ₂ ) +220 ……… (3)
Indicate
In the above formula (3),
H ₂ O: H ₂ O concentration (vol%) in the atmospheric gas during heating (reheating) after pickling removal of the concentrated layer of steel components
H ₂ : H ₂ concentration (vol%) in atmospheric gas during heating (reheating) after pickling removal of concentrated layer of steel components
Indicates.
[0074]
That is, as described above, in the present invention, an oxidizable alloy element such as Mn is once sufficiently concentrated on the surface of the steel sheet in the annealing furnace, and after the resulting concentrated layer is pickled and removed, before galvanizing. Then, it is heated again in a reducing atmosphere.
As a result, since the alloy elements in the surface layer of the base metal are deficient during reheating, concentration of oxidizable alloy elements on the steel sheet surface and formation of oxide films during reheating are suppressed, preventing the occurrence of non-plating defects. It becomes possible to do.
[0075]
In contrast, the heating temperature in the annealing furnace is a heating of the first step: If T ₁ is low, becomes insufficient depletion of alloying elements in the base steel surface layer, easily oxidized during reheating in a heating of the second stage Concentration of the alloying element on the steel sheet surface and formation of an oxide film occur, and non-plating defects occur.
For this reason, the lowering of the allowable lower limit value of the heating temperature: T ₂ of the steel plate during reheating occurs with respect to the lowering of the heating temperature: T ₁ of the steel plate in the annealing furnace.
[0076]
In addition, when the H ₂ O / H ₂ ratio, which is the ratio between the moisture concentration and the hydrogen concentration of the atmospheric gas during reheating, decreases, the surface concentration increases due to the decrease in oxygen potential, so H ₂ O / H ₂ the heating temperature of the steel sheet at the time of re-heating with decreasing ratio: reduction in the T ₂ of the allowable lower limit value occurs.
As a result, the condition under which the occurrence of non-plating defects can be prevented and excellent plating adhesion can be obtained is represented by the following formula (1).
[0077]
T ₂ ≦ T ₁ + K ………… (1)
[K = 50log (H ₂ O / H ₂ ) +220 ……… (3)]
Further, when the heating temperature of the steel sheet during reheating: T ₂ exceeds 900 ° C., the structure formed in the annealing furnace is transformed, so that a desired structure and material cannot be obtained.
Furthermore, in the present invention, for the following reasons, the heating temperature T ₂ at the time of reheating needs to satisfy the following formula (2) corresponding to the P content in steel.
[0078]
T ₂ ≧ −T ₁ + L ……… (2)
However, in the above formula (2),
L = 1000P + 1440 ………………… (4)
Indicate
In the above formula (4),
P: P content in steel (wt%)
Indicates.
[0079]
That is, during pickling of a rolled steel sheet, a P-based oxide, which is a Fe-P-based pickling residue, is generated on the steel sheet surface along with the elution of the base iron, and the residue is completely reduced to improve the plating property. The temperature during reheating: T ₂ must be increased.
Further, the amount of P-based oxide produced is substantially proportional to the amount of P in steel.
For this reason, the heating temperature: T ₂ must be increased according to the above-described formula (2) as the amount of P in the steel increases.
[0080]
Furthermore, in the present invention, when the hydrogen concentration in the atmospheric gas at the time of reheating is less than 1 vol%, the P-based oxide on the steel sheet surface is slightly less thermodynamically reduced, and heating for a long time is required. The hydrogen concentration in the atmospheric gas during reheating is preferably 1 to 100 vol%.
Moreover, it is preferable that the dew point of atmospheric gas at the time of reheating is -50 degreeC-0 degreeC.
[0081]
This is because when the dew point of the atmospheric gas at the time of reheating exceeds 0 ° C., the P-based oxide on the surface of the steel plate is slightly less thermodynamically reduced, and heating for a long time is necessary. This is because it is industrially difficult to make the dew point lower than −50 ° C.
As described above, the atmosphere during reheating is compatible with both the promotion of the reduction of P-based oxides, which are Fe-P-based pickling residues on the steel sheet surface, and the suppression of the surface concentration of easily oxidizable elements such as Mn. Thus, good plating properties can be secured for the first time by simultaneously controlling the heating temperature, dew point, and hydrogen concentration.
[0082]
[5] Conditions for hot dip galvanizing and heat alloying treatment:
In the present invention, the base steel plate is heated and reduced as described above, and then galvanized in a hot dip galvanizing bath.
The hot dip galvanizing bath is suitably a plating bath containing 0.08 to 0.2 wt% of Al, and the bath temperature is suitably 460 to 500 ° C.
[0083]
The plate temperature of the steel sheet when entering the bath is suitably 460-500 ° C.
Moreover, it is preferable that the plating adhesion amount of a hot dip galvanized steel plate is 20-120 g / m < ² > per one surface of a steel plate.
When the adhesion amount of the hot dip galvanizing is less than 20 g / m ² , the corrosion resistance is lowered. Conversely, when the adhesion amount of the plating exceeds 120 g / m ² , the effect of improving the corrosion resistance is practically saturated and is not economical.
[0084]
In addition, the above-mentioned adhesion amount per one side of the steel sheet indicates an adhesion amount per unit area obtained by dividing the plating adhesion amount by the plating adhesion area.
That is, in the case of normal double-sided plating, the amount of adhesion per unit area divided by the amount of plating adhered on both sides is shown. In the case of single-sided plating, the unit of plating adhesion divided by the area of plating adhesion on one side The amount of adhesion per area is shown.
[0085]
In the present invention, the heat alloying treatment is preferably performed under the following conditions.
That is, the maximum reached plate temperature of the steel sheet during the heat alloying treatment is preferably in the range of 465 to 510 ° C, and the maximum reached plate temperature is more preferably in the range of 470 to 505 ° C. .
This is because when the maximum temperature of the steel sheet during the heat alloying treatment is less than 465 ° C., the ζ phase and further the Γ phase described later are easily generated on the surface layer of the Fe-Zn alloy layer, and the plating adhesion is reduced. On the other hand, when the maximum plate temperature exceeds 510 ° C., the Γ phase is easily generated and the plating adhesion is lowered.
[0086]
In the present invention, the coating amount of the galvannealed steel plate is preferably ²⁰ to 120 g / m ² as the amount of coating per one surface of the steel plate defined above.
When the coating weight of alloyed hot dip galvanizing is less than 20 g / m ² , the corrosion resistance decreases, and conversely, when the coating weight exceeds 120 g / m ² , the corrosion resistance improvement effect is practically saturated and is not economical. .
[0087]
In addition, the plating adhesion amount of the above-mentioned alloyed hot dip galvanizing was analyzed by dissolving the plating layer in an alkali-containing solution such as NaOH or KOH or an acid-containing solution such as HCl or H ₂ SO ₄ and analyzing the resulting plating solution. Can be measured.
[III. Fe content, P content, phase structure, surface shape (number of recesses) of alloyed hot-dip galvanized alloy layer:]
When the hot-dip galvanized steel sheet produced by the method for producing a hot-dip galvanized steel sheet according to the present invention is heat-alloyed, in order to improve the plating adhesion after alloying, in the plated layer after heat-alloying The amount of Fe diffusion, that is, the Fe content in the plating layer is preferably 7 to 13 wt%, and more preferably 8 to 11 wt%.
[0088]
If the Fe content in the plated layer after alloying is less than 7 wt%, not only unevenness of the burn will occur, but peeling (flaking) of the plated layer will occur, and if the Fe content exceeds 13 wt%, It causes deterioration of plating adhesion due to the alloy.
Furthermore, the present invention controls the alloying temperature after hot dip galvanizing according to the P content in the base steel sheet, regulates the phase structure of the alloyed plating layer, and ensures excellent plating adhesion. It is what.
[0089]
In the present invention, the phase structure of the alloyed plating layer is quantified by the following method.
That is, after a plating layer is bonded to an iron plate using an epoxy adhesive and the adhesive is cured, the plating layer is mechanically pulled to peel the plating layer together with the adhesive from the base iron interface.
Next, the peeled plating layer is subjected to X-ray diffraction under the following conditions from the interface side of the plating layer / steel plate (plated steel plate), and the diffraction peak due to the alloy phase is measured.
[0090]
[Conditions for X-ray diffraction:]
Measurement surface: 15mm diameter circular shape θ / 2θ method X-ray tube: Cu tube Voltage: 50kV
Tube current: 250mA
Among diffraction peaks due to alloy phases, diffraction intensity (cps) of crystal lattice spacing d = 2.59 ら れ る considered to be derived from Γ phase (Fe ₃ Zn ₁₀ ) and Γ ₁ phase (Fe ₅ Zn ₂₁ ): Γ (2.59 Å ), Diffractive intensity (cps) of crystal lattice spacing d = 2.13 Å considered to be derived from δ ₁ phase (FeZn ₇ ): δ (2.13 Å), and crystal lattice plane considered to be derived from ζ phase (FeZn ₁₃ ) Diffraction intensity (cps) at interval d = 1.26 ：: ζ (1.26 測定) measured, ratio of Γ (2.59 Å) and δ (2.13 Å), ratio of ζ (1.26 Å) and δ (2.13 Å) Ask for.
[0091]
Incidentally, it fractionating gamma phase and gamma ₁ phase for crystallographically difficult, in the present invention is labeled as gamma phases combined gamma phase and gamma ₁ phase.
In the present invention, the strength of the Γ phase preferably satisfies the following formula (5) with respect to the P content (wt%) in the steel, and the strength of the ζ phase satisfies the following formula (6): It is preferable to do.
[0092]
Γ (2.59mm) / δ (2.13mm) ≤ 0.008 × [{P content in steel (wt%)} ^-0.8 ] …………… (5)
ζ (1.26Å) / δ (2.13Å) ≦ 0.03 ……… (6)
Since the Γ phase is hard and brittle, and it forms at the interface between the plating layer and the ground iron, if the strength of the Γ phase is greater than the range of the above formula (5) with respect to the strength of the δ phase, alloyed hot dip galvanizing When the steel sheet is pressed, the Γ phase is destroyed, and the plating layer peels off at the Γ phase as a boundary.
[0093]
Furthermore, the allowable upper limit of the strength ratio of the Γ phase to the δ phase decreases as the P content in the steel increases because the P-added steel generally does not easily generate the Γ phase.
In addition, since the ζ phase has poor slidability, when the strength ratio of the ζ phase generated on the upper surface of the plating layer to the δ phase is larger than the range of the above formula (6), the plating layer is easily peeled off during pressing.
[0094]
The reason why both the strength of the Γ phase and the strength of the ζ phase are limited by the above-described production method of the present invention and a high-strength galvannealed steel sheet with good plating adhesion can be produced is as follows. is there.
That is, when a steel plate is once heated in a heating furnace before plating, an alloying delay occurs during hot galvanization heating alloying.
[0095]
On the other hand, in the case of a steel sheet containing P, P in the steel that has segregated at the grain boundary once concentrates on the surface when the steel sheet before plating is heated, so that P that segregates at the grain boundary decreases.
As a result, the concentrated layer is removed by pickling and then heated again in the hot dip galvanizing line (reduction heating). High-speed alloying becomes possible at a low temperature of 500 ° C. or lower, and the formation of Γ phase is suppressed.
[0096]
Further, since alloying can be performed in the vicinity of 500 ° C. in a short time, formation of ζ phase can be suppressed.
For this reason, for the first time by the production method of the present invention, the formation of a Γ phase and a ζ phase that adversely affect the plating adhesion is suppressed, and the high-strength galvannealed steel sheet having the above-described phase structure and excellent plating adhesion It became possible to get.
[0097]
In the galvannealed steel sheet according to the present invention, the P content in the galvannealed layer is preferably 0.0011 wt% or more and 0.03 wt% or less.
The reason will be described below.
That is, according to the production method of the present invention, P segregated at the grain boundary moves from the grain boundary to the surface of the iron core by heating in the first annealing furnace. P easily diffuses into the surface.
[0098]
The plated steel sheet in which P is diffused in the plating layer has extremely good plating adhesion compared to the plated steel sheet in which P is not diffused.
The exact reason for this is unknown, but can be estimated as follows. That is, when diffusion of P into the plating layer at the time of heating alloying is easy, a p-based oxide is generated on the plating surface in addition to the Zn-based oxide at the time of alloying, and this oxide works as a lubricant. It is presumed that the slidability is improved and plating peeling (flaking) is less likely to occur.
[0099]
Since this diffusion amount depends on the delicate surface state of the steel sheet surface immediately before plating, it is not necessarily proportional to the Fe diffusion amount.
Furthermore, when P is concentrated during annealing, it may be greater than the amount of P diffusion in the steel and the amount of P diffusion that can be simply estimated from the amount of Fe diffusion during alloying. In the alloy hot-dip galvanized steel sheet of the present invention, Even in the case of steel with a P content of 0.1 wt%, the upper limit of the amount of P diffusion into the plated layer after heat alloying (: P content in the plated layer) was 0.03 wt%.
[0100]
According to the present invention, when the P content in the plated layer after heating alloying satisfies 0.0011 wt% or more and 0.03 wt% or less, the high strength alloyed hot dip galvanizing excellent in plating adhesion improvement effect. It was found that a steel plate was obtained.
If the amount of P diffusion into the plating layer (: P content in the plating layer after heat alloying) is less than 0.0011 wt%, the effect of improving the plating adhesion is not seen, and conversely exceeds 0.03 wt% In this case, the P content in the steel must be more than 0.1 wt%, and other performances such as spot weldability deteriorate.
[0101]
In the present invention, the P content in the plated layer after heat alloying is more preferably 0.0011 wt% or more and 0.020 wt% or less.
Furthermore, according to the present invention, by performing two-stage heating, an outburst occurs during alloying of the hot dip galvanized steel sheet, and a crater is formed on the surface of the alloyed hot dip galvanized layer. Oil retention is improved during the press working of the plated steel sheet, and slidability is improved.
[0102]
As a result, according to the present invention, flaking such as plating peeling at the time of press working of the galvannealed steel sheet can be prevented.
In the present invention, the surface area of the plated layer of the galvannealed steel sheet is observed with an optical microscope, a scanning electron microscope (SEM), or a laser microscope. It is preferable that 4 to 100 recesses exist.
[0103]
When the number of recesses in the surface area: 100 μm × 100 μm is less than 4, it is difficult to obtain an effect of improving press formability by improving oil retention.
Conversely, when the number of concave portions in the surface region: 100 μm × 100 μm exceeds 100, the volume of the crater convex portion (the outer ring surrounding the edge of the crater concave portion) increases as the number of concave portions increases.
[0104]
As a result, the actual volume of the crater recess is reduced, the absolute amount of oil retained is reduced, and the sliding improvement effect is reduced.
In the present invention, the depth of the concave portion of the crater-like uneven portion on the surface of the plated layer of the galvannealed steel sheet is 2 to 10 μm, and the height of the convex portion (the outer ring surrounding the edge of the crater concave portion) is 3 μm or less. More preferably, the number of recesses is 4 to 100 in the surface region: 100 μm × 100 μm.
[0105]
This is because if the depth of the recess is less than 2 μm, the oil retainability is poor during pressing, if the depth of the recess exceeds 10 μm, if the height of the protrusion exceeds 3 μm, This is because the effect of improving the slidability by the retained oil is reduced.
The depth of the concave portion and the height of the convex portion can be determined by observing the alloyed hot-dip galvanized steel sheet obliquely with an optical microscope or a scanning electron microscope (SEM), and can be determined with a laser microscope. .
[0106]
In the present invention, in order to obtain a very excellent slidability improving effect, it is more preferable that the upper limit value of the number of recesses in the surface region: 100 μm × 100 μm is 70.
[0107]
【Example】
Hereinafter, the present invention will be described more specifically based on examples.
A 300 mm thick continuous cast slab having the chemical composition shown in Table 1 (steel type: A to N) is heated to 1200 ° C., roughly rolled for 3 passes, and then rolled with a 7-stand finish rolling mill to obtain a thickness of 2.3 mm. A hot rolled steel sheet was obtained.
[0108]
Then, it wound up at the temperature (: CT) shown in Table 2.
After pickling the obtained hot-rolled steel sheet, after steel plate A was hot-rolled steel sheet (Example 1, Comparative Examples 5 and 6) or cold-rolled to a thickness of 1.0 mm (Comparative example 1-4), and Tsuban a continuous annealing line, the heating temperature shown in Table 2: was annealed at T _1.
[0109]
For steel types B to N, after pickling the hot-rolled steel sheet, it was cold-rolled to a thickness of 1.0 mm, then passed through a continuous annealing line, and the heating temperatures shown in Table 2 were: and annealed by T ₁ (example 2 to 14, Comparative examples 7 to 10).
Then, the obtained rolled steel plate of each steel type was passed through a continuous hot dip galvanizing line, and pretreatment pickling, heat reduction, hot dip galvanizing, and heat alloying treatment were performed under various conditions shown in Table 2.
[0110]
Moreover, in Example 6, the performance of the obtained hot dip galvanized steel sheet was evaluated based on the evaluation method and evaluation criteria described later without performing the heat alloying treatment.
In addition, manufacturing conditions other than the manufacturing conditions shown in Table 2 are shown in the following (1) to (3).
(1) Pickling (pretreatment pickling) in continuous galvanizing line:
Pickling (pretreatment pickling) in the continuous hot dip galvanizing line shown in Table 2 is as follows: liquid temperature: 60 ° C., HCl concentration: 5 wt% pickling solution (pH = 1 or less) or liquid temperature: 60 ° C., H ₂ SO ₄ concentration: An experiment was carried out by pickling for 10 seconds using a pickling solution (pH: 1 or less) of 5 wt%, and an effect for improving the plating property was recognized under either condition.
[0111]
(2) Heat reduction in continuous hot dip galvanizing line:
The heat reduction in the continuous hot dip galvanizing line was performed in an H ₂ -N ₂ gas atmosphere having the H ₂ concentration shown in Table 2.
(3) Plating adhesion amount of hot dip galvanizing, Plating adhesion amount of alloyed hot dip galvanizing:
The coating amount of hot dip galvanizing was 40 g / m ^{2 on} both sides of the steel plate.
[0112]
In addition, the coating amount of the alloyed hot dip galvanizing was in the range of 30 to 60 g / m ^{2 on} both sides of the steel sheet.
Next, plating performance, plating adhesion, press formability, appearance after alloying, degree of alloying, P content in alloyed plating layer, alloying of the obtained hot dip galvanized steel sheet, galvannealed steel sheet The phase structure, workability, corrosion resistance, and spot weldability of the plating layer were evaluated based on the following evaluation methods and evaluation criteria.
[0113]
The obtained evaluation results are shown in Table 3.
In addition, the presence or absence of the reduction | restoration of the P-type oxide in Table 2 was judged by analyzing the steel plate surface with ESCA (: photoelectron spectrometer), and whether the peak of the P compound considered to couple | bond with oxygen is recognized clearly. .
The P compound that is considered to be bonded to oxygen described above is phosphate group (PO ₄ ³⁻ ), hydrogen phosphate group (HPO ₄ ²⁻ , H ₂ PO ₄ ⁻ ), hydroxyl group (OH ⁻ ) and iron ion ( The following iron phosphate compounds having Fe ³⁺ and Fe ²⁺ ) as main constituent elements.
[0114]
Iron phosphate compounds: Fe ^III (PO ₄ ) · nH ₂ O, Fe ^III ₂ (HPO ₄ ) ₃ · nH ₂ O, Fe ^III (H ₂ PO ₄ ) ₃ · nH ₂ O, Fe ^II ₃ (PO ₄ ) ₂ · nH ₂ O, Fe ^II (HPO ₄ ) · nH ₂ O, Fe ^II (H ₂ PO ₄ ) ₂ · nH ₂ O, Fe ^III (HPO ₄ ) (OH) · nH ₂ O, Fe ^III ₄ {( PO ₄ ) (OH)} ₃ · nH ₂ O (n is an integer of 0 or more)
In addition, ESCA is measured by a conventional method, and is described as an actual measurement example in a general spectrum collection, focusing on the spectral intensity of P at a position considered to be combined with O, corresponding to the iron phosphate compound described above, The peak height is clearly recognized when the height H from the base of the peak position satisfies the relationship of H ≧ 3N as compared with the average amplitude N of the noise part other than the peak.
[0115]
[Plating properties:]
Visually evaluate the appearance of the hot-dip galvanized steel sheet (unalloyed hot-dip galvanized steel sheet).
○: No plating defects (good plating properties)
×: Non-plating defect occurred [plating adhesion:]
The plated steel sheet was bent back 90 degrees, the compression-side plating layer was peeled off with cellophane tape, and the amount of plating film adhered to the cellophane tape was evaluated.
[0116]
(Unalloyed plated steel sheet)
○: No peeling of plating layer (good plating adhesion)
×: Plating layer peeled off (Poor plating adhesion)
(Alloyed plated steel sheet)
○: Less plating peeling (good plating adhesion)
×: Large amount of plating peeling (Poor plating adhesion)
[Surface shape (number of recesses):]
The number of recesses with a depth of 2 to 10 μm on the surface of the plated layer of the alloyed hot-dip galvanized steel sheet and the height of the protrusions surrounding the edges of the recesses is 3 μm or less is the surface area: 100 μm × 100 μm Was measured with a laser microscope.
[0117]
[Press formability:]
The oiled galvannealed steel sheet was pressed to investigate whether it could be processed with little peeling of the plating.
That is, after press working, the plating adhering to the mold was peeled off with a cellophane tape, and the amount of the plating film adhering to the cellophane tape was evaluated.
[0118]
○: Plating peeling amount is small (good press formability)
X: Large amount of plating peeling (poor press formability)
[Appearance after alloying:]
Visually evaluated.
○: Uniform appearance was obtained with no alloying unevenness ×: Alloying unevenness occurred [degree of alloying, P content in alloyed plating layer:]
The plating layer was dissolved by a general plating layer dissolution method using an alkaline solution or an acidic solution, and the Fe content and the P content in the alloyed hot-dip galvanized layer were analyzed and measured by analyzing the obtained solution.
[0119]
[Phase structure of alloyed plating layer:]
Quantification was performed as described above.
[Machinability:]
Those satisfying TS ≧ 590 MPa and satisfying El ≧ 30% were evaluated as good, and the others were regarded as defective.
[0120]
[Corrosion resistance:]
The corrosion resistance test was evaluated by the weight loss by the salt spray test (SST).
[Spot weldability:]
Pressure: 2.01 kN, Current: 3.5 kA, Energizing time: Ts = 25 cyc., Tup = 3 cyc., Tw = 8 cyc., Th = 5 cyc., To = 50 cyc. Those that could be welded were considered excellent, and those that could not be welded were considered defective.
[0121]
As shown in Tables 2 and 3, none of the alloyed hot-dip galvanized steel sheets of Examples 1 to 5 and Examples 7 to 14 manufactured by the manufacturing method of the present invention is free from non-plating defects and is plated. In addition to excellent properties, there was no problem with respect to plating adhesion, press formability, appearance after alloying, workability, and spot weldability.
In addition, the hot dip galvanized steel plate of Example 6 was excellent in plating properties with no unplating defects, and had no problems with respect to plating adhesion, workability, and spot weldability.
[0122]
On the other hand, the alloyed hot dip galvanized steel sheets of Comparative Examples 1 to 10 are the heating reduction temperature prior to hot dip galvanizing, the alloying temperature during hot alloying after hot dip galvanizing, the degree of alloying, and the P content in the plating layer. The amount, the phase structure of the alloyed plating layer, the surface shape (number of recesses) or the steel composition are different from the conditions of the present invention or more suitable conditions, so that non-plating defects occur, plating quality, press formability Or workability and spot weldability were poor.
[0123]
Furthermore, as shown in Table 3, in the present invention, it was found that the plated steel sheet containing Mo in the base material steel sheet has little corrosion weight loss and excellent corrosion resistance.
This is presumably because a corrosion inhibiting effect is obtained by diffusion and addition of Mo into the plating layer.
[0124]
[Table 1]

[0125]
[Table 2]

[0126]
[Table 3]

[0127]
[Table 4]

[0128]
[Table 5]

[0129]
【The invention's effect】
According to the present invention, it is possible to obtain a high-strength hot-dip galvanized steel sheet and a high-strength galvannealed steel sheet that are excellent in workability and plating adhesion by preventing the occurrence of non-plating defects.
Furthermore, according to the present invention, a high-strength galvannealed steel sheet excellent in press formability can be obtained.
[0130]
As a result, by applying the plated steel sheet according to the present invention, it is possible to reduce the weight and fuel consumption of the automobile, thereby contributing greatly to the improvement of the global environment.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between steel plate temperature during heating in an annealing furnace: T ₁ and steel plate temperature during heating reduction before hot dip galvanizing: T ₂ and plating properties and workability.
[Fig. 2] Steel plate temperature during hot reduction before hot dip galvanization: T ₂ and ratio of H ₂ O concentration to H ₂ concentration in atmospheric gas during heating before hot dip galvanization: H ₂ O / H ₂ It is a graph which shows the relationship with plating property.
FIG. 3 is a graph showing the relationship between the steel plate temperature at the time of heat reduction before hot dip galvanizing: T ₂ and the P content in steel and the plating properties.
FIG. 4 is a graph showing the phase structure of an alloyed plating layer and the relationship between P content in steel and plating adhesion.

Claims (7)

P: hot-rolled steel slab containing 0.10 wt% or less, and hot-rolled steel sheet obtained after pickling, or after cold rolling, heating temperature: T ₁ is 750 ° C or higher After heating at 950 ° C. or lower and cooling, the concentrated layer of steel components on the steel plate surface is removed by pickling, and the resulting steel plate is heated in a reducing atmosphere at a heating temperature: T ₂ of 650 ° C. or higher. High-strength hot-dip zinc alloy with excellent workability and plating adhesion, characterized by hot reduction under 900 ℃ or less and satisfying both of the following formulas (1) and (2). Manufacturing method of plated steel sheet.
Record
T ₂ ≦ T ₁ + K ………… (1)
T ₂ ≧ −T ₁ + L ……… (2)
However, in the above formulas (1) and (2),
K = 50log (H ₂ O / H ₂ ) +220 ……… (3)
L = 1000P + 1440 ………………… (4)
Indicate
In the above formulas (1) to (4),
T ₁ : heating temperature before cooling (° C.)
T ₂ : Heating temperature (° C.) at the time of heat reduction after pickling removal of the concentrated layer of the steel components
H ₂ O: H ₂ O concentration (vol%) in the atmospheric gas at the time of heating reduction after pickling removal of the concentrated layer of the above-mentioned components in steel
H ₂ : H ₂ concentration (vol%) in the atmospheric gas at the time of heating reduction after pickling removal of the concentrated layer of the above-mentioned components in steel
P: P content in steel (wt%)
Indicates.   The dew point of the atmospheric gas at the time of the heating reduction after pickling removal of the concentrated layer of the above-described component in steel is -50 ° C to 0 ° C, and the hydrogen concentration in the atmospheric gas is 1 to 100 vol%. Item 2. A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and plating adhesion according to Item 1.   The pickling method for the concentrated layer of the above-mentioned steel component is a pickling method in which pickling is performed in a pickling solution of pH ≦ 1, liquid temperature: 40 to 90 ° C. for 1 to 20 seconds. Item 3. A method for producing a high-strength hot-dip galvanized steel sheet excellent in workability and plating adhesion according to Item 1 or 2.   A heat-alloying treatment is further applied to the hot-dip galvanized steel sheet obtained by the method for producing a high-strength hot-dip galvanized steel sheet according to any one of claims 1 to 3. A method for producing excellent high-strength galvannealed steel sheets.   5. The high-strength galvannealed high-galvanized steel excellent in workability and plating adhesion according to claim 4, wherein the maximum temperature of the hot-dip galvanized steel sheet during the heat-alloying treatment is 465 to 510 ° C. Manufacturing method of steel sheet.   The high-strength galvannealed steel sheet with excellent workability and plating adhesion according to claim 4 or 5, wherein the Fe content in the galvannealed layer is 7 wt% or more and 13 wt% or less. Manufacturing method. 7. The processability and plating adhesion according to claim 4, wherein the surface of the galvannealed layer has 4 to 100 recesses in a surface region of 100 μm × 100 μm. A method for producing excellent high-strength galvannealed steel sheets .

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