JP5799819B2 - Method for producing hot-dip galvanized steel sheet with excellent plating wettability and pick-up resistance - Google Patents

Description

  The present invention relates to a method for producing a hot dip galvanized steel sheet, and more specifically, the production of a hot dip galvanized steel sheet that is excellent in plating wettability and pick-up resistance and can be applied as a member in the automotive field, home appliance field, and building material field. It is about the method.

  In members of the automobile field, the home appliance field, and the building material field, surface treated steel sheets imparted with rust prevention properties are used, and hot dip galvanized steel sheets are used because they can be manufactured at low cost and are excellent in rust prevention properties.

  Generally, a hot dip galvanized steel sheet is manufactured by the following method using a continuous hot dip galvanizing facility. First, using a thin steel plate obtained by hot-rolling, cold-rolling or heat-treating a slab, degreasing and / or pickling in the pre-treatment process for the purpose of cleaning the surface of the base steel plate, or preheating by omitting the pre-treatment step After the oil on the surface of the base steel plate is burned and removed in the furnace, recrystallization annealing is performed by heating. The atmosphere during recrystallization annealing is heated in a reducing atmosphere of Fe because the oxide of Fe inhibits the wettability of plating when the subsequent plating is processed. After the recrystallization annealing, the steel sheet is continuously cooled to a temperature suitable for plating in a reducing atmosphere of Fe, and hot dip galvanized by being immersed in a hot dip zinc bath without being exposed to the air.

  In continuous hot dip galvanizing equipment, the types of furnaces that perform recrystallization annealing include DFF (direct flame type), NOF (non-oxidation type), all radiant tube type (total reduction type), or combinations thereof. .

  However, when the Fe oxide scale is formed on the surface of the steel sheet being heated, it is picked up by a roll in the heating furnace, and the wrinkles are generated on the steel sheet. This is a serious problem for parts such as automotive steel plates. For this reason, all radiant tube types, which suppress the generation of oxidized scale of Fe by making the entire inside of the furnace a reducing atmosphere of Fe and hardly pick up the rolls in the heating furnace, are mainly used. Thus, a hot dip galvanized steel sheet having a beautiful plated appearance that is difficult to pick up an oxide scale on a roll in a heating furnace and has no push-up is called a hot dip galvanized steel sheet having excellent pick-up resistance.

  In recent years, especially in the automobile field, in order to achieve both weight saving for the purpose of improving fuel consumption as well as the function of protecting passengers in the event of a collision, the molten zinc has been strengthened by adding elements such as Si and Mn. The use of plated steel sheets is increasing.

  However, since Si and Mn are elements that are more easily oxidized than Fe, Si and Mn are oxidized even in a reducing atmosphere of Fe during heating of recrystallization annealing in the all radiant tube type. Therefore, a steel sheet containing Si or Mn is oxidized in the process of recrystallization annealing, and Si and Mn on the surface of the steel sheet are oxidized. Further, Si and Mn other than the steel sheet surface are oxidized if they reach the surface by thermal diffusion. Mn oxide is concentrated on the surface of the steel sheet.

  When the oxides of Si and Mn are concentrated on the surface of the steel sheet after annealing, contact between the molten zinc and the steel sheet is hindered in the process of being immersed in the molten zinc bath, which causes the wettability of the plating to decrease. If the wettability of the plating is lowered, non-plating defects occur, resulting in poor appearance and poor rust prevention. Further, when the oxides of Si and Mn are concentrated, there is a problem that pick-up is performed on a roll in a heating furnace as in the case of Fe oxide scale and the pick-up resistance is inferior.

  As a technique for suppressing the concentration of the oxides of Si and Mn, as a technique focusing on the recrystallization annealing process, Patent Document 1, after oxidizing the steel surface to have a thickness of 400 to 10,000 mm. The method of reducing and plating Fe in a furnace atmosphere containing hydrogen is disclosed in Patent Document 2 in which, after oxidizing the steel sheet surface, the oxygen potential in the reduction furnace is controlled to reduce Fe. In addition, a method of plating after suppressing surface concentration of Si oxide by internally oxidizing Si is shown. However, these techniques have the problem that the pick-up resistance is inferior because an Fe oxide scale is formed on the surface of the steel plate during annealing.

Further, in Patent Document 3, in the recrystallization annealing in the all radiant tube type in order to suppress the formation of Fe oxide scale, the oxygen potential of the annealing atmosphere is lowered to a value at which Fe, Si, and Mn are reduced, and then plating is performed. How to do is shown. However, in this technology, in order to reduce Si and Mn, it is necessary to extremely reduce the water vapor concentration in the atmosphere or extremely increase the hydrogen concentration. However, in addition to the problem that industrial feasibility is poor, oxidation is not possible. Without remaining on the steel plate surface, Si and Mn hinder the reaction between the plating and the base metal, and when immersed in the plating bath, it reacts with the oxide on the surface of the bath to form Si and Mn oxides. It has a problem that the performance is lowered. Here, the oxygen potential is a value represented by the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the water vapor partial pressure and the hydrogen partial pressure in the atmosphere. Oxidized and reduced.

  Patent Document 4 discloses a method of plating after raising the oxygen potential of the annealing atmosphere until Si and Mn are internally oxidized. However, when the oxygen potential is increased, Si and Mn are internally oxidized, but Fe is oxidized, so that the plating wettability and pick-up resistance are poor. On the other hand, when the oxygen potential is increased to such an extent that Fe is not oxidized, the internal oxidation of Si and Mn is insufficient, and the oxides of Si and Mn are concentrated on the surface, resulting in poor plating wettability and pick-up resistance. Techniques that simply increase or decrease the oxygen potential are not sufficient to solve both the wettability and pick-up resistance problems of the plating.

  Furthermore, as a technique for suppressing the concentration of the oxides of Si and Mn, it has been noted in Patent Document 5 that the oxygen potential is controlled in the hot rolling process as focusing on internal oxidation in advance in the hot rolling process. A technique for producing a hot dip galvanized steel sheet in a continuous hot dip galvanizing facility using a thin steel sheet in which Si is internally oxidized is shown. However, in the present technology, at the time of rolling in the cold rolling process or the like, the internal oxidation layer is also rolled together, and the thickness of the internal oxidation layer is reduced. Therefore, Si oxide is concentrated on the surface during the recrystallization annealing process, which is insufficient for improving the plating wettability, and Fe oxide formed when Si is internally oxidized in the hot rolling process. However, it has the problem of picking up a roll in a later process and generating an appearance defect.

JP 55-122865 A JP 2001-323355 A JP 2010-126757 A JP 2008-7842 A JP 2000-309847 A

  This invention makes it a subject to provide the manufacturing method of the hot dip galvanized steel plate which is excellent in plating wettability and pick-up resistance, using the steel plate containing Si and Mn which is an easily oxidizable element as a base material.

In order to solve the above-mentioned problems, the present inventors provided an all-radiant tube-type heating furnace when manufacturing a hot-dip galvanized steel sheet based on a steel sheet containing Si and Mn, which are oxidizable elements. In continuous hot dip galvanizing equipment, if annealing is performed in order in the following first, second and third steps, it is excellent in plating wettability by suppressing the concentration of surface oxides on the steel sheet after annealing, and further annealing treatment. In the middle of the process, when there is a steel sheet in the heating process and soaking process, it has been found that a hot-dip galvanized steel sheet with excellent pick-up resistance can be produced by suppressing the concentration of the surface oxide of the steel sheet, and the present invention It came.
First step (preheating):
In a nitrogen atmosphere where the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ to the water vapor partial pressure PH ₂ O is −4 or more and −2 or less, the time a ₁ second (10 ≦ a ₁ ≦ 500) ) To a plate temperature T ₁ ° C. (200 ≦ T ₁ ≦ 500).
Second step (heating):
In a nitrogen atmosphere where the logarithm log (PH ₂ O / PH ₂ ) of the ratio between the hydrogen partial pressure PH ₂ and the water vapor partial pressure PH ₂ O is −1.5 or more and +0.5 or less, the time a ₂ seconds (20 ≦ a heated from ItaAtsushi _{T 1} ° C. to _{T 2 ℃ (600 ≦ T 2} ≦ 1000) between the ₂ ≦ 1000).
Third step (soaking):
Time a ₃ seconds (30 ≦ a ₃ ≦ 1500) in a nitrogen atmosphere in which the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ to the water vapor partial pressure PH ₂ O is −4 to −2 ) Is soaked at a plate temperature T ₃ ° C. (−100 ≦ T ₃ −T ₂ ≦ 100).

That is, the gist of the present invention is as follows.
(1) In mass%,
C is 0.05% or more and 0.50% or less,
Si is 0.1% or more and 3.0% or less,
Mn from 0.5% to 5.0%,
P is 0.001% to 0.5%,
S is 0.001% or more and 0.03% or less,
The Al containing 1.0% 0.005% or more, the steel sheet consisting of residues Fe and unavoidable impurities, Casting, hot rolling, pickling, after a cold-rolled steel sheet is subjected to cold rolling, Plating wettability, characterized by annealing in the following first, second, and third steps when performing an annealing treatment and a treatment for applying a hot dip galvanizing layer on the cold-rolled steel sheet in a continuous hot dip galvanizing facility And the manufacturing method of the hot-dip galvanized steel plate excellent in pick-up resistance.
First step (preheating):
In a nitrogen atmosphere where the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ to the water vapor partial pressure PH ₂ O is −4 or more and −2 or less, the time a ₁ second (10 ≦ a ₁ ≦ 500) ) To a plate temperature T ₁ ° C. (200 ≦ T ₁ ≦ 500).
Second step (heating):
In a nitrogen atmosphere where the logarithm log (PH ₂ O / PH ₂ ) of the ratio between the hydrogen partial pressure PH ₂ and the water vapor partial pressure PH ₂ O is −1.5 or more and +0.5 or less, the time a ₂ seconds (20 ≦ a heated from ItaAtsushi _{T 1} ° C. to _{T 2 ℃ (600 ≦ T 2} ≦ 1000) between the ₂ ≦ 1000).
Third step (soaking):
Time a ₃ seconds (30 ≦ a ₃ ≦ 1500) in a nitrogen atmosphere in which the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ to the water vapor partial pressure PH ₂ O is −4 to −2 ) Is soaked at a plate temperature T ₃ ° C. (−100 ≦ T ₃ −T ₂ ≦ 100).

  (2) The steel sheet further contains one or more elements of Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM as component compositions. The method for producing a hot-dip galvanized steel sheet having excellent plating wettability and pick-up resistance according to (1), comprising 0.0001% or more and 1% or less.

  (3) The hydrogen concentration in the first step is 3% by volume to 40% by volume, and the hydrogen concentration in the second and third steps is 1% by volume to 15% by volume. The manufacturing method of the hot dip galvanized steel plate which is excellent in the plating wettability and pick-up resistance as described in 1) or (2).

(4) The plating wettability according to any one of (1) to (3), wherein the times a ₁ , a ₂ , and a ₃ satisfy the following relational expressions (i) and (ii): A method for producing hot-dip galvanized steel sheets with excellent pick-up resistance.
a ₁ ≦ a ₂ ≦ 4a ₁ (i)
(A ₂ + a ₁ ) / 4 ≦ a ₃ ≦ 2 (a ₂ + a ₁ ) (ii)

  According to the production method of the present invention, a hot-dip galvanized steel sheet having excellent plating wettability and pick-up resistance can be obtained.

These are logPH ₂ O / PH ₂ in the first step and preheating time in the first step. The logPH ₂ O / PH ₂ in the first step and the ultimate plate temperature in the first step. These are logPH ₂ O / PH ₂ in the second step and the heating time in the second step. The logPH ₂ O / PH ₂ in the second step and the ultimate plate temperature in the second step. The log PH ₂ O / PH ₂ in the third step and the soaking time in the third step. This is the relationship between T ₃ and (T ₃ -T ₂ ) in the temperature range T ₃ during soaking in the third step.

Hereinafter, the present invention will be described in detail.
First, the steel component of the steel plate provided with the hot dip galvanized layer of the present invention as a premise is as follows. In the following description, “%” means “% by mass” unless otherwise specified.

C: 0.05% or more and 0.50% or less C is an element that stabilizes the austenite phase, and is an element necessary for increasing the strength of the steel sheet. If the amount of C is less than 0.05%, the strength is insufficient, and if it exceeds 0.50%, the workability deteriorates. For this reason, the amount of C is 0.05% or more and 0.5% or less, preferably 0.10% or more and 0.40% or less.

Si: 0.1% or more and 3.0% or less Si improves the strength of the steel sheet by concentrating solute C in the ferrite phase in the austenite phase and increasing the temper softening resistance of the steel. If the Si content is less than 0.1%, the strength is insufficient, and if it exceeds 3.0%, the workability is lowered, and the plating wettability and pick-up resistance are not sufficiently improved. For this reason, Si amount is 0.1% or more and 3.0% or less, Preferably it is 0.5% or more and 2.0% or less.

Mn: 0.5% or more and 5.0% or less Mn is an element useful for enhancing the hardenability and increasing the strength of the steel sheet. If the amount of Mn is less than 0.5%, the strength is insufficient, and if it exceeds 5.0%, the workability is lowered, and the plating wettability and pick-up resistance are not sufficiently improved. For this reason, the amount of Mn is 0.5% or more and 5.0% or less, preferably 1.0% or more and less than 3.0%.

P: 0.001% or more and 0.5% or less P contributes to the improvement of strength, and may be contained according to the required strength level. However, if the P content exceeds 0.5%, the material deteriorates due to grain boundary segregation, so the upper limit is made 0.5%. on the other hand. To make the P content less than 0.001%, a great cost increase at the steel making stage is required, so 0.001% is made the lower limit.

S: 0.001% or more and 0.03% or less S is an inevitably contained impurity element, and is less because it reduces the workability by producing plate-like inclusions MnS after cold rolling. However, excessive reduction is accompanied by an increase in the desulfurization cost of the steelmaking process. For this reason, the amount of S is 0.001% or more and 0.03% or less.

Al: 0.005% or more and 1.0% or less Al has a high affinity with N in steel, and has an effect of fixing solid solution N as a precipitate to improve workability, but is excessive. On the other hand, such addition deteriorates workability, and further reduces plating wettability and pickup resistance. For this reason, the amount of Al is 0.005% or more and 1.0% or less.

  The balance other than the above component composition is Fe and unavoidable impurities, but in the present invention, Ti, Nb, Cr, Mo, Ni, Cu, One or more (← *) elements selected from Zr, V, W, B, Ca, and REM may be appropriately contained in an amount of 0.0001% to 1%, respectively.

  The method for producing steel is not particularly limited from casting to cold rolling, and is subjected to general casting, hot rolling, pickling, and cold rolling to form a cold rolled steel sheet, and the thickness is preferably 0. .1 mm or more and 3 mm or less.

  Next, the manufacturing method of the hot dip galvanized steel sheet excellent in the plating wettability and pick-up resistance of this invention is demonstrated.

  The authors have conducted sincere studies on a method for producing a hot-dip galvanized steel sheet having excellent pick-up resistance. As a result, it has been found that pick-up resistance is reduced when an oxide is present on the surface of the steel sheet in the course of annealing. At this time, the pick-up resistance of both the oxide on the surface of the steel sheet is reduced by either the oxide existing before annealing or the oxide formed during annealing. This is presumably because the oxide is more fragile than the metal and therefore easily peeled off, and the peeled oxide is picked up by a roll in the furnace, so that the pick-up resistance is lowered. In particular, the oxide concentrated on the surface during annealing is likely to be peeled off due to residual stress at the interface with the surrounding metal elements due to the difference in molar volume between the oxidized state and the metal state, and the pick-up resistance is reduced.

  Therefore, as a manufacturing method for improving the pick-up resistance of a hot-dip galvanized steel sheet, it is very important to suppress oxides on the surface of the steel sheet during the course of annealing.

  Furthermore, the authors have made sincere investigations on the method of manufacturing hot-dip galvanized steel sheets with excellent plating wettability. As a result, if oxides are present on the steel sheet surface before plating after annealing, the plating wettability decreases. I found out. The oxide on the surface of the steel plate at this time is reduced in plating wettability by either an oxide existing before annealing or an oxide formed during annealing. It is considered that when the steel plate surface is covered with an oxide, the wettability of the plating is lowered because the reactivity between the oxide and the plating is low when the hot dip galvanizing is processed.

  Therefore, as a manufacturing method for improving the plating wettability of the hot dip galvanized steel sheet, it is very important to suppress oxides on the surface of the steel sheet before the plating treatment after the end of annealing.

Therefore, as a manufacturing method, after making a cold-rolled steel plate of a predetermined component using a commonly used method, annealing treatment and treatment for applying a hot-dip galvanized layer in a continuous hot-dip galvanizing facility equipped with a heating furnace and a soaking furnace In the heating furnace and soaking furnace in which annealing is performed, annealing in the following first, second, and third steps can produce a hot dip galvanized layer that is excellent in plating wettability and pickup resistance. It is important for manufacturing the steel plate provided.
First step (preheating):
In a nitrogen atmosphere where the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ to the water vapor partial pressure PH ₂ O is −4 or more and −2 or less, the time a ₁ second (10 ≦ a ₁ ≦ 500) ) To a plate temperature T ₁ ° C. (200 ≦ T ₁ ≦ 500).
Second step (heating):
In a nitrogen atmosphere where the logarithm log (PH ₂ O / PH ₂ ) of the ratio between the hydrogen partial pressure PH ₂ and the water vapor partial pressure PH ₂ O is −1.5 or more and +0.5 or less, the time a ₂ seconds (20 ≦ a heated from ItaAtsushi _{T 1} ° C. to _{T 2 ℃ (600 ≦ T 2} ≦ 1000) between the ₂ ≦ 1000).
Third step (soaking):
Time a ₃ seconds (30 ≦ a ₃ ≦ 1500) in a nitrogen atmosphere in which the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ to the water vapor partial pressure PH ₂ O is −4 to −2 ) Is soaked at a plate temperature T ₃ ° C. (−100 ≦ T ₃ −T ₂ ≦ 100).

First, the first step will be described below. In the first step, the steel sheet is preheated and the log (PH ₂ O / PH ₂ ) is lowered to reduce Fe oxide on the surface of the steel sheet. The purpose is to improve sex. The amount of Si and Mn oxide formed at this time is controlled and made very small so that the plating wettability and pick-up resistance are not lowered.

When the log (PH ₂ O / PH ₂ ) in the first step is less than −4, the reduction effect is saturated and uneconomical, and the surface of the roll in an annealing furnace generally composed of an oxide is used. The reactivity with the steel sheet that is reduced and passed through increases and pick-up resistance decreases. If it exceeds -2, the reduction of Fe oxide present on the surface of the steel sheet becomes insufficient, and furthermore, Si and Mn are oxidized and concentrated on the surface, resulting in poor plating wettability and pick-up resistance. log (PH ₂ O / PH ₂ ) is more preferably −3.5 or more and −2.5 or less.

When the time a ₁ is less than 10 seconds, the reduction of the Fe oxide present on the surface of the steel sheet becomes insufficient in terms of the reduction rate, resulting in poor pick-up resistance. If it exceeds 500 seconds, productivity is lowered and uneconomical, and oxides of Si and Mn are concentrated on the surface, so that plating wettability and pickup resistance are poor. The time a ₁ is more preferably 40 seconds or longer and 400 seconds or shorter.

Wherein the temperature T ₁ is lower than 200 ° C., the reduction of Fe oxides present on the surface of the steel sheet becomes insufficient, poor resistance to pick-up resistance. Above 500 ° C., oxides of Si and Mn are concentrated on the surface, so that the plating wettability and pick-up resistance are poor. The temperature T ₁ is more preferably 250 ° C. or higher and 450 ° C. or lower.

Next, the second step will be described below. In the second step, the steel plate is heated and the log (PH ₂ O / PH ₂ ) is raised to suppress the concentration of oxide on the surface by internally oxidizing Si and Mn on the surface of the steel plate, The plating wettability and pick-up resistance of the hot-dip galvanized steel sheet could be improved. The amount of Fe oxide formed at this time is controlled and made very small so that the plating wettability and pick-up resistance are not lowered.

When the log (PH ₂ O / PH ₂ ) in the second step is less than −1.5, Si and Mn on the surface of the steel sheet are not sufficiently oxidized internally, and are concentrated as oxides on the surface, and the plating wettability Inferior to pick-up resistance. If it exceeds +0.5, the oxide of Fe increases on the steel sheet surface, and in addition to poor plating wettability and pick-up resistance, the surface of the steel sheet is easily peeled off due to excessive internal oxidation of Si and Mn. Inferior pick-up resistance. log (PH ₂ O / PH ₂ ) is more preferably −1.5 or more and 0 or less.

When the time a ₂ is less than 20 seconds, Si and Mn are not sufficiently oxidized internally, and thus concentrate as an oxide on the surface of the steel sheet during heating in the subsequent process, resulting in poor plating wettability and pick-up resistance. If it exceeds 1000 seconds, productivity is lowered and uneconomical, and Si and Mn are excessively oxidized internally, so that the surface of the steel sheet is easily peeled off and the pick-up resistance is poor. Time _{a 2} is more preferably 600 seconds or less than 60 seconds.

Wherein the temperature T ₂ is less than 600 ° C., since the Si and Mn is not sufficiently internal oxidation, for concentrated as an oxide on the surface of the steel sheet during the heating in the subsequent step, poor plating wettability and resistance to pickup property. Above 1000 ° C., Si and Mn in the steel are more easily diffused on the surface of the steel sheet, and oxides of Si and Mn are concentrated on the surface, so that the plating wettability and pickup resistance are poor. Temperature _{T 2} is more preferably 950 ° C. or less 650 ° C. or higher.

Next, the third step will be described below. In the third step, the steel sheet is soaked and the log (PH ₂ O / PH ₂ ) is lowered to reduce Fe oxide remaining on the steel sheet surface, and excessive Si causing peeling of the steel sheet surface. It aims at improving the plating wettability and pick-up resistance of a hot-dip galvanized steel sheet by suppressing the internal oxidation of Mn. At this time, since Si and Mn on the surface of the steel sheet form internal oxidation in the second step and are not concentrated on the surface of the steel sheet, plating wettability and pick-up resistance are not lowered.

If the log (PH ₂ O / PH ₂ ) in the third step is less than −4, the reduction effect of Fe oxide is saturated and uneconomical, and generally in an annealing furnace generally composed of oxides. The surface of the roll is reduced, the reactivity with the steel sheet to be passed through increases, and the pick-up resistance decreases. If it exceeds −2, the Fe oxide on the steel sheet surface is not sufficiently reduced or Fe is oxidized, and in addition, Si and Mn are excessively oxidized internally, and the steel sheet surface is peeled off to deteriorate the pick-up resistance. log (PH ₂ O / PH ₂ ) is more preferably −3.5 or more and −2.5 or less.

And in the time a ₃ is less than 30 seconds, since the Fe oxide is oxidized sufficiently not reduced or Fe, poor plating wettability and resistance to pickup property. In excess of 1500 seconds, productivity is lowered and uneconomical. In addition, Si and Mn diffuse from the lower layer to the steel plate surface from the layer where Si and Mn are internally oxidized, so the oxides of Si and Mn are concentrated. Therefore, the plating wettability and pickup resistance are poor. Time _{a 3} is more preferably not more than 1000 seconds 60 seconds.

When the relational expression (T ₃ -T ₂ ) of the temperature T ₃ is less than −100 ° C., the Fe oxide is not sufficiently reduced or the Fe is oxidized, so that the plating wettability and pickup resistance are poor. Fe oxide in this case, since the Fe oxide formed during the second step is included, the upper limit of the temperature T ₃ is a temperature related to the temperature T ₂ of the second step. Above 100 ° C, solute Si and Mn diffuse from the lower layer to the surface of the steel sheet, and the oxides of Si and Mn concentrate, resulting in increased plating wettability and pick-up resistance. Inferior. Internal oxidized layer at this time of Si and Mn is because it contains an internal oxide formed during the second step, the lower limit of the temperature T ₃ is a temperature related to the temperature T ₂ of the second step.

  Further, the hydrogen concentration in the first step is 3% by volume or more and 40% by volume or less, and the hydrogen concentration in the second or third step is 1% by volume or more and 15% by volume or less. Excellent pick-up performance. If the hydrogen concentration in the first step is less than 3% by volume, the reduction of Fe oxide does not proceed sufficiently and the wettability of the plating and pick-up resistance deteriorates. If it exceeds 40% by volume, hydrogen penetrates into the steel sheet and the surface of the steel sheet becomes hydrogen brittle. Therefore, pick-up resistance is reduced. The hydrogen concentration in the second and third steps, where the plate temperature is higher than that in the first step, is less than 1% by volume, the reduction of the Fe oxide does not proceed sufficiently and the plating wettability and pick-up resistance are reduced. Since hydrogen penetrates into the steel sheet and the steel sheet surface becomes hydrogen embrittled, pick-up resistance is reduced.

In addition, with respect to the times a ₁ , a ₂ , and a ₃ of the first, second, and third steps, as indicated by relational expressions (i) and (ii), a ₁ ≦ a ₂ ≦ 4a ₁ , (a ₂ + a ₁ ) / 4 ≦ a ₃ ≦ 2 (a ₂ + a ₁ ), the plating wettability and pickup resistance are excellent. When a ₁ > a ₂ , Si and Mn are not sufficiently internally oxidized, and when a ₃ > 2 (a ₂ + a ₁ ), solid solution Si and Mn are formed on the steel sheet surface from a lower layer than the layer where Si and Mn are internally oxidized. Due to diffusion, plating wettability and pick-up resistance are reduced. More preferable ranges are the following relational expressions (iii) and (iv).
_{3a 1/2 ≦ a 2 ≦} 3a 1, ··· (iii)
(A ₂ + a ₁ ) / 2 ≦ a ₃ ≦ 3 (a ₂ + a ₁ ) / 2 (iv)

  In continuous hot dip galvanizing equipment, in order to control the atmospheric conditions in the first, second and third steps of preheating, heating and soaking, the concentration of nitrogen, water vapor and hydrogen is controlled in each step. It is necessary to throw it in. Therefore, when the gas is flowing from the first process to the second process and from the second process to the third process, the hydrogen concentration is lower or higher than the first process from between the first process and the second process. An atmosphere with a water vapor concentration is introduced so as to flow toward the second step, and an atmosphere with a higher hydrogen concentration or a lower water vapor concentration than the second step is directed to the third step from between the second step and the third step. It may be introduced so that it flows. When gas flows from the third step to the second step and from the second step to the first step, between the third step and the second step, a hydrogen concentration lower than the third step, or a high water vapor concentration. An atmosphere is introduced to flow toward the second step, and an atmosphere having a higher hydrogen concentration than the second step or a lower hydrogen concentration than the second step is added to the first step between the second step and the first step. What is necessary is just to introduce so that it may flow toward.

  The dew point in the annealing furnace was measured using a dew point meter (Nippon Yakin Kagaku Kogyo Co., Ltd., DSP-Ex), and the hydrogen concentration was measured using a high concentration gas detector (New Cosmos Electric Co., Ltd., XP-3140). .

  After the preheating, heating, and soaking processes in the first, second, and third steps, a general process may be performed from a necessary cooling process to a process of applying a hot-dip galvanized layer. For example, the cooling process, the rapid cooling process, the overaging process, the second cooling process, the water quench process, the reheating process, or the like may be performed alone or in combination.

  After annealing, it is immersed in a hot dip galvanizing bath to apply a hot dip galvanized layer.

  The steel plate temperature during immersion in the hot dip galvanizing bath is preferably 400 ° C or higher and 600 ° C or lower. If the temperature is lower than 400 ° C, the bath temperature of the hot dip galvanizing bath is lowered and the plating bath may be solidified, which is not suitable, and if it exceeds 600 ° C, the bath temperature of the hot dip galvanizing bath is increased and the evaporation of molten zinc is intense. Therefore, there is an operational problem that vaporized zinc adheres to the furnace.

  The hot dip galvanizing bath uses a galvanizing bath having a bath temperature of 440 ° C. or higher and 550 ° C. or lower and an Al concentration in the bath of 0.10% or higher and 0.24% or lower. After immersion, it is heated at 440 ° C. or higher and 600 ° C. or lower.

  If the bath temperature is less than 440 ° C, the molten zinc may be solidified in the bath, which is not suitable. If the bath temperature exceeds 550 ° C, the molten zinc vaporizes strongly on the surface of the bath, and the operating cost and vaporized zinc adhere to the furnace. There are operational problems.

  When the sum of the Al concentration in the bath and the cation concentration of Al is less than 0.10%, there is a problem in operation because the evaporation of molten zinc is intense on the bath surface and the operating cost and vaporized zinc adhere to the furnace. If it exceeds 0.24%, a large amount of Al oxide adheres to the steel plate when immersed in the bath, thereby reducing the wettability of the plating.

  The heating temperature for the alloying treatment is optimally 440 ° C or higher and 600 ° C or lower. If it is less than 440 ° C, the alloying progresses slowly, and if it exceeds 600 ° C, a hard and brittle Zn-Fe alloy layer formed at the iron-iron interface due to overalloy is formed too much, resulting in deterioration of plating adhesion, and residual austenite phase. Since it decomposes, the balance between the strength and ductility of the steel sheet also deteriorates.

  In addition, Mg may be added to the plating bath in order to improve the corrosion resistance. This technique is applicable not only to hot dip galvanizing but also to manufacturing methods such as hot dip Al, hot dip Zn-5% Al, hot dip Zn-55% Al plating.

  Hereinafter, the present invention will be specifically described based on examples.

  Continuous hot-dip galvanizing equipped with an all-radiant tube heating furnace was used for specimens 1 to 72 of 1 mm thick cold-rolled plates shown in Table 1 subjected to normal casting, hot rolling, pickling, and cold rolling. An annealing treatment and a treatment for applying a plating layer were performed in the equipment. By using an all radiant tube type, as mentioned above, roll pick-up is difficult and productivity is good.

Logarithm of the ratio of the water vapor partial pressure and the hydrogen partial pressure by introducing nitrogen gas containing hydrogen and water vapor as the conditions of the first, second and third steps of preheating, heating and soaking, and the atmosphere in each step Log (PH ₂ O / PH ₂ ) and hydrogen concentration are shown in levels A1 to A84 in Table 2 and Table 3 (continuation of Table 2). Comparative examples are shown in levels B1 to B54 in Tables 4 and 5 (continuation of Table 4).

  Moreover, as a comparative example, B55-60 of Table 4 and Table 5 show the levels when manufactured without passing through any one or more of the first step, the second step, and the third step as annealing conditions.

  After the third step, it was immersed in a hot dip galvanizing bath through general cooling, rapid cooling, overaging, and second cooling step. The hot dip galvanizing bath contains a plating bath temperature of 460 ° C. and 0.13% by mass of Al. After dipping the steel plate in a hot dip galvanizing bath, the plating thickness is adjusted to 8 μm per side by nitrogen gas wiping, and then the alloying furnace Then, alloying was performed by heating at a steel plate temperature of 500 ° C. for 30 seconds, and the plating wettability and plating adhesion of the obtained plated steel plate were evaluated. The results of Examples are shown in Tables 2 and 3, and the results of Comparative Examples are shown in Tables 4 and 5.

  Plating wettability is N2 times or more when there is no Zn and Fe is exposed by EPMA mapping Zn and Fe of the surface of hot dip galvanized steel sheet 200 μm × 200 μm with N number of 10 times. When there are poor wettability (×), when there are N1 times, the wettability is generally good (◯), and when all of N10 times are covered with Zn and the area where Fe is exposed is not recognized Was evaluated as having good wettability (◎).

  The pick-up resistance is determined by visually observing the surface 100 mm × 100 mm of the hot-dip galvanized steel sheet with N number of 10 times. ), The pick-up resistance was generally good (◯) when there were N1 times, and the pick-up resistance was good (◎) when no prickling was observed in any of N10 times.

  Regarding the item of “judgment result of relational expression (i)” in Tables 3 and 5, “◯” is described when the condition satisfies the relational expression (i), and “x” is not satisfied.

  Regarding the item of “judgment result of relational expression (ii)” in Tables 3 and 5, “◯” is described when the condition satisfies the relational expression (ii), and “x” is not satisfied.

  As a result of the investigation of plating wettability and plating adhesion of Examples and Comparative Examples of the present invention, the levels A1 to A84 in Tables 2 and 3 as examples of the present invention are the levels of Tables 4 and 5 as Comparative Examples. It turned out that it is excellent in plating wettability and pick-up resistance compared with B1-B60.

  The hot-dip galvanized steel sheet produced by the method of the present invention is excellent in plating wettability and pick-up resistance, and is expected to be used as a surface treatment member in the automotive field, the home appliance field, and the building material field.

Claims (4)

% By mass
C is 0.05% or more and 0.50% or less,
Si is 0.1% or more and 3.0% or less,
Mn from 0.5% to 5.0%,
P is 0.001% to 0.5%,
S is 0.001% or more and 0.03% or less,
The Al containing 1.0% 0.005% or more, the steel sheet consisting of residues Fe and unavoidable impurities, Casting, hot rolling, pickling, after a cold-rolled steel sheet is subjected to cold rolling, Plating wettability, characterized by annealing in the following first, second, and third steps when performing an annealing treatment and a treatment for applying a hot dip galvanizing layer on the cold-rolled steel sheet in a continuous hot dip galvanizing facility And the manufacturing method of the hot-dip galvanized steel plate excellent in pick-up resistance.
First step (preheating):
In a nitrogen atmosphere where the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ to the water vapor partial pressure PH ₂ O is −4 or more and −2 or less, the time a ₁ second (10 ≦ a ₁ ≦ 500) ) To a plate temperature T ₁ ° C. (200 ≦ T ₁ ≦ 500).
Second step (heating):
In a nitrogen atmosphere where the logarithm log (PH ₂ O / PH ₂ ) of the ratio between the hydrogen partial pressure PH ₂ and the water vapor partial pressure PH ₂ O is −1.5 or more and +0.5 or less, the time a ₂ seconds (20 ≦ a heated from ItaAtsushi _{T 1} ° C. to _{T 2 ℃ (600 ≦ T 2} ≦ 1000) between the ₂ ≦ 1000).
Third step (soaking):
Time a ₃ seconds (30 ≦ a ₃ ≦ 1500) in a nitrogen atmosphere in which the logarithm log (PH ₂ O / PH ₂ ) of the ratio of the hydrogen partial pressure PH ₂ to the water vapor partial pressure PH ₂ O is −4 to −2 ) Is soaked at a plate temperature T ₃ ° C. (−100 ≦ T ₃ −T ₂ ≦ 100).   The steel sheet further contains, as a component composition, 0.001% by mass of one or more elements of Ti, Nb, Cr, Mo, Ni, Cu, Zr, V, W, B, Ca, and REM. The method for producing a hot-dip galvanized steel sheet having excellent plating wettability and pick-up resistance according to claim 1, comprising: 1% to 1%.   The hydrogen concentration in the first step is 3% by volume or more and 40% by volume or less, and the hydrogen concentration in the second or third step is 1% by volume or more and 15% by volume or less. 2. A method for producing a hot-dip galvanized steel sheet having excellent plating wettability and pickup resistance as described in 2. The time a _1, a _2, a ₃ are the following relations (i), and satisfies the (ii), is excellent in plating wettability and resistance to pickup property according to any one of claims 1 to 3 Manufacturing method of hot dip galvanized steel sheet.
a ₁ ≦ a ₂ ≦ 4a ₁ (i)
(A ₂ + a ₁ ) / 4 ≦ a ₃ ≦ 2 (a ₂ + a ₁ ) (ii)