JP3915345B2 - Manufacturing method of high-tensile hot-dip steel sheet - Google Patents

Description

【００３１】
【表２】

【００３２】
得られた結果を表２に示す。表２から明らかなように、本発明に従う発明例は比較例に比べて良好なめっき性を有していることがわかる。そのうえ、合金化処理性にも優れていることがわかる。これらの結果は、いずれも鋼板表面における表面濃化を抑制することによって得られたものであると思われる。
【００３３】
【発明の効果】
以上説明したように、本発明によれば、高張力でありながら、不めっき欠陥のない溶融亜鉛めっき鋼板をはじめとする溶融めっき鋼板を提供することができる。また、本発明によれば、合金化処理性のよい溶融亜鉛めっき鋼板をも提供することが可能になる。しがって、本発明は自動車の軽量化、低燃費化に大きく寄与するものといえる。[0001]
BACKGROUND OF THE INVENTION
The present invention provides a high-tensile hot-dip galvanized steel sheet (including alloyed ones ) suitable for use in automobile bodies, etc., in which zinc, aluminum, zinc-aluminum alloy or the like is hot-plated on the surface of a high-strength steel sheet . It relates to a manufacturing method.
[0002]
[Prior art]
In recent years, there has been an increasing trend to apply high-tensile hot-dip galvanized steel sheets that have been hot-dip galvanized as steel sheets for automobiles in order to achieve both weight reduction for improving fuel economy and improved collision safety. Yes. Such a high-tensile hot-dip galvanized steel sheet needs to have plating properties as well as desired strength and workability (such as press formability). Hereinafter, the high-tensile hot dip galvanized steel sheet will be mainly described.
[0003]
By the way, as a steel plate excellent in strength and workability, an ultra-low carbon high-tensile steel plate obtained by adding a strengthening element such as Si, Mn, P to ultra-low carbon steel is well known. However, when trying to plate a steel sheet to which such strengthening elements are added with a continuous hot dip galvanizing line (CGL), a concentrated layer of Si, Mn, etc. is formed on the steel sheet surface in the annealing process before plating, thereby reducing the plating performance. It is known to let
This concentration phenomenon occurs when annealing in a reducing atmosphere before plating, even if it is reducing for Fe, it is oxidizing to Si, Mn, etc. in the steel. Is selectively oxidized to form an oxide layer, and the concentration of these elements occurs on the surface. This surface oxide significantly reduces the wettability of the molten zinc. As a result, the high-tensile hot-dip galvanized steel sheet has a problem that the plating properties are lowered, and particularly when the content of Si, Mn, P, etc. is high, non-plating that is not partially plated tends to occur.
[0004]
As a proposal for improving the deterioration of the plating property in such a high-tensile steel sheet, Japanese Patent Application Laid-Open No. 55-122865 discloses that a steel sheet is forcibly oxidized under a high oxygen partial pressure prior to heating during plating and then reduced. Japanese Patent Laid-Open No. 58-104163 discloses a method of performing pre-plating before performing hot-dip plating.
[0005]
[Problems to be solved by the invention]
However, in the former method, if the surface oxide is not sufficiently controlled by forced oxidation, there is a problem that stable plating cannot always be performed depending on the components in the steel and the plating conditions. As a result, the manufacturing cost increases.
Further, in JP-A-7-70723 and JP-A-8-85858, recrystallization annealing is performed in advance before plating to form a surface oxide, and after this oxide is pickled and removed, hot dip galvanization is performed. A way to do it has been proposed. This method has made it possible to prevent non-plating defects in a considerable number of high-strength steels. However, even with these methods, there remains a problem that the steel type having a high Si content cannot be completely prevented.
[0006]
Therefore, the present invention is a proposal for solving the above-mentioned problems of the prior art, and is a high-tensile melt that has excellent plating properties even for high-strength steel, particularly high-strength steel containing high Si and Mn. and to provide a method of manufacturing a plated steel plate.
[0007]
[Means for Solving the Problems]
When the inventors combined use of Nb oxidation inhibition and pickling removal of the oxide film generated by annealing in the continuous annealing line (CAL) (abbreviated as “recrystallization annealing”), the continuous hot dip galvanizing line (CGL) It has been found that the concentration of Si and Mn on the surface is reduced by heating at the time of plating (abbreviated as “plating annealing”), and a significant improvement in plating properties can be achieved.
The present invention has been completed on the basis of such knowledge, and the gist of the present invention is as follows.
[0008]
Delete [0009]
Delete [0010]
( 1 ) C: 0.0005 to 0.0050 mass%, Si: 0.10 to 1.2 mass%, Mn: 0.50 to 2.5 mass%, P: 0.13 mass% or less, S: 0.010 mass% Hereinafter, Al: 0.10 mass% or less, N: 0.005 mass% or less, B: 0.005 mass% or less, and Nb: 0.02 to 0.20 mass%, with the balance from the composition of Fe and inevitable impurities The resulting steel sheet is recrystallized and annealed in a reducing atmosphere with a dew point of 0 ° C. or less and at 780 ° C. or more, and the oxide formed on the surface after cooling is removed by pickling, and then the reducing property with a dew point of −20 ° C. or less. A method for producing a high-tensile hot-dip galvanized steel sheet, which is heated to 650 ° C. or higher in an atmosphere and lower than or equal to the temperature in the recrystallization annealing, and is hot-dipped in the middle of cooling from this temperature.
[0011]
( 2 ) C: 0.0005 to 0.0050 mass%, Si: 0.10 to 1.2 mass%, Mn: 0.50 to 2.5 mass%, P: 0.13 mass% or less, S: 0.010 mass% Hereinafter, Al: 0.10 mass% or less, N: 0.005 mass% or less, B: 0.005 mass% or less, Nb: 0.02 to 0.20 mass% and Ti: 0.020 mass% or less, the balance being A steel plate composed of Fe and inevitable impurities is subjected to recrystallization annealing in a reducing atmosphere with a dew point of 0 ° C. or lower and at a temperature of 780 ° C. or higher, and the oxide formed on the surface after cooling is removed by pickling. Is heated at a temperature of 650 ° C. or higher in a reducing atmosphere of −20 ° C. or lower and lower than the temperature in the recrystallization annealing, and is subjected to hot dipping in the middle of temperature lowering from this temperature. Method of manufacturing a Kki steel plate.
[0012]
( 3 ) A method for producing a high-tensile hot-dip galvanized steel sheet, characterized by subjecting it to alloying after hot-dip plating by the method described in (1) or (2 ) above .
[0013]
DETAILED DESCRIPTION OF THE INVENTION
As described above, the present invention is mainly characterized in that Nb is added to the steel, the oxide film formed by recrystallization annealing is pickled and removed, and then plated through plating annealing. The reason why the component composition, recrystallization annealing conditions, plating annealing conditions, and the like in this invention are limited to the ranges shown in the summary configuration will be described below.
[0014]
C: 0.0005 to 0.0050 mass%
C is preferably reduced to improve the elongation and the r value. However, if it is less than 0.0005 mass%, the secondary work brittleness resistance deteriorates and the strength of the welded portion (welding heat affected zone) decreases. Therefore, it is not preferable, and the production cost is high. On the other hand, if the C content exceeds 0.0050 mass%, even if an equal amount of Ti or Nb is contained, it is difficult to obtain an effect of improving the material (particularly ductility) by these elements. Therefore, the C content is in the range of 0.0005 to 0.0050 mass%.
[0015]
Si: 0.10 to 1.2 mass%
Si is an element effective for strengthening steel, but its effect is small when added less than 0.10 mass%. On the other hand, if the content exceeds 1.2 mass%, Si oxide is generated on the surface of the steel sheet after plating annealing, and non-plating defects are likely to occur. Therefore, Si is contained in the range of 0.10 to 1.2 mass% according to the required strength.
[0016]
Mn: 0.50 to 2.5 mass%
Mn is an element that contributes to an increase in strength, but if it is less than 0.50 mass%, sufficient strength cannot be obtained. On the other hand, if added in excess of 2.5 mass%, Mn oxide is generated on the surface of the steel sheet after the plating annealing, and non-plating defects are likely to occur, and the steel is hardened so that cold rolling becomes difficult. Therefore, the Mn content is in the range of 0.50 to 2.5 mass%.
[0017]
P: 0.13 mass% or less P is an element that contributes to an improvement in workability (mainly r value) as well as an increase in strength. However, if the P content exceeds 0.13 mass%, the increase in strength is saturated, segregation during solidification becomes prominent, resulting in deterioration of workability, and further resistance to secondary work brittleness is greatly deteriorated. Therefore, P is added at an upper limit of 0.13 mass%.
[0018]
S: 0.010 mass% or less Since S is a harmful element that causes uneven alloying in the alloying treatment after hot dipping, it is desirable to reduce it as much as possible. Moreover, the reduction of the S amount contributes to the improvement of workability due to the reduction of S precipitates in the steel and the increase of the effective Ti amount for fixing C. Therefore, S is limited to 0.010 mass% or less.
[0019]
Al: 0.10 mass% or less Al is an element effective for cleaning steel, and if inclusions can be excluded from the steel, it may not be substantially contained. However, when the content exceeds 0.1 mass%, the surface properties are deteriorated, so it is necessary to limit the content to 0.1 mass% or less.
[0020]
N: 0.005 mass% or less N is desirably reduced as much as possible in order to secure materials such as ductility and r value. While an almost satisfactory characteristic can be obtained when the N content is 0.005 mass% or less, further reduction causes a cost increase, which is disadvantageous. Therefore, the upper limit is set to 0.005 mass%.
[0021]
B: 0.005 mass% or less
B is an element effective for improving secondary work brittleness resistance. Even if this effect is added in excess of 0.005 mass%, there is no further increase in effect, and depending on the annealing conditions, there is a possibility that workability may be lowered. Moreover, when it adds excessively, a hot-rolled mother board will also be hardened and rolling will become difficult. Therefore, B is added with 0.005 mass% as the upper limit. In addition, although there is no limitation in particular about the minimum of addition amount, what is necessary is just to contain according to the required improvement degree of secondary work brittleness resistance, and it is desirable to contain 0.001 mass% or more normally.
[0022]
Nb: 0.02 to 0.20 mass%
Nb is an important element that suppresses oxide formation (surface concentration) on the steel sheet surface after plating annealing and improves plating properties. Such an effect cannot be obtained when the Nb addition amount is less than 0.02 mass%. On the other hand, when the amount of Nb added is increased, the recrystallization temperature rises. In particular, when it exceeds 0.20 mass%, the recrystallization temperature becomes too high, and the surface oxide generated by annealing becomes thick, so that pickling removal is difficult. It becomes. Therefore, the Nb addition amount is in the range of 0.02 to 0.20 mass%.
[0023]
Ti: 0.020 mass% or less Ti is an element effective for improving the formability of steel, and is added as necessary. However, if it is added excessively, it will be mixed into the surface oxide produced at the stage of recrystallization annealing by CAL, making it difficult to remove the oxide by pickling. Therefore, Ti is added in a range of 0.02 mass% or less.
[0024]
A steel sheet for plating is manufactured by a known method using the steel having the above-described component composition. That is, hot rolling is performed at a slab heating temperature of 1100 to 1300 ° C. and a finishing temperature of 800 to 1000 ° C., followed by pickling and cold rolling. The cold rolling reduction ratio is preferably 50% or more.
Next, after recrystallization annealing, pickling, and plating annealing described below, hot dipping is performed, and in some cases, alloying is further performed.
[0025]
Recrystallization annealing Recrystallization annealing releases the strain introduced by cold rolling by heating above the recrystallization temperature (usually using CAL) and imparts the necessary mechanical properties and workability to the steel sheet. In addition to the role, in order to remove Si and Mn immediately below the surface of the steel sheet, this is performed for the role of once generating oxides of Si and Mn on the surface of the steel sheet. If the recrystallization annealing is less than 780 ° C., the formation of oxides is insufficient, and therefore, it is performed at 780 ° C. or higher. The atmosphere of recrystallization annealing is performed under the condition that the dew point is 0 ° C. or less. Because, when the dew point is higher than 0 ° C., the oxide is mainly Fe oxide, Si and Mn oxides is because hardly form raw. Dew point 0 ℃ the reducing atmosphere is nitrogen gas, argon gas, alone or these gases of hydrogen gas may be assumed that engaged on mixing two or more kinds. As a temperature history at the time of recrystallization annealing, a pattern of cooling at a rate of 1 to 100 ° C./s after being held at 820 to 900 ° C. for 0 to 120 s is preferable.
[0026]
Pickling removal of oxide Pickling is performed to remove oxides of Si and Mn formed on the surface of the steel sheet by recrystallization annealing in a reducing atmosphere. Thus, by removing the oxides of Si and Mn by pickling, a deficient layer of Si and Mn can be formed directly under the steel sheet, so that surface enrichment of Si and Mn is suppressed even by subsequent plating annealing. be able to.
As the pickling solution, it is preferable to use 3 to 20 % hydrochloric acid, and the pickling time is preferably 3 to 60 seconds.
[0027]
Plating annealing After removing the surface Si and Mn oxides by pickling, plating annealing (heating before plating) is performed. In the case of hot dip galvanizing, this plating annealing may be performed using CGL. And the conditions of plating annealing need to be performed at 650 degreeC or more and below the temperature in the said recrystallization annealing in a reducing atmosphere with a dew point of -20 degrees C or less.
This is because, in an atmosphere having a dew point higher than −20 ° C., a thick Fe oxide is generated on the surface of the steel sheet. In addition, when the annealing temperature is less than 650 ° C., the steel sheet surface is not activated. On the other hand, when the annealing temperature is equal to or higher than the recrystallization annealing temperature, Si or Mn inside the steel sheet is generated just under the steel sheet surface by recrystallization annealing. This is because these elements diffuse to the surface beyond the Mn-deficient layer and form oxides of Si and Mn again to lower the plating property.
Dew point -20 ° C. or less of the reducing atmosphere, nitrogen gas, argon gas, alone or these gases of hydrogen gas may be assumed that engaged on mixing two or more kinds. As a temperature history at the time of plating annealing, a pattern of cooling at a rate of 1 to 100 ° C./s after being held at 730 to 760 ° C. for 0 to 180 s is preferable.
[0028]
【Example】
The steel ingot which consists of various component composition shown in Table 1 was heated at 1150-1250 degreeC, and the hot rolling was hot-rolled at 850-950 degreeC. This hot-rolled steel strip is pickled and cold-rolled at a reduction rate of 77% to obtain a cold-rolled sheet having a thickness of 0.7 mm. Further, under the conditions shown in Table 2, recrystallization annealing and pickling and plating annealing are performed. After the process, hot dip galvanization was performed with CGL under the following conditions. In addition, the holding time in recrystallization annealing and plating annealing was set to 60 seconds. _5% H 2 + _{N 2} gas in the recrystallization annealing, the plated annealed using _7% H ₂ + _N 2 gas.
-Hot dip galvanizing conditions Bath temperature: 470 ° C
Infiltration plate temperature: 470 ° C
Al content: 0.15 mass%
Plating adhesion amount: 60 g / m ² (single side)
Plating time: 1 second [0029]
Ten pieces each of 40 mm × 80 mm test pieces were collected from the hot dip galvanized steel sheet thus obtained, the surface of the specimen was observed with a scanner, and the area ratio of the non-plated portion obtained by image processing was 0.1% or less. Was passed. The same determination was performed on 10 test pieces, and the pass rate was determined from the ratio of the pass number.
Moreover, about invention 2 and 3, the alloying process for 60 second was performed at 520 degreeC, and it confirmed that there was no alloying nonuniformity at all.
[0030]
[Table 1]

[0031]
[Table 2]

[0032]
The obtained results are shown in Table 2. As is apparent from Table 2, it can be seen that the inventive examples according to the present invention have better plating properties than the comparative examples. Moreover, it can be seen that the alloying processability is also excellent. These results seem to be obtained by suppressing the surface concentration on the steel sheet surface.
[0033]
【The invention's effect】
As described above, according to the present invention, it is possible to provide a hot dip galvanized steel sheet including a hot dip galvanized steel sheet having no unplating defects while having high tension. Further, according to the present invention, it is possible to provide a hot-dip galvanized steel sheet having a good alloying processability. Therefore, it can be said that the present invention greatly contributes to the reduction in weight and fuel consumption of automobiles.

Claims (3)

C: 0.0005 to 0.0050 mass%,
Si: 0.10 to 1.2 mass%,
Mn: 0.50 to 2.5 mass%
P: 0.13 mass% or less,
S: 0.010 mass% or less,
Al: 0.10 mass% or less,
N: 0.005 mass% or less,
B: 0.005 mass% or less and Nb: 0.02 to 0.20 mass%, the balance being a steel plate composed of Fe and inevitable impurities, in a reducing atmosphere with a dew point of 0 ° C. or less and 780 ° C. or more Then, the oxide formed on the surface after cooling is removed by pickling, and then heated in a reducing atmosphere having a dew point of −20 ° C. or lower to 650 ° C. or higher and lower than the temperature in the recrystallization annealing. A method for producing a high-tensile hot-dip galvanized steel sheet, comprising hot-dip plating in the middle of temperature drop from temperature. C: 0.0005 to 0.0050 mass%,
Si: 0.10 to 1.2 mass%,
Mn: 0.50 to 2.5 mass%
P: 0.13 mass% or less,
S: 0.010 mass% or less,
Al: 0.10 mass% or less,
N: 0.005 mass% or less,
B: 0.005 mass% or less,
A steel plate containing Nb: 0.02 to 0.20 mass% and Ti: 0.020 mass% or less, the balance being composed of Fe and inevitable impurities, in a reducing atmosphere with a dew point of 0 ° C. or less and 780 ° C. or more Then, the oxide formed on the surface after cooling is removed by pickling, and then heated in a reducing atmosphere with a dew point of −20 ° C. or lower to 650 ° C. or higher and lower than the temperature in the recrystallization annealing. A method for producing a high-tensile hot-dip galvanized steel sheet, comprising hot-dip plating in the middle of temperature drop from temperature. After dip plated by the method according to claim 1 or claim 2, the method of producing a high tensile hot dip plated steel sheet, characterized in that the alloying.