JP2001192795A - High-strength hot-dip coated steel sheet and method for producing the same - Google Patents

Abstract

(57) [Problem] To provide a high-strength hot-dip coated steel sheet having excellent plating properties even for high-tensile steel containing high Si and Mn, and a method of manufacturing the same. SOLUTION: C: 0.008% by mass or less, Si: 0.10 to 1.2% by mass, Mn: 0.50 to 2.5% by mass, P: 0.13% by mass or less,
S: 0.010% by mass or less, Al: 0.10% by mass or less, N: 0.
005% by mass or less, B: 0.005% by mass or less and Se: 0.
001-0.05% by mass, and Nb: 0.02-0.20% by mass,
Ti: A steel sheet containing one or two kinds of 0.02 to 0.20 mass%, the balance being composed of Fe and unavoidable impurities is recrystallized and annealed in a reducing atmosphere having a dew point of 0 ° C or less and at 780 ° C or more. Then, the oxides formed on the surface of the steel sheet after cooling are pickled and removed, and then heated to 700 to 900 ° C. in a reducing atmosphere having a dew point of −20 ° C. or less, and hot-dip plating is performed while the temperature is lowered from this temperature.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength hot-dip coated steel sheet (alloy-processed) suitable for use in automobile bodies and the like, in which the surface of a steel sheet is hot-dip coated with zinc, aluminum, a zinc-aluminum alloy or the like. And a method of manufacturing the same.

[0002]

2. Description of the Related Art In recent years, excellent corrosion resistance has been imparted to the surface of a steel sheet which has been subjected to a high tensile force in order to achieve both light weight for improving fuel efficiency of a car and improvement of collision safety as a steel sheet for an automobile. There is an increasing tendency to use so-called high-strength hot-dip galvanized steel sheets that have been subjected to hot-dip galvanizing. Such a high-strength hot-dip coated steel sheet needs to have good plating properties as well as desired strength and workability (such as press formability).

As a steel sheet having excellent strength and workability, an ultra-low carbon high-strength steel sheet obtained by adding a strengthening element such as Si, Mn, or P to an ultra-low carbon steel is well known. However, when a steel sheet to which such a strengthening element is added is plated by, for example, a continuous hot-dip galvanizing equipment (hereinafter, referred to as CGL), an oxide such as Si, Mn, etc. Is known to decrease the plating property. That is, when a steel sheet is annealed in a reducing atmosphere before plating, easily oxidizable elements such as Si and Mn in the steel are selectively oxidized to form an oxide layer on the steel sheet surface. Since this surface oxide significantly reduces the wettability of the molten metal, in high-strength hot-dip galvanized steel sheets, the plating properties are reduced, especially when the content of Si, Mn, P, etc. is high, partially. There has been a problem that non-plating, which is not plated, tends to occur.

[0004] As a proposal for improving the deterioration of the plating property in such a high-strength steel sheet, Japanese Patent Application Laid-Open No. 55-12 / 1982 has been proposed.
No. 2865 discloses a method in which a steel sheet is forcibly oxidized under a high oxygen partial pressure prior to heating during plating and then reduced,
JP-A-58-104163 discloses a method of performing pre-plating before hot-dip plating.

[0005]

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. In the latter method, there is a problem that an extra process is added to increase the manufacturing cost. Further, Japanese Unexamined Patent Publication No.
No. 70723, Japanese Patent Application Laid-Open No. 8-85858,
Before plating, recrystallization annealing is performed to generate surface oxides,
A method has been proposed in which this oxide is removed by pickling and then hot-dip galvanized. This method has enabled non-plating defects to be prevented for a significant number of high strength steels. However, even with these methods, there still remains a problem that a steel type having a high Si content has not yet been completely prevented.

Accordingly, the present invention is a proposal for solving the above-mentioned problems of the prior art, and has excellent plating properties even for high-strength steels, particularly high-tensile steels containing high Si and Mn. An object is to provide a high-strength hot-dip coated steel sheet and a method for producing the same.

[0007]

Means for Solving the Problems The present inventors have determined that a predetermined amount of Se
When the steel sheet to which is added is annealed by a continuous annealing line (CAL) (abbreviated as “recrystallization annealing”), the form of the surface oxide changes to a spherical shape. Heating before plating treatment (abbreviated as "plating annealing") in a hot-dip galvanizing line (CGL) or the like showed that the concentration of Si and Mn oxides on the surface was reduced, and the plating properties could be greatly improved. . The present invention has been completed based on such knowledge, and the gist configuration thereof is as follows.

(1) C: 0.008% by mass or less, Si: 0.10 to
1.2 mass%, Mn: 0.50-2.5 mass%, P: 0.13 mass%
S: 0.010% by mass or less, Al: 0.10% by mass or less,
N: 0.005% by mass or less, B: 0.005% by mass or less and
A steel sheet containing Se: 0.001 to 0.05% by mass, Nb: 0.02 to 0.20% by mass, Ti: one or two types of 0.02 to 0.20% by mass, and the balance being Fe and inevitable impurities. In a reducing atmosphere having a dew point of 0 ° C. or lower and recrystallization annealing at 780 ° C. or higher, oxides formed on the surface of the steel sheet after cooling are removed by pickling, and then in a reducing atmosphere having a dew point of −20 ° C. or lower. A high-strength hot-dip coated steel sheet having a hot-dip layer formed on at least one surface by heating to about 900 ° C. and hot-dip coating while the temperature is lowered from this temperature.

(2) C: 0.008% by mass or less, 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% by mass or less, B: 0.005% by mass or less and
A steel sheet containing Se: 0.001 to 0.05% by mass, Nb: 0.02 to 0.20% by mass, Ti: one or two types of 0.02 to 0.20% by mass, and the balance being Fe and inevitable impurities. In a reducing atmosphere having a dew point of 0 ° C. or lower and recrystallization annealing at 780 ° C. or higher, oxides formed on the surface of the steel sheet after cooling are removed by pickling, and then in a reducing atmosphere having a dew point of −20 ° C. or lower. A method for producing a high-strength hot-dip coated steel sheet, wherein the hot-dip steel sheet is heated to about 900 ° C. and hot-dip coated while the temperature is lowered from this temperature.

(3) A method for producing a high-strength hot-dip coated steel sheet, comprising hot-dip coating according to the method described in (2) above, followed by alloying treatment.

[0011]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS As described above, the present invention mainly involves adding Se to steel, removing the oxide film formed by recrystallization annealing by pickling, and then plating through plating annealing. It is a characteristic. Hereinafter, the reasons for limiting the component composition, recrystallization annealing conditions, plating annealing conditions, and the like in the present invention to the ranges shown in the summary configuration will be described. Hereinafter, “mass%” is described as “%”.

C: 0.008% or less C is preferably as small as possible for improving elongation and r value.
In particular, when the content exceeds 0.008%, these processing characteristics are deteriorated. Therefore, the content is made 0.008% or less.

Si: 0.10 to 1.2% Si is an effective element for strengthening steel, but its effect is small when it is added less than 0.10%. On the other hand, when the content exceeds 1.2%, Si oxide is generated on the steel sheet surface after plating annealing,
Non-plating defects are likely to occur. Therefore, Si is 0.
It is contained in the range of 10 to 1.2% depending on the required strength.

Mn: 0.50 to 2.5% Mn is an element that contributes to an increase in strength, but if it is less than 0.50%, sufficient strength cannot be obtained. On the other hand, if it is added in excess of 2.5%, Mn oxide is generated on the steel sheet surface after plating annealing to easily cause non-plating defects, and the steel is hardened too much to make cold rolling difficult. Therefore, the Mn content is in the range of 0.50 to 2.5%.

P: 0.13% or less P is an element contributing to the increase in strength. However, if added in excess of 0.13%, the effect on the increase in strength is saturated, and segregation during solidification becomes remarkable, resulting in processing. This leads to a deterioration of the brittleness, and further to a significant deterioration of the secondary working brittleness resistance. Therefore, P is added with an upper limit of 0.13%.

S: 0.010% or less S is a harmful element that causes uneven alloying in the alloying treatment after hot-dip plating, so it is desirable to reduce S as much as possible. The reduction in the amount of S leads to a reduction in the amount of S precipitates in steel, thereby improving workability, and also contributes to an increase in the amount of effective Ti for fixing C. Therefore, the amount of S is 0.
Limit to 010% or less.

Al: 0.10% or less Al is an element effective for cleaning steel, and may not be substantially contained if inclusions can be eliminated from the steel. However, when the content exceeds 0.10%, deterioration of the surface properties is caused. Therefore, it is necessary to limit the content to 0.10% or less.

N: 0.005% or less N is desirably reduced as much as possible in order to secure materials such as ductility and r value. When the N content is 0.005% or less, almost satisfactory characteristics can be obtained, and the cost is sufficiently applicable to actual operation. Therefore, the upper limit of the amount of N is set to 0.005%.

B: 0.005% or less B is an element effective for improving secondary work brittleness resistance. Even if this effect is added in excess of 0.005%, there is no further increase in the effect, and depending on the annealing conditions, the workability may be reduced. Therefore, B is added up to 0.005%. The lower limit of the amount of addition is not particularly limited, but may be contained depending on the required degree of improvement in the resistance to secondary working embrittlement. Usually, it is desirable to contain 0.0010% or more.

Se: 0.001 to 0.05% Se has an effect of changing the form of the oxide formed on the surface of the steel sheet into a sphere by mixing it into the surface oxide, thereby improving the hot-dipability. are doing. Such an effect can be obtained by adding 0.001% or more,
%, The deep drawability deteriorates. Therefore, the amount of Se added is set to 0.001 to 0.05%.

Nb: 0.02 to 0.20%, Ti: 0.02 to 0.20% Nb or Ti is an element effective for improving the formability of a steel sheet. Such effects can be obtained by adding 0.02% or more, respectively. If any of them is added in excess of 0.20%, the recrystallization temperature becomes too high, making annealing itself difficult or causing surface defects. Therefore, Nb and Ti are added in one or two of them in the range of 0.02 to 0.20%.

A steel sheet for hot-dip plating is manufactured by a known method using the steel having the above-mentioned composition. That is,
Set the slab heating temperature to 1100 to 1300 ° C and the finishing temperature to 800
Hot rolling at ~ 1000 ° C is performed, followed by pickling and cold rolling. The cold rolling reduction is preferably 50% or more. Next, after the recrystallization annealing, pickling, and plating annealing described below, hot-dip plating is performed, and in some cases, alloying treatment is further performed.

Recrystallization annealing In recrystallization annealing, the strain introduced by cold rolling is released by heating (usually using CAL) above the recrystallization temperature to reduce the mechanical properties and workability required for the steel sheet. In addition to the role of imparting, to remove Si and Mn just below the steel sheet surface,
This is performed for the role of once generating oxides of Si and Mn on the steel sheet surface. If the recrystallization annealing is performed at a temperature lower than 780 ° C., the generation of oxides is insufficient. The atmosphere for the recrystallization annealing is performed under the condition that the dew point is 0 ° C. or less. This is because if the dew point is higher than 0 ° C., the oxide is mainly composed of Fe oxide, and it is difficult to generate Si or Mn oxide. The reducing atmosphere having a dew point of 0 ° C. or less is nitrogen gas,
Argon gas or hydrogen gas alone or two of these gases
What is necessary is just to mix the seeds or more. As the temperature history during the recrystallization annealing, a pattern in which the temperature is maintained at 820 to 900 ° C. for 0 to 120 s, and then cooled at a rate of 1 to 100 ° C./s is preferable.

Pickling removal of oxides Formed on the steel sheet surface by recrystallization annealing in a reducing atmosphere
Pickling is performed to remove oxides of Si and Mn. Thus,
By pickling and removing oxides of Si and Mn,
Since a layer containing less Si and Mn can be formed, the surface concentration of Si and Mn can be suppressed even by plating annealing in a later step. As the pickling solution, it is preferable to use 3 to 20% hydrochloric acid, and the pickling time is preferably 3 to 60 seconds.

Plating Annealing After removing oxides of Si and Mn on the surface by pickling, plating annealing (heating before plating) is performed, for example, by CGL.
The conditions of the plating annealing need to be performed at a temperature of 700 to 900 ° C. in a reducing atmosphere having a dew point of −20 ° C. or less.
In an atmosphere where the dew point is higher than -20 ° C, thick Fe
This is because an oxide is generated. Also, if the annealing temperature is 700
When the temperature is lower than 900C, the surface of the steel sheet is not activated. On the other hand, when the temperature exceeds 900C, a thick surface concentrate is generated, the wettability of the molten metal is deteriorated, and non-plating defects occur. Dew point -20 ° C
The following reducing atmosphere may be nitrogen gas, argon gas or hydrogen gas alone or a mixture of two or more of these gases. The temperature history at the time of plating annealing is as follows.
A pattern of cooling at a rate of s is preferred.

[0026]

EXAMPLE Steel ingots having various component compositions shown in Table 1
It was heated to 00 ° C. and hot rolled at a finish rolling temperature of 900 ° C. After pickling this hot-rolled steel strip, it is cold-rolled to a sheet thickness of 0.7 mm.
And then subjected to hot-dip galvanizing under the following conditions through a recrystallization annealing-pickling-plating annealing (CGL) process under the conditions shown in Table 2. The temperature histories in the recrystallization annealing and the plating annealing were such that the temperature was raised at 10 ° C./s, held for 60 seconds, and cooled at 20 ° C./s. The atmosphere was 5 vol% H ₂ + N _{2 by} recrystallization annealing.
The gas and plating annealing were changed to 7 vol% H ₂ + N ₂ gas. -Hot dip galvanizing conditions Bath temperature: 470 ° C Immersion plate temperature: 470 ° C Al content: 0.15% Coating weight: 60 g / m ² (one side) Plating time: 1 second

From the hot-dip galvanized steel sheet thus obtained, a test piece of 40 mm × 80 mm was sampled, the surface thereof was observed with a scanner, and the area ratio of the unplated portion determined by image processing was determined. Also, for these examples,
Alloying treatment was performed at 520 ° C. for 60 seconds, and it was confirmed that the invention example had no alloying unevenness.

[0028]

[Table 1]

[0029]

[Table 2]

The results obtained are shown in Table 2. As is clear from Table 2, it can be seen that the inventive examples according to the present invention have better plating properties than the comparative examples. In addition, it can be seen that the alloying property is excellent. These results are considered to have been obtained by suppressing the surface concentration on the steel sheet surface.

[0031]

As described above, according to the present invention,
It is possible to provide a hot-dip coated steel sheet having high plating strength and no non-plating defects. Further, according to the present invention, it is possible to provide a hot-dip coated steel sheet having good alloying property. Therefore, the present invention greatly contributes to weight reduction and fuel economy of automobiles.

Continued on the front page (51) Int.Cl. ⁷ Identification code FI Theme coat II (Reference) C23C 2/28 C23C 2/28 (72) Inventor Kazuo Mochizuki 1 Kawasaki-cho, Chuo-ku, Chiba-shi, Chiba Kawasaki Steel Corporation 4K027 AA02 AA23 AB02 AB07 AB28 AB42 AB44 AB48 AC02 AC12 AC73 AE12 AE33 AE34 4K037 EA01 EA02 EA04 EA15 EA16 EA18 EA19 EA23 EA25 EA27 EA28 EA31 EB02 F05 FJ02 F09

Claims (3)

[Claims] C: 0.008% by mass or less, Si: 0.10 to 1.2% by mass, Mn: 0.50 to 2.5% by mass, P: 0.13% by mass or less, S: 0.010% by mass or less, Al: 0.10% by mass or less, N : 0.005% by mass or less, B: 0.005% by mass or less and Se: 0.001 to 0.05% by mass, Nb: 0.02 to 0.20% by mass, Ti: 0.02 to 0.20% by mass, and the balance Is recrystallized and annealed in a reducing atmosphere having a dew point of 0 ° C. or less and at a temperature of 780 ° C. or more to remove oxides generated on the steel sheet surface after cooling, 7 in a reducing atmosphere with a dew point of -20 ° C or less
A high-strength hot-dip coated steel sheet having a hot-dip coating layer formed on at least one surface by being heated to 00 to 900 ° C. and hot-dip-coated while being cooled from this temperature. 2. C: 0.008% by mass or less, Si: 0.10 to 1.2% by mass, Mn: 0.50 to 2.5% by mass, P: 0.13% by mass or less, S: 0.010% by mass or less, Al: 0.10% by mass or less, N : 0.005% by mass or less, B: 0.005% by mass or less and Se: 0.001 to 0.05% by mass, Nb: 0.02 to 0.20% by mass, Ti: 0.02 to 0.20% by mass, and the balance Is recrystallized and annealed in a reducing atmosphere having a dew point of 0 ° C. or less and at a temperature of 780 ° C. or more to remove oxides generated on the steel sheet surface after cooling, 7 in a reducing atmosphere with a dew point of -20 ° C or less
A method for producing a high-strength hot-dip coated steel sheet, wherein the hot-dip coated steel sheet is heated to 00 to 900 ° C. and hot-dip coated while the temperature is lowered from this temperature. 3. A method for producing a high-strength hot-dip coated steel sheet, comprising hot-dip coating by the method according to claim 2 and then performing alloying treatment.