JPH04173924A - Production of hot-dip galvanized steel sheet excellent in baking hardenability and pitting corrosion resistance and having superior workability - Google Patents

Abstract

PURPOSE:To produce a hot-dip galvanized steel sheet excellent in baking hardenability and pitting corrosion resistance and having superior workability by subjecting a slab of a steel, having a specific composition in which respective contents of P and Cu are specified, to hot rolling and cold rolling and then continuously applying hot-dip galvanizing to the resulting steel sheet in a specific temp. region. CONSTITUTION:A slab of a steel having a composition consisting of, by weight, 0.001-0.010% C, <=1.5% Si, 0.05-1.8% Mn, 0.03-0.20% P, <=0.02% S, 0.05-1.50% Cu, 0.005-0.10% solAl, 0.007% N, and the balance iron with inevitable impurities is hot- rolled and cold-rolled. Subsequently, after subjected or not subjected to iron plating by means of a continuous electroplating line, the resulting cold rolled steel sheet is passed through a continuous hot-dip galvanizing line and subjected to hot-dip galvanizing while regulating the annealing temp. in the line to 670-950 deg.C. By this method, the hot-dip galvanized steel sheet in which pitting corrosion resistance is improved and also workability, baking hardenability, and adhesive strength of plating are simultaneously satisfied can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for manufacturing a hot-dip galvanized steel sheet having bake hardenability and excellent pitting corrosion resistance and good workability.

[Conventional technology]

In the field of thin steel sheets used for automobile steel sheets, etc., in addition to excellent workability, in order to improve Punt resistance, the yield stress of the steel sheet increases after painting is baked, that is, bake hardenability is required. Often. Demand for surface-treated steel sheets is increasing from the viewpoint of corrosion resistance. A typical surface-treated steel sheet is a hot-dip galvanized steel sheet. In this case, hot-dip galvanized steel sheets with increased coating weight are used to reduce pitting corrosion.

Conventionally, this type of hot-dip galvanized steel sheet has been made of low-carbon aluminum-killed steel sheet, ultra-low carbon steel with Ti added to its base steel sheet.

Alternatively, a typical example is to use a high tensile strength steel plate to which Si, Mn, P, Cr, etc. are added to increase the strength, and there are many proposals regarding these.

For example, Japanese Patent Publication No. 1-54413 discloses a hot-dip galvanized steel sheet in which P is added to a low carbon aluminum killed steel.

Japanese Unexamined Patent Publication No. 58-31035 discloses a hot-dip galvanized steel sheet having bake hardenability, which is made by adding Si and P to a low carbon aluminum killed steel. Japanese Patent Publication No. 1-37468 discloses a hot-dip galvanized steel sheet in which S r + M n + P is added in combination to ultra-low carbon Ti-added steel.

[Problem that the invention seeks to solve]

Generally, in hot-dip galvanizing, molten zinc and the base steel react to form an alloy layer, and this alloy layer bonds the zinc layer and the base steel. Furthermore, those that are actively subjected to alloying treatment to make the entire plating layer an alloy layer are also frequently used because they provide good paint adhesion. This alloy layer is hard and brittle.

Therefore, when the coating layer becomes thicker, the workability of hot-dip galvanized steel sheets is dominated by the workability of the coating layer rather than the workability of the base metal, so even if the workability of the base material is improved, If the plating layer, which does not have good workability, peels off or is damaged during processing, the base steel base material is exposed and rusts, resulting in the problem of so-called pitting corrosion. Furthermore, the plating layer peels off in powder form during press molding, which causes surface defects called stars.

As described above, the pitting corrosion of hot-dip galvanized steel sheets originates from the hard and brittle alloy layer, and when they are subjected to deep drawing such as automotive steel sheets, solving this problem requires good workability and good workability. This is an important issue as well as providing bake hardenability.

Although low-carbon aluminum killed steel is used as the base metal base material, very high bake hardenability can be obtained, but there is a problem in that elongation deteriorates significantly due to aging at room temperature. Bake-hardening properties cannot be obtained when ultra-low carbon Ti-added steel is used as the base metal.On the other hand, bake-hardening is achieved by adding St and P to low-carbon aluminum-killed steel, as disclosed in JP-A No. 58-31035. When imparting properties, it is necessary to thicken the plating layer since this alone cannot solve the problem of pitting corrosion resistance.

In view of the above circumstances, the present invention aims to provide a hot-dip galvanized steel sheet that can simultaneously improve the perforation resistance, which is the most important corrosion resistance of hot-dip galvanized steel sheets, and also satisfy workability, bake hardenability, and plating adhesion. It was done for the purpose of obtaining.

Means to solve problem C] According to the invention, at 9% by weight.

C: 0.001-0.010%.

Si: 1.5% or less.

Mn: 0.05% to 1.8%.

P: 0.03-0.20%.

S: 0.02% or less.

Cu: 0.05-1.50%.

sol, A1; 0.005-0.100%.

N　i　O,007% or less.

and, in some cases, further.

Up to 1% Ni, up to 1% Mo or up to 7% Cr
Contains one or more of the following.

Slabs of steel, the remainder consisting of iron and unavoidable impurities, are hot-rolled, cold-rolled, and then with or without iron plating in a continuous electroplating line.

The sheet is passed through a continuous hot-dip galvanizing line, and hot-dip galvanizing is carried out at an annealing temperature in the line of 670 to 950°C. The present invention provides a method for manufacturing a hot-dip galvanized steel sheet with excellent workability and excellent bake hardenability and pitting corrosion resistance.

At that time, the hot-dip galvanizing process reduces the galvanizing layer to 400%
It may include alloying at a temperature of ~650<0>C.

[Effect]

The present invention achieves the above object by appropriately considering the chemical composition of the steel of the base steel plate and at the same time adopting appropriate manufacturing conditions. First, the reasons for regulating the content ranges of each component in the base steel sheet will be explained together with their effects.

Since C significantly deteriorates ductility, it is preferable to have less C, but if C is less than 0.0%, sufficient bake hardenability cannot be obtained. On the other hand, if C exceeds 0.010%, the deterioration of ductility due to room temperature aging becomes significant. Therefore, C is 0.001 to 0
．． 010%.

Si is a preferable element for improving the strength of steel without impairing its workability. According to research by the present inventors, Si in steel is If the content exceeds about 0.10%, non-plating will occur. Therefore, it is desirable that St be 0.10% or less. However, this problem can be solved by applying iron plating with a coating weight of about 2 g/M" by electroplating before passing the plate through the Sendzimer type continuous galvanizing line. However, if Si is 1.5% If it exceeds it, it becomes hard and the ductility deteriorates, so the upper limit was set at 1.5%.

Mn is effective in preventing hot embrittlement caused by S,
For this purpose, a minimum content of 0.05% or more is required. Also,
It is also a desirable element for improving the strength of steel. However, when the content exceeds 1.8%, ductility and deep drawability decrease. For this reason, P and Cu are characteristic elements in the present invention, with a lower limit of Mn content of 0.05% and an upper limit of 1.80%. The combined addition of both elements provides the effect of significantly improving the pitting corrosion resistance of hot-dip galvanized steel sheets. For this purpose, P should be at least 0.03% and Cu should be at least 0.05%.
% or more is required. However, even if the P content exceeds 0.20% and the Cu content exceeds 1.5%, the effect of improving pitting corrosion resistance is saturated and the ductility deteriorates. Therefore, P is 0.03% to 0.20%, Cu is 0.05% to
It is contained within a range of 1.5%.

S is an element that is essentially harmful to the base steel, and is preferably as small as possible; however, in the case of the steel of the present invention, up to 0.02% is permissible, so it was set to 0.02% or less.

A1 not only plays a role as a deoxidizing side.

It has the function of fixing N in steel as AIN and preventing room temperature aging due to solid solution N. For this purpose, an AI of 0.005% or more is required, but if it exceeds 0.10%, the A1.
Since inclusions such as O increase and deteriorate workability and surface quality, the lower limit is set to 0.005% and the upper limit is set to 0.10%.
And so.

N is an element that is essentially harmful to the steel of the present invention, and it is preferable to have less N, but up to 0.007% is permissible.
．． 0.007% or less.

Furthermore, it is preferable to contain one or more of Ni up to 1%, Mo up to 1%, or Cr up to 7%, thereby improving the strength and pitting corrosion resistance of the steel sheet.

Ni effectively prevents hot embrittlement caused by Cu and improves pitting corrosion resistance, but its effect is saturated even if it is added in an amount exceeding 1%. Therefore, the manufacturing cost becomes unnecessarily high.

Therefore, the upper limit is set at 1%.

Mo effectively acts to increase the strength of the steel sheet and improve the pitting corrosion resistance, but if it is added in an amount exceeding 1%, the effect will be saturated and the manufacturing cost will increase, so the upper limit is set at 1%.

Cr has an effective effect on improving pitting corrosion resistance, but if it exceeds 7%, manufacturing costs become extremely high, so the upper limit is set at 7%.

In the present invention, a slab of steel having two or more component compositions is hot-rolled and then cold-rolled to produce a cold-rolled steel strip, which is passed through a continuous hot-dip galvanizing line to be hot-dip galvanized. . At that time, it is also advantageous to pass the sheet through a continuous electroplating line and apply an appropriate amount of iron plating before passing the sheet through the continuous galvanizing line. This makes it possible to completely eliminate the occurrence of non-plating and improve corrosion resistance. Especially among the copper strips in the above chemical composition value range, S
This iron plating is advantageous for materials with a high i content (20.1% Si). The amount of iron plating is 2
It may be extremely thin, on the order of g/m".

Continuous hot-dip galvanizing is performed using well-known in-line annealing equipment (
Sendzimer type continuous hot-dip plating equipment) can be used, but it is important that the annealing temperature is 670 to 950°C. At temperatures below 670°C, the recrystallization of the rope does not proceed sufficiently, making it difficult to achieve good workability.

However, even if the annealing temperature exceeds 950° C., the effect of improving workability is saturated and surface flaws are likely to occur, making it difficult to obtain food products.

Further, in the present invention, alloying treatment of the plating layer of the hot-dip galvanized steel sheet can also be actively performed. As mentioned above, if the plating layer is alloyed, the plating layer will become hard and the problem of pitting corrosion will generally occur during processing. It can be said that this treatment is preferable in terms of the intended use of the present invention because it improves coating film adhesion and lap resistance weldability.

usually.

Alloying takes place in a continuous hot-dip galvanizing line.

Heating to a temperature range of 400 DEG to 650 DEG C. is sufficient to achieve alloying. That is, if the temperature is lower than this range, there will be insufficient alloying, and if the temperature is higher than this range, there will be overalloying, which will impair the adhesion of the plating layer.

EXAMPLES The effects of the present invention will be specifically illustrated by examples below.

[Example 1] Steel having the chemical composition values shown in Table 1 was hot rolled under the conditions shown in Table 2, and further cold rolled to obtain a cold rolled steel strip having a plate thickness of 0.8 a+m. The obtained cold-rolled steel strip was annealed on a continuous hot-dip galvanizing line under the conditions shown in Table 2 to give a coating weight of 30 g/fi.
Hot-dip galvanized copper strips are produced by hot-dip galvanizing with or without 9-alloying treatment, and then the elongation rate is 0.
．． 8% skin pass rolling was performed. The mechanical properties and corrosion resistance of the obtained hot-dip galvanized steel sheets were investigated, and the results are also listed in Table 2. For mechanical properties, a JIS Z 2201 No. 5 test piece was used. Corrosion resistance test is 70X150m+m
A test piece was cut out and subjected to a composite corrosion test. The composite corrosion test was conducted according to JISZ 2371 salt spray test. That is, [salt spray test with salt water concentration changed to 1.0% for 5 hours] → [hot air drying at 60°C for 4 hours] → [5
The maximum erosion depth due to corrosion was measured after carrying out 30 cycles of 14 hours of wet test at 0° C. → 1 hour of blow drying, each cycle having a total of 24 hours.

As can be seen from the results in Table 2, the hot-dip galvanized steel sheets produced using comparison steel No. 1, which has high C and P and Co that are lower than the amounts specified in the present invention, have low tensile strength (TS). The yield point (YS) is relatively high and the elongation (EL) is low.
Furthermore, since the bake hardenability (BH) is too high, room temperature aging occurs. And corrosion resistance is poor.

The hot-dip galvanized steel sheet produced using the N112 comparative steel, which contains less C and more P, has relatively good tensile strength (TS), elongation (E) and bake hardenability (Bl). , the corrosion resistance is still poor.

On the other hand, hot-dip galvanized steel sheets manufactured using N113 to 11tI4 having a composition within the range specified in the present invention have a low yield point (YS) and a low elongation (EL) even though their tensile strength (TS) is high. , good bake hardenability (BH) and excellent corrosion resistance.

[Example 2] Steel having the chemical composition values shown in Table 3 was hot rolled under the conditions shown in Table 4, and further cold rolled to obtain a cold rolled steel strip having a plate thickness of 0.8□. The obtained cold-rolled steel strip was coated with Fe-0,01%B with a coating weight of 2 g7m2 on a continuous electroplating line, and then annealed on a continuous hot-dip galvanizing line under the conditions shown in Table 4. Hot-dip galvanizing was applied with a coating weight of 30 g/+n'', followed by skin pass rolling with an elongation rate of 0.8%. The mechanical properties and corrosion resistance of the obtained hot-dip galvanized steel sheet were evaluated in the same manner as in Example 1. The results are also listed in Table 4.

As can be seen from the results in Table 4, the hot-dip galvanized steel sheets manufactured using the comparison steel of 12, which has higher C and lower P and Cu than those specified in the present invention, have relatively low tensile strength (TS). Since the yield point (YS) is high, the elongation (EL) is low, and the bake hardenability (B)I) is too high, room temperature aging occurs. And it has poor corrosion resistance.

Also, although it has low C and contains P, Cu or Ni,
The hot-dip galvanized copper sheet manufactured using the comparison steel of Nα13 without addition of Cr, Mo, etc. has relatively good tensile strength (TS), elongation (EL), and bake hardenability (8H).
2, but poor corrosion resistance.

On the other hand, hot-dip galvanized steel sheets manufactured using N014 to 21111 having a composition within the range specified in the present invention have a high yield point (YS) despite their high tensile strength (TS).
), good elongation (II:L), good bake hardenability (BH), and excellent corrosion resistance.

Claims (3)

[Claims] (1) In weight%, C: 0.001 to 0.010%, Si: 1.5% or less, Mn: 0.05% to 1.8%, P: 0.03 to 0.20%, S ; 0.02% or less, Cu; 0.05 to 1.50%, sol. Al: 0.005-0.100%, N: 0.0
After hot rolling and cold rolling, a continuous electroplating line with or without iron plating is applied to a slab of steel containing up to 0.07% and the remainder consisting of iron and unavoidable impurities. A method for manufacturing a hot-dip galvanized steel sheet with excellent workability and excellent bake hardenability and pitting corrosion resistance, which comprises passing the sheet through a hot-dip galvanizing line and hot-dip galvanizing the sheet at an annealing temperature of 670 to 950°C in the line. . (2) In weight%, C: 0.001 to 0.010%, Si: 1.5% or less, Mn: 0.05% to 1.8%, P: 0.03 to 0.20%, S ; 0.02% or less, Cu; 0.05 to 1.50%, sol. Al: 0.005-0.100%, N: 0.0
07% or less, and up to 1% Ni, up to 1% Mo or up to 7% Cr
After hot rolling and cold rolling, a steel slab containing one or more of , a well-workable hot-dip galvanized steel sheet with excellent bake hardenability and pitting corrosion resistance, which is passed through a continuous hot-dip galvanizing line and hot-dip galvanized at an annealing temperature of 670 to 950°C in this line. manufacturing method. (3) Hot-dip galvanizing process has a galvanized layer of 400 to 6
Claim 1 or 2 comprising an alloying treatment at a temperature of 50°C.
The manufacturing method described in.