JPS5896822A - Production of painted steel plate having strain aging hardness - Google Patents

Abstract

PURPOSE:To obtain a surface-treated steel plate which is provided with both of strain aging hardness and high corrosion resistance by subjecting a steel plate of specified contents of C, Mn, solAl which are essential components to recrystallization annealing and skin pass rolling then baking org. films contg. zinc powder under specific conditions. CONSTITUTION:The steel slab contg. <=0.03% C, <=0.4% Mn, <=0.10% solAl, and if necessary, contg. <=0.150% P and 0.80% Si is rolled to a desired thickness. This steel plate is subjected to recrystallization annealing in a box annealing furnace then to <=1.5% skin pass rolling. In succession, the steel plate is coated with org. films contg. powder of metallic zinc as it is or after the same is applied with zinc electroplating or the like. The films are baked by using IR lamps, etc. at >=5 deg.C/sec heating rate and 30 deg.C/sec cooling rate at the cycles at which the relation wherein the baking temp. T and the baking time (t) is expressed by the equation are satisfied.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention aims to provide a method for creating a surface-treated steel sheet that has both strain age hardenability and high corrosion resistance and is used in corrosion-resistant structures.

In recent years, the automobile industry has strongly promoted the use of high-strength cold-rolled steel sheets to reduce the weight of car bodies in order to raise tuition fees, and the use of surface-treated steel sheets to prevent rust in car bodies. Furthermore, there is an increasing demand for high-strength surface-treated steel sheets that have both of these performances.

Currently, high-strength, longitudinally-cooled gfM plates are attracting particular attention because they are soft before forming and can be easily processed into presses, etc., and at the same time, their strain-age hardening properties significantly increase their longitudinal strength through baking the paint after forming. This strain age hardenability is due to strain aging caused by C that is forced into the steel sheet.

On the other hand, there are various types of highly corrosion-resistant surface-treated steel sheets, such as bath-dip galvanized steel sheets, electro-galvanized steel sheets, alloyed hot-dip galvanized steel sheets, and alloy electro-galvanized and galvanized steel sheets such as Zn-N1. Currently, steel sheets that are coated with an organic coating containing metallic zinc powder are currently attracting attention as highly corrosion-resistant steel sheets. Especially as a base for painting, Zn-Nl, Zn-Nl-Cr or Zn
-Ni -Cr -F* alloy electrogalvanized steel sheets offer not only corrosion resistance but also peeling resistance! It also has excellent moldability.

However, applying heat treatment called baking during the manufacturing of steel sheets in this way will harden the steel sheets before they reach the customer, which can lead to forming defects during forming at the customer. , it will lose its value as a processing material.

The present applicant previously filed a patent application (Patent Application No. 144987/1987) for an invention for restoring the material quality of steel material to one close to the material quality before coating baking by subjecting it to temper rolling after baking treatment. However, this prior invention is very effective when there is a temper rolling mill on the same line as the coating baking equipment, but if there is no KIJj 4 quality rolling mill on the same line, such processing '
Applying t-coating will lead to a decrease in productivity and an increase in costs. Therefore, the present inventors paid special consideration to the baking treatment of the organic coating and invented a method for minimizing the deterioration of the material without performing temper rolling after baking the coating as described above.

The present invention provides a special method for baking an organic coating when surface-treating a steel sheet in which sufficient Cl remains as a solid solution to impart strain age hardenability by adjusting the composition and process of the starting steel sheet. The object of the present invention is to provide a surface-treated steel sheet having high corrosion resistance t- without impairing strain age hardenability.

The gist of the present invention is that C: 0.03 or less, M
n: 0.40% or less, mol, At: 0.10%
In addition to the following, if necessary, P: 0.150 or less, 8
1: Steel containing any of the following O, SO*, either singly or in combination, with the remainder F and unavoidable impurities, is tapped into slag, and hot-rolled and cold-rolled into the final desired plate. A thick steel plate is manufactured, and then this steel plate is recrystallized and annealed in a box annealing furnace, and then subjected to temper rolling of 1.5 or less, and then subjected to as-is, electrolytic galvanizing, or electrolytic composite plating mainly consisting of zinc. Or, after applying electroalloy plating, apply an organic coating mainly containing metallic zinc powder and heat and bake using near-infrared rays or infrared yfk at a heating rate of 5℃/1.5℃ or higher, The cooling rate is 30℃/s@e or more, and the baking temperature T (C) and baking time S (S・C) are O≦Log t≦-0,024+5.52...
...(1) A method for manufacturing a coated steel sheet having strain age hardenability, characterized by baking in a baking cycle that satisfies the following relationship.

The reasons for limiting the components and manufacturing conditions in the present invention will be explained in detail below.

If C exceeds 0.03, the entire amount will precipitate as carbide due to slow cooling after annealing, making it difficult for solid solution C to remain.
% or less. In addition, when annealing is performed in a tight coil, in order to further suppress precipitation of solid mC, it is preferable that C is 0.021 or less.

Mn is an essential element for strengthening steel and suppressing hot embrittlement, but if its content exceeds 0.40%, it promotes the precipitation of F.30 during cooling, and the Solid solution CJl of
'fr: Since it is reduced, it is set to 0.40% or less.

Aj is added for the purpose of deoxidizing, but the lid containing it is
If AA exceeds o, i o s, the surface quality will deteriorate due to an increase in alumina clusters, so mo4 AJ!
0.10- or less.

P is very effective as a solid solution strengthening element, but if the content exceeds 0.15-0, the steel will become extremely brittle, so the amount added is preferably 0.150- or less. On the other hand, in the case of ordinary steel base without adding P, the amount of P is F to ensure elongation.
i0.01- or less is preferable.

Since Sl increases solid solution Ct, which causes strain age hardening, during box annealing, it is preferable to add a large amount of Sl, but if the content exceeds 0.5 ost, it will impair weldability, so when adding it, it should be within O. SOs. The addition of is preferred.

Then, recrystallization annealing is performed in a box annealing furnace, and the annealing conditions at that time are usually a heating rate of 10 to 250°C/h.

The annealing temperature is 650 to 5 ooc, the annealing time is 1 to 12 hours, and the cooling rate is 10 to 250 °C/h.In addition, the precipitation of solid solute C during cooling is further suppressed, leaving a large amount of solid solute C in the steel to prevent strain. In order to increase the age hardenability, an o-non-coil annealing method is used, and the cooling rate is set to 50 to 50°C.Then, the heat-treated steel sheet is subjected to temper rolling. It is essential to take precautions to prevent this from occurring, which causes unevenness and impairs the smoothness of the surface. However, if this temper rolling rate exceeds 1.5 inches in terms of elongation rate, the mechanical properties will deteriorate significantly at this stage.
Preferably it is O,S~1.2-.

Next, this steel plate can be used as is or electrogalvanized,
Or coated steel plate with zinc-based electrocomposite plating or electroalloy plating for painting an organic coating') as a base steel plate, which has excellent pressability such as corrosion resistance and peeling resistance, and weldability. In order to achieve this, it is preferable to use a Zn-based cold-weather alloy plating that has excellent corrosion resistance and pressability. It is preferable to use the base steel plate as a base steel plate.

Further, as the organic coating 2, a sink rich paint mainly containing metallic zinc powder is usually used, but from the viewpoint of weldability, it is particularly preferable to include 1 to 40% of stainless steel powder in addition to the metallic zinc powder.

As a heat source for heating and baking after application of an organic film, it is preferable to use an infrared lamp or a near-infrared lamp 'f1, but the output energy density is particularly high, and the time required to reach a steady output after energization is 1. It is preferable to use a near-infrared lamp with a very short duration of about seconds.

The heating rate and cooling rate were set to 5°C/l@C or higher and 30°C/1°C or higher, respectively, taking into account the length and height of the baking line, but heating and cooling required a long time, and the steel plate was kept at high temperatures for a long time. Exposure to the lower temperature will accelerate the strain aging of the steel plate, leading to greater deterioration of the material, so the heating rate should be 30℃/■e or higher.
The cooling rate is preferably 50°C/s@e or more.

The baking conditions for organic coatings are limited to a certain range for the reasons described below. Regarding the baking temperature and baking time, when the sole purpose is to bake the coating film, it is preferable to use a high temperature and a long time.

However, when a steel plate with strain age hardening properties is used as the base steel plate for a coating, heating at high temperatures and for a long period of time significantly accelerates strain aging, resulting in deterioration due to aging such as an increase in the yield point and the appearance of elongation at the yield point during the baking stage. This results in molding defects occurring during molding at the customer. After this baking process, the reduction rate is 1
．． It is possible to release the dislocations fixed by solid solution C by performing temper rolling of 51 or less, and to introduce new dislocations to restore the material quality of the steel plate to that close to that before the baking treatment. If there is no temper rolling machine for this purpose in the baking line, performing such temper rolling will lead to a decrease in productivity and an increase in cost.
A number of experiments were conducted by varying the baking temperature and baking time of the organic coating, and the results were analyzed and the following findings were made.

In other words, baking temperature T (℃) and baking time t (sea)
is 0≦Log t≦−0,024+5.52
If the relationship (1) is satisfied, the 4 yield point will not appear after the baking treatment, and the increase in the 1+ yield point can be suppressed to about l Kyf /IIm''.

EXAMPLES The effects of the present invention will be specifically explained below with reference to Examples. The chemical composition of the steel used is shown in Table 1.

In addition, the annealing cycles used in this example are shown in Figures @1 and 2, but annealing with a tight coil (1st rg4)
The heating rate for both oven coil annealing ($2 figure) is 40
℃/h, the annealing temperature is 700℃, the holding time at the annealing temperature is 4 hours, and the average cooling rate is 40℃/h for the tight coil.
h, with an oven coil at 150° C./h. Annealing edge: Temper rolling was performed at the pressure adjustment rate shown in Table 3.

Next, such a steel plate was coated with Zn 1i as the main component and N
l is 15 weight - 1F is 5 weight value, Cr t-3 weight -
After electroplating the alloy containing 20 iP/m”, 3
1 of zinc-rich paint containing 5 weight of stainless steel powder
It was painted and baked to a thickness of 0μ. As a heating source for baking,
Particularly preferred is the use of nine near-infrared run ff, and the heating rate and cooling rate are both 50°C/■.

The baking cycle is shown in FIG. 2, and the baking temperature and baking time #i are combined as shown in Table 3.

To evaluate the corrosion resistance of painted steel sheets manufactured in this way,
A cyclic corrosion test, which is considered to be an accelerated method of the salt spray test specified in JI8 Z 2371, was used, and the conditions are shown in Table 2.

Table 2 Cyclic corrosion test conditions' were set to 9, and spot weldability was evaluated based on change in r and diameter at 2000 continuous welding points or more.

The evaluation results are shown in Table 11!3.

The superiority of the material of the present invention is clear from table wE3.

In other words, the film baking time is longer than the time determined by conditional expression (1) of the present invention (7) (A2, 5, 7, 10
.

12, 14, and 16), as the steel plate hardens through strain aging, the yield point elongation recovers and
For those (A3, 4) in which the pressure regulation rate before coating baking is higher than the range of the present invention and K is out of range, the yield point and
The elongation is significantly deteriorated compared to the material of the present invention.

The material of the present invention (AI, 6.8, 9, 11, 13.15) has excellent mechanical properties, corrosion resistance, and weldability, and is effective for automotive parts that require severe press processing and also require corrosion resistance. It is clear that it can be used effectively.

[Brief explanation of drawings]

1 and 2 are diagrams showing the annealing cycle of the embodiment of the present invention, and FIG. 1 shows the annealing cycle with a tight coil,
The figure shows the baking cycle with an oven coil. FIG. 3 is a diagram showing a baking treatment cycle for an organic film according to an embodiment of the present invention. In the figure, T is baking temperature (°C), t is baking time (-・e
).

Claims (1)

[Claims] C: 0.03% or less, Mn: 0.4% or less,
In addition to sol, AL: 0, IO, below, if necessary, P: 0.150- or below, sol: O, SO, or below, alone or in combination of both, the remainder F, and The fII4 containing unavoidable impurities is tapped into a slab, hot-smoked and cold-rolled to produce a steel plate with the final desired thickness, then recrystallized in a box annealing furnace, and then 1,5- The following temper rolling is applied, and after that, electrogalvanizing, electrolytic composite plating or electroalloy plating mainly consisting of zinc is applied, and then an organic thinning film containing mainly metallic zinc powder is applied, and near-infrared rays are applied. It is recommended to heat and bake using a rung or an infrared rung, heating speed 5
C/s・C or more, cooling rate 30℃/s@e, and baking temperature T (C) and baking time t (see) are 1 O≦Lo@t≦−〇, 024T+5.52 −...
A method for manufacturing a coated steel sheet having strain age hardening properties, which comprises baking in a baking cycle that satisfies the following relationship: -.(1).