JPH07278775A - Production of hot dip aluminum coated steel sheet for deep drawing excellent thermal discoloration resistance - Google Patents

Abstract

PURPOSE:To simultaneously satisfy discoloration resistance and good workability by subjecting steel having specific components to finish rolling, cold rolling, annealing and nitriding treatments, then to a heat treatment under specific conditions. CONSTITUTION:The components of the steel sheet are composed to contain, by weight, <=0.05% C, <=0.005% N, <=1.5% Mn, <=0.5% Si, <=0.1% P, <=0.05% Al and B under condition of 0.7<B/N<1.5 and the balance Fe with inevitable impurities. This steel is subjected to finish rolling at Ar3 or above and is then coiled up at >=500 to <=780 deg.C. The steel sheet is then subjected to recrystallization annealing after cold rolling, then to a nitriding treatment at >=20 to <=500 in the product of the concn. (%) of NH3 and residence time (sec) in a nitriding atmosphere kept at >=600 to <=800 deg.C. The steel sheet is subjected to hot dip Al coating, then to the heat treatment for >=2 to <=10 hours at >=300 to <=450 deg.C. As a result, the hot dip aluminum coated steel sheet which is suppressed in the deterioration of merchandise value by thermal discoloration as far as possible is produced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention requires the production of hot-dip aluminized steel sheet which is required to have excellent deep drawability for household heating appliances and the like, while suppressing deterioration of commercial value due to thermal discoloration caused by exposure to high temperatures. It is about law.

[0002]

2. Description of the Related Art A hot-dip aluminized steel sheet has been developed as a surface-treated steel sheet having excellent oxidation resistance and corrosion resistance. However, when exposed to high temperatures for a long time during use, iron atoms diffuse from the base metal into the aluminum plating. It is known that the alloying of Fe-Al progresses to the surface of the plating phase and the surface layer is darkened and both the oxidation resistance and the corrosion resistance deteriorate. As a measure to suppress this alloying, Japanese Patent Publication No. 2-61544 discloses that solid solution nitrogen is left in the base material and an AlN phase is formed between the base material and the plating phase in the molten aluminum plating step or the subsequent heat treatment. , Fe and Al
A method for suppressing alloying by suppressing the mutual diffusion of

However, in the component system in which solid solution nitrogen is left in the steel, this interstitial element influences the crystal rotation accompanying rolling during cold rolling, and crystals with a {111} orientation which is advantageous for deep drawability after annealing. In the present situation, when such a steel sheet is used as a base material in order to suppress the generation of grains, a hot-dip aluminized steel sheet having excellent deep drawability cannot be produced.

In order to achieve excellent deep drawability, steel IF
(Interstitial atoms free) is known to be effective, and many steel sheets have been developed in which Ti, Nb, etc. are added to precipitate C and N as carbonitrides, and most of the solid solution C and N are removed. However, the production cost becomes high because of the increase in steel making cost due to the extremely low carbonization and the addition of expensive elements such as Ti and Nb.

As a method of producing a deep-drawing steel sheet without using IF steel, there is a method of winding aluminum killed steel at a high temperature to reduce the solute C and N as much as possible. However, when such a steel plate is used as a base plate for hot dip aluminum plating, solid solution nitrogen does not exist in the base metal even after the heat treatment disclosed in Japanese Patent Publication No. 2-61544 after hot dip aluminum plating. Therefore, an AlN phase is not formed at the interface between the mother phase and the plating phase, and alloying proceeds during high temperature use.

[0006]

DISCLOSURE OF THE INVENTION The present invention provides a method for producing a hot-dip aluminized steel sheet which simultaneously satisfies excellent deep drawability and heat discoloration resistance.

[0007]

Means for Solving the Problems The present inventors cold-rolled steel in which solute nitrogen hardly exists in a hot-rolled sheet, recrystallized by continuous annealing, and then nitrided under appropriate conditions.
It was found that an AlN phase can be formed between the base material and the plating phase of the hot-dip aluminized steel sheet, and as a result, it was found that a hot-dip aluminized steel sheet satisfying both excellent deep drawability and heat discoloration resistance can be produced.

In principle, the interstitial elements are formed in the form of precipitates during cold rolling as much as possible so as not to adversely affect the texture formation during cold rolling-annealing, and then solute nitrogen is introduced into the base metal by nitriding. Then, by controlling the heat history of the molten aluminum plating, AlN is formed between the base material and the plating phase, and the subsequent alloying is suppressed. At this time, the necessary formation state of AlN is achieved. It is necessary to select an appropriate composition and nitriding condition range.

That is, the present invention has been made from such a viewpoint, and the gist thereof is C: 0.05% or less, N: 0.005% or less, Mn: 1.5% by weight. % Or less, Si: 0.5% or less, P: 0.1% or less, Al: 0.05%
Hereinafter, steel containing B so as to satisfy the condition of 0.7 <B / N <1.5, and the balance Fe and unavoidable impurities,
Finish rolling is performed at a temperature above the Ar ₃ transformation point, 500 ° C
As described above, after winding at a temperature of 780 ° C. or lower, cold rolling, and recrystallization annealing in a continuous annealing furnace, 600 ° C. or higher, 800 ° C.
In the following nitriding atmosphere, the nitriding treatment is performed under the condition that the product of NH ₃ concentration (%) and the residence time (second) in this temperature range is 20 or more and 500 or less, followed by hot dip aluminum plating,
Then, it is a method for producing a deep-drawn hot-dip aluminized steel sheet having excellent heat discoloration resistance, which is characterized by performing heat treatment at a temperature of 300 ° C. or higher and 450 ° C. or lower for 2 hours or more and 100 hours or less.

The present invention will be described in detail below.
The reasons for limiting the components of the present invention are as follows. C: 0.0
The reason why the content is 5% or less and N: 0.005% or less is that if C and N are added in excess of these amounts, the workability of the product is impaired. As other components, the components usually contained for improving strength, that is, the upper limits of Mn and P are respectively M.
The reason why n: 1.5% or less and P: 0.1% or less is that the addition of more than this deteriorates the workability.

Since Al and Si form a nitride in steel and control the formation of AlN between the base metal and the plating phase, the upper limits were made 0.05% and 0.5%, respectively. Further, the addition amount of B is limited to 0.7 <B / N <1.5 because B <0.7
This is because a large amount of solid solution N exists in N during cold rolling, smooth crystal rotation is hindered, and it is difficult to form a texture that is preferable for deep drawability after annealing. On the other hand, excessive solid solution B deteriorates the workability, so the upper limit was set to B <1.5N.

Next, the limitation of the manufacturing process will be described. Since the characteristics of the steel of the present invention do not largely depend on the heating conditions of hot rolling, there is no particular need to limit it. That is, it does not matter whether the slab is reheated and then hot-rolled or the direct slab is hot-rolled. However, low temperature heating is advantageous because it promotes precipitation of AlN. Further, a thin cast piece from which hot rolling is omitted may be used.

Since hot rolling must be completed in the austenite region in order to obtain excellent deep drawability, the finishing temperature is A
It must be above the r ₃ transformation point. Winding temperature is 500
The reason why the temperature is limited to not less than 0 ° C is that the material becomes hard and the workability deteriorates at a temperature lower than this. On the other hand, the reason why the upper limit of the winding temperature is 780 ° C. is that the pickling property is deteriorated when the winding temperature is higher than this. The winding temperature is preferably 650 ° C. or lower in order to minimize variations in the material.

The cold rolling conditions are also strong and need not be limited,
A cold rolling rate of 60% or more and 85% or less is preferable in order to develop a texture having a preferable deep drawability. Regarding the recrystallization annealing conditions, if the steel sheet is exposed to a nitriding atmosphere before being recrystallized, recrystallization is remarkably suppressed. Therefore, it is necessary to perform recrystallization before entering the nitriding atmosphere.

The most important process condition of the present invention is the nitriding condition, in which the concentration of NH ₃ (%) and the residence time (seconds) in this temperature range in the nitriding atmosphere of the steel of the present invention of 600 ° C. to 800 ° C. By nitriding under the condition that the product is 20 or more and 500 or less, a relatively large amount of solute nitrogen can remain in the original plate, and the heat treatment described later causes mutual diffusion of Fe and Al between the base metal and the plating phase. It is possible to form AlN having a sufficient thickness to suppress the above. NH in nitriding atmosphere
The lower limit of the product of the concentration (%) of ₃ and the residence time (second) in the nitriding temperature range between 600 ° C and 800 ° C was set to 20 because nitriding did not proceed sufficiently and solid solution in the original plate was observed. This is because the amount of nitrogen is small and AlN having a sufficient thickness is not formed between the base material and the plating phase. On the other hand, the upper limit is set to 500 because the workability of the steel sheet deteriorates when nitriding is excessively increased.

The heat treatment condition after hot dip aluminum plating is 300
The reason why the temperature is lower than or equal to 450 ° C. and lower than or equal to 2 hours and lower than or equal to 100 hours is that the temperature is low, and if the treatment time is short, a sufficient thickness of AlN is not formed between the base metal and the plating phase. However, if a high temperature treatment is performed for a long time, not only is the possibility that the plating phase may peel during processing due to the transformation of the plating phase or the progress of alloying due to the mutual diffusion of Fe and Al, but also discoloration may occur. This is because.
The aluminum plating described here does not impair the gist of the present invention even if the aluminum plating bath contains an element such as Si in order to suppress the alloying reaction that occurs in the hot dipping process.

[0017]

EXAMPLES Examples of the present invention will be described below together with comparative examples. Table 1 shows the composition of the steel of the present invention used in the examples and the comparative material. Table 2 shows manufacturing conditions and quality characteristics.
The material here is that the continuous cast slab is heated to 1200 ° C., the 4 mm thick hot-rolled sheet obtained by hot rolling is cooled by 80%, and this cold-rolled sheet is continuously annealed. , F, I at 800 ° C. for 30 seconds and materials A, D, E, G, H at 750 ° C. for 30 seconds by recrystallization annealing, then nitriding treatment, and melting at 690 ° C. containing Si. 5 for aluminum bath
It is soaked for a ^second and plated with aluminum of about 30 g / m ² . Then, the post heat treatment was performed by batch annealing.

The same materials were used in Experiment Nos. 1 to 17 to clarify the influence of process conditions. Experiment number 1
From 8 to 26, the effect of steel grade was clarified. In the discoloration test, heat treatment was performed at a temperature of 550 ° C. for 50 hours, and then the surface quality was observed. Experiment numbers 1, 4, 5, 7 within the scope of the present invention,
8, 10, 13, 15, 16, 17, 18, 19, 2
0, 21, 22 have good discoloration resistance and excellent workability.

On the other hand, in Experiment No. 2, the workability was significantly deteriorated because the finishing temperature was below the transformation point. In Experiment No. 3, the coiling temperature was low and carbides were finely precipitated on the hot-rolled sheet, so that excellent deep drawability could not be obtained. In Experiment No. 6, ductility was remarkably deteriorated due to excessive nitriding. In Experiment No. 9, nitriding was not sufficiently performed, a sufficient amount of AlN layer was not formed at the interface between the plating and the base material by the subsequent heat treatment, and blackening was observed after the discoloration test. In Experiment No. 11, since the post heat treatment temperature was high, blackening was already observed after the post heat treatment. On the contrary, in Experiment No. 12, the post-heat treatment temperature was too low, so that a sufficient amount of the AlN layer was not formed at the interface between the plating and the base material by the heat treatment, and blackening was observed after the discoloration test. The same thing was observed in Experiment No. 14 in which the post heat treatment time was short.

In Experiment No. 23, the B / N of Steel G used was 1.
It can be seen that the workability is inferior to that of Experiment No. 1 in which the material A of 5 or more and other components are almost the same and the experiment conditions are almost the same. In Experiment No. 24, a large amount of C was not obtained and excellent workability was not obtained. In Experiment No. 25, since the B / N of Steel I used was as small as 0.61 and a large amount of solute N remained, smooth crystal rotation was hindered during cold rolling and a texture preferable for deep drawability was formed after recrystallization. No, experiment number 18 under similar conditions
Compared with, the workability is deteriorated. In Experiment No. 26 using a material containing a large amount of Al, Al and N attracted each other in the base material and suppressed the movement of N, so that a sufficient amount of AlN was not formed at the interface between the plating and the base material. Was recognized. In Experiment No. 27, which used a material containing a large amount of Si, Si and N attracted each other in the base material and suppressed the movement of N, so a sufficient amount of AlN was not formed at the interface between the plating and the base material. It seems that the change has appeared.
Experiment No. 28 is a conventional discoloration-resistant steel plate, but excellent deep drawability cannot be obtained because a large amount of solute N exists during cold rolling.

[0021]

[Table 1]

[0022]

[Table 2]

[0023]

Industrial Applicability According to the present invention, a hot-dip aluminized steel sheet satisfying both discoloration resistance and good workability, which have hitherto been incompatible, can be produced and is industrially extremely useful.

Continuation of the front page (51) Int.Cl. ⁶ Identification code Office reference number FI technical display location // C22C 38/00 301 T 38/06

Claims (1)

[Claims] 1. By weight ratio, C: 0.05% or less, N: 0.005% or less, Mn: 1.5% or less, Si: 0.5% or less, P: 0.1% or less, Al: 0.05% or less, B is included so as to satisfy the condition of 0.7 <B / N <1.5, and the balance Fe and unavoidable impurities are subjected to finish rolling at a temperature of Ar ₃ transformation point or higher. , 500 ° C or higher, 7
After winding at 80 ° C or lower, cold rolling, and recrystallization annealing in a continuous annealing furnace, NH ₃ concentration (%) and staying in this temperature range in a nitriding atmosphere of 600 ° C or higher and 800 ° C or lower Nitriding is performed under the condition that the product of time (seconds) is 20 or more and 500 or less, followed by hot dip aluminum plating, and then
At a temperature of 300 ° C to 450 ° C for 2 hours or more, 10
A method for producing a hot-dip aluminized steel sheet for deep drawing, which is characterized by heat treatment for 0 hour or less.