JPH08100217A - Production of hot dip aluminum coated steel sheet excellent in heat resistance - Google Patents

Abstract

PURPOSE: To produce a hot dip aluminum coated steel sheet excellent in heat resistance (darkish discoloration resistance), workability, weldability, etc., on an industrial scale of production by using an aluminum killed steel sheet as a steel sheet for base material. CONSTITUTION: The low carbon aluminum killed steel has a composition consisting of 0.03-0.08% C, 0.1-0.4% Mn, 0.004-0.014% Al, 0.0025-0.005% N, and the balance Fe with inevitable impurities. A hot rolled plate of this steel is coiled at 500-600 deg.C, and a cold rolled sheet of this steel is used as a steel sheet for base material for plating. After hot dip aluminum coating, heat treatment is done at 250-500 deg.C for >=1min holding time. By this method, the hot dip aluminum coated steel sheet, increased in the luster retentivity of a plating layer surface in a high temp. environment, can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention uses an aluminum-killed steel sheet as a base metal sheet for plating, has excellent heat resistance, has high gloss retention (blackening resistance) on the surface of the plating layer in a high temperature environment, and has good workability and welding. The present invention relates to a method for producing a hot-dip aluminized steel sheet having properties and the like.

[0002]

2. Description of the Related Art Hot-dip aluminized steel sheet is a material excellent in high-temperature corrosion resistance, and is used for heat-resistant applications such as automobile exhaust system members (muffler pipes, etc.) and petroleum stove panels. Traditionally, hot-dip aluminized steel sheets are
The rimmed steel sheet has been manufactured as a plated base steel sheet,
In recent years, with the progress of continuous casting of slabs, the proportion of aluminum-killed steel sheets used as a base steel sheet has significantly increased. However, in the case of the hot-dip aluminized steel sheet using the rimmed steel sheet, while maintaining a beautiful silver-white glossy surface even in the temperature range of about 550 ° C. About 350 ° C
When heated to the temperature of 1, there is a tendency for the silver-white glossy surface to be lost in a short time and to change to grayish black (black change). The black discoloration on the surface of the plating layer is a phenomenon accompanying the interdiffusion reaction of Fe-Al (alloying of the aluminum plating layer), which occurs at the interface between the aluminum plating layer and the base steel plate. In the case of a hot-dip aluminized steel sheet using a rimmed steel sheet, at the interface between the plating layer and the base material steel sheet, there is a barrier layer that prevents the Fe-Al interdiffusion reaction between the base material steel sheet and the aluminum plating layer. ,
In a hot-dip aluminized steel sheet having an aluminum-killed steel sheet as a base steel sheet, since such a barrier layer is not formed, blackening (loss of silver-white luster) occurs rapidly under the influence of heating in a relatively low temperature range.

As a countermeasure, Japanese Patent Publication No. 5-26864 discloses that C: 0.2% or less, Mn: 0.1 to 0.4.
%, Al: 0.005-0.02%, N: 0.0005
~ 0.006%, the balance is Fe aluminum killed steel sheet as the base material steel sheet, hot-dip aluminum plating (Si content 1 ~ 15
%), And at a temperature rising rate of 300 ° C./hr or less, 350 to 50%.
A method of manufacturing a hot dip aluminized steel sheet has been proposed in which heat treatment is performed by heating to 0 ° C. and holding for 30 minutes or more.
As a result of the aluminum content of the base material steel plate, the composition of the molten Al plating bath (Si content), and the regulation of the heat treatment conditions after plating, the manufacturing method has the effect of Fe- at the interface between the plating layer and the base material steel plate. By forming an AlN layer as a barrier layer that prevents mutual diffusion of Al, alloying of the plating layer is suppressed and prevented.

[0004]

DISCLOSURE OF THE INVENTION In the method for producing a hot-dip aluminized steel sheet disclosed in the above publication, as heat treatment for increasing its heat resistance, as described therein, a temperature of 300 ° C./Hr or less is used. Slow heating rate 350 ~
It is necessary to perform a heat treatment of heating to 500 ° C. and holding for 30 minutes or more. However, it is impossible to carry out the heat treatment in a continuous annealing furnace type heat treatment furnace (the heating temperature rising time is extremely short, usually about 1 to 2 minutes). Therefore, the batch annealing method should be adopted. Inevitably, even when batch annealing is adopted, the open coil annealing furnace (usually the heating rate is 300
(It is significantly faster than ℃ / Hr) is difficult, and it is unavoidable to use a tight coil annealing furnace. Therefore, the heat treatment requires a long time, is extremely inefficient, lacks flexibility in production planning, and is subject to many restrictions when implemented on an industrial scale.
Moreover, although the hot-dip aluminized steel sheet has improved heat resistance, the limit is around 550 ° C., and in a high temperature environment exceeding that, a rapid black change occurs and a silver-white glossy surface. Lost. Further, when the C content of the base steel sheet is a high C content exceeding about 0.08%, the workability is remarkably low, and for example, in the case of the use of a muffler for automobiles, it is difficult to expand the pipe, while the C content is high. When the amount is a low C amount of less than about 0.03%, there is a problem that the weld heat-affected zone is coarsened and the workability and toughness are impaired. The present invention further enhances the heat resistance (blackening resistance) of a hot-dip aluminized steel sheet using an aluminum-killed steel sheet as a base steel sheet, and withstands the weldability required as a structural part material and strong processing such as pipe expansion processing. It is intended to provide a novel method for producing a hot-dip aluminized steel sheet having high workability.

[0005]

The method for producing a hot-dip aluminized steel sheet according to the present invention comprises a method of rolling a steel strip by hot rolling 500 times.
C: 0.03 to 0.08 performed in a temperature range of to 600 ° C.
%, Mn: 0.1 to 0.4%, Al: 0.004 to 0.
014%, N: 0.0025 to 0.0050%, balance F
It is characterized in that a cold-rolled steel sheet of low carbon aluminum killed steel consisting of e and unavoidable impurities is used as a base metal sheet for galvanizing, hot-dip aluminum plating is performed, and then heat treatment is performed at a temperature of 250 to 500 ° C. for a holding time of 1 minute or more. .

[0006]

The hot-dip aluminized steel sheet produced by the method of the present invention is rich in solid solution N in the steel as a result of the regulation of the Al and N contents of the base steel sheet and the regulation of the hot rolling coiling temperature. Then, the solid solution N remaining in the steel diffuses and transfers to the surface of the steel sheet by the heat treatment performed after the hot dip aluminum plating to form an AlN layer at the interface with the plating layer. Due to the formation of the AlN layer, Fe-Al interdiffusion at the interface (alloying reaction of the plating layer) is effectively suppressed and prevented, and the surface of the plating layer is
Even in a high temperature range of about 600 ° C, no black change occurs,
A beautiful silver-white glossy surface is held stably. The heat treatment for forming the AlN layer is much less restricted than the prior art disclosed in the above publication and is 250 to 500 ° C.
It can be sufficiently achieved by holding in the temperature range of 1 minute or more, which means that the heat treatment can be performed in a continuous annealing furnace type heat treatment furnace, and even in the case of batch annealing, This means that an open coil annealing furnace can be used instead of the tight coil annealing furnace. Further, the hot-dip aluminized steel sheet according to the present invention has improved workability and weldability as an effect of defining the C content and the like of the base steel sheet.

[Reasons for Limiting Components of Plated Base Steel Sheet] The reasons for limiting the chemical composition of the low carbon aluminum killed steel sheet used as the base steel sheet in the present invention are as follows. Al: 0.004 to 0.014% Al is added as a deoxidizing element in the steel melting process. If the amount is less than 0.004%, the deoxidizing action becomes insufficient and it becomes difficult to maintain stable continuous casting operation. On the other hand, if it exceeds 0.014%, it reacts with the solid solution N in the steel and causes a shortage of the residual amount of the solid solution N in the steel necessary for forming the barrier layer (AlN layer) at the plating layer interface. The effect of improving heat resistance (blackening resistance) becomes poor. For this reason,
The amount of Al is 0.004 to 0.014%.

FIG. 1 shows the effects of Al content and N content of the base steel sheet on the heat resistance (blackening resistance) of the hot dip aluminum plated steel sheet. [Test-plated steel sheet] Base steel sheet (low-carbon aluminum-killed steel cold-rolled sheet) Chemical composition (Wt%): C 0.05, Mn 0.20, Al 0.004-0.020, N
0.0005-0.0050, Fe Bal, Plate thickness: 1.0 mm Hot rolling coiling temperature: 505 ° C Plating layer: 8.5% Si-Al, Layer thickness 19 µm Heat treatment after plating: 400 ° C x 3 Hr It shows the presence or absence of black color change on the surface of the plating layer after the test piece cut out from the test material was held at 100 Hr in a heating furnace set to a constant temperature. ○: No black change at 600 ° C (holding silver-white glossy surface) Δ: Black change at 550 ° C (no black change at 550 ° C) ×: Black change at 550 ° C As shown in the drawing, The N content is defined by the present invention (Al content 0.004 to 0.014%, N content 0.002).
5 to 0.005%) by adjusting the range of 60
A high degree of heat resistance that can withstand heating at 0 ° C is ensured.

N: 0.0025 to 0.0050% N is an element necessary for improving the heat resistance (blackening resistance) of the hot dip aluminum plated steel sheet, and the solid solution N in the base steel sheet is hot dip plated. In a heat treatment to be performed later, an AlN layer (barrier layer that prevents black discoloration due to Fe-Al interdiffusion) is formed at the interface with the plated layer by diffusion transfer to the surface of the steel sheet. In order to fully enhance the heat resistance improving effect by the formation of this AlN layer, the N content needs to be at least 0.0025%. However, if it exceeds 0.0050%, the ductility of the base steel sheet is greatly deteriorated due to aging and the workability is impaired, so this is made the upper limit.

FIG. 2 shows the influence of the N content of the base steel sheet on the ductility of the hot-dip aluminized steel sheet and the reduction in elongation (ΔEl) before and after aging. [Test-plated steel sheet] Base steel sheet (low-carbon aluminum-killed steel cold-rolled sheet) Chemical composition (Wt%): C 0.05, Mn 0.20, Al 0.004, and 0.
014, N 0.0005-0.0065, Fe Bal, Plate thickness: 1.0 mm Hot rolling Winding temperature: 505 ℃ Plating layer: 8.0% Si-Al, Layer thickness 20 μm Heat treatment after plating: 400 ℃ × 3 Hr Aging treatment: Test Material (plate thickness 1.0 mm) to test piece (JIS Z220
Cut out No. 15) and carry out artificial aging treatment at 100 ° C x 1 hr. In the figure, (1) is the measurement result of the Al 0.004% and (2) is the Al 0.014%. If the N content of the base steel sheet exceeds the upper limit value of 50 ppm (0.0050%) specified by the present invention, the ductility is greatly reduced by aging, but by limiting it to 50 ppm or less, the reduction of ductility by aging is suppressed. It is understood that good ductility is maintained.

C: 0.03 to 0.08% C greatly affects the workability and weldability of the hot-dip aluminized steel sheet. If the amount of C is less than 0.03%, the metal structure of the heat-affected zone of the welding becomes coarse, and the workability and toughness of that portion deteriorate, and the workability after welding is greatly impaired. For example, a muffler for automobiles. In the manufacture of pipes, it is difficult to secure the workability necessary for pipe expansion processing after pipe making by welding. Therefore, the C content is at least 0.03%.
Need. However, if the C content exceeds 0.08%,
Since the ductility of the steel sheet is greatly reduced and good workability cannot be ensured in this case as well, the upper limit is set.

FIG. 3 shows the effect of the C content of the base steel sheet on the ductility (elongation) and workability (pipe expanding workability) of the hot-dip aluminized steel sheet. [Test plated steel sheet] Base material steel sheet (low carbon aluminum killed steel cold rolled sheet) Chemical composition (Wt%): C 0.05-0.20, Mn 0.20, Al 0.008, N
0.0035, Fe Bal, Plate thickness: 1.0 mm Hot rolling coiling temperature: 505 ℃ Plating layer: 8.7% Si-Al, Layer thickness 18 μm Heat treatment after plating: 400 ℃ × 3 Hr Pipe forming process: Pipe by high frequency welding ( Pipe diameter 28.6
mm), and then pipe expansion processing (cold processing) using a mandrel to finish the pipe with a pipe diameter of 37.2 mm (expansion rate: 1.3). In the figure, ○: No cracking during pipe expansion, ●: Cracking occurred. As shown in the figure, when the C content of the base steel sheet exceeds 0.08%, the ductility of the steel sheet suddenly decreases and the cracks in the pipe expanding process are unavoidable. Although the ductility is good, the workability required for pipe expansion cannot be secured. On the other hand, 0.03 to 0.08 specified in the present invention
In the range of%, high elongation characteristics and good workability capable of preventing pipe expansion cracking are secured.

Mn: 0.1 to 0.4% Mn fixes S in the steel as MnS and renders it harmless, prevents red hot embrittlement of the steel, and improves hot workability. For this,
0.1% or more is required, but if it exceeds 0.4%, the workability of the steel deteriorates, so this is made the upper limit.

[Reason for Limiting Winding Temperature of Hot Rolled Steel Sheet] The upper limit of the steel sheet winding temperature in hot rolling of low carbon aluminum killed steel is defined as 600 ° C. Most of the solid solution N in the steel reacts with Al added to the steel as a deoxidizer and is fixed in the steel as AlN. As a result, the barrier layer (Al
This is because the residual amount of solid solution N necessary for forming the N layer) is insufficient, and heat resistance (blackening resistance) cannot be ensured. On the other hand, 5
The lower limit temperature of 00 ° C. is that if it is wound in a lower temperature range, the ductility is lowered due to the refinement of the crystal grains of the steel sheet, and it becomes difficult to secure good workability. Is.

[Reason for limiting heat treatment conditions after hot-dip aluminum plating] By heat treatment performed after hot-dip aluminum plating, solid solution N in the base steel sheet diffuses and migrates to the surface, and an AlN layer (barrier layer) is formed at the interface with the plating layer. To form. The heat treatment is 25
It is achieved by keeping the temperature range of 0 to 500 ° C. for 1 minute or more. The lower limit of the treatment temperature was set to 250 ° C., because at lower temperatures, a sufficient improvement effect of heat resistance could not be obtained even after long-time treatment, while 500 ° C. was set as the upper limit. What is more, high temperature treatment exceeding that not only impairs thermo-economic efficiency, but also causes re-solution of cementite (Fe ₃ C) in the steel of the base material steel sheet, and increases the amount of solute C to improve workability of the steel sheet. This is because it causes a decline.

FIG. 4 shows the effect of heat treatment temperature on the effect of improving the heat resistance (blackening resistance) of hot dip aluminum plated steel sheet,
The processing time is shown as a parameter. The horizontal axis of the figure is
The processing temperature in the heat treatment after plating (a in the figure is a processing time of 1 minute, b is the same 30 minutes, and c is the same 180 minutes), and the vertical axis represents the blackening start temperature of the plating layer surface in the heat resistance test. is there. [Test plated steel sheet] Base steel sheet (low carbon aluminum killed steel cold rolled sheet) Chemical composition: C 0.05, Mn 0.20, Al 0.008, N 0.0035, Fe Ba
l, (Wt%) Plate thickness: 1.0 mm Hot rolling coiling temperature: 505 ℃ Plating layer: 8.7% Si-Al, Layer thickness 18 μm Heat treatment after plating: Temperature 100-600 ℃, Time 1 min, 30
Minutes, 180 minutes. Heat resistance test: heating the test material in a furnace (inside furnace temperature 400-625 ℃)
After 100 hours of holding, the presence of black discoloration on the plating layer surface is visually judged.

As shown in the figure, when the temperature of the heat treatment performed after the plating is lower than 250 ° C., the effect of improving the heat resistance is small even if the treatment time is extended, but the treatment temperature is 250 ° C.
It can be seen that the heat resistance (blackening start temperature) can be increased to a high temperature region around 600 ° C. by setting the temperature to the above (lower limit temperature of the present invention) or more (from the viewpoint of the heat resistance improving effect, the treatment temperature is 500 ° C. or less). However, the upper limit is 500 ° C. from the viewpoint of ensuring the heat economy and workability as described above.). Further, as is clear from the figure, the heat resistance improving effect can be achieved by a heat treatment for a short time of about 1 minute, and this can be applied to a continuous annealing furnace or the like as already described. I mean.

The cold rolling following the hot rolling of the aluminum-killed steel sheet used as the base steel sheet of the hot-dip aluminized steel sheet of the present invention may be carried out in accordance with a conventional method, and the hot-dip aluminum coating step also has special restrictions and conditions. Need not be added,
It is carried out in a continuous hot dip coating line according to a conventional method, and a plating bath adjusted to a Si-Al alloy bath composition containing about 0.5 to 10% of Si is used, if desired.

[0019]

【Example】

[1] Manufacture of hot-dip aluminized steel sheet (i) Base material steel sheet: Hot-rolled continuously cast slab of low carbon aluminum killed steel, pickled hot rolled steel sheet (thickness 3.2 mm),
It is subjected to cold rolling to obtain a cold rolled steel plate with a plate thickness of 1.0 mm. (ii) Hot-dip aluminum plating and heat treatment after plating: The cold-rolled steel sheet is supplied to a continuous hot-dip aluminizing line, and reduction annealing treatment is carried out in accordance with a conventional method, passing through a plating bath, and the amount of coating weight on the plating bath. Adjustment, etc. (plating layer composition: Al-8.5% Si, coating weight 80 g / m ² ·
Both sides). After plating, heat treatment is performed under various heating conditions to obtain a test piece of hot dip aluminized steel sheet. Table 1 and Table 3
Shows the chemical composition of the base steel sheet, the hot rolling coiling temperature, and the heat treatment conditions (treatment temperature / time) after plating.

[2] Evaluation of various characteristics Tables 2 and 4 show the measurement results of heat resistance (blackening resistance), mechanical properties, and workability (tube expansion workability) of each hot-dip aluminized steel sheet. The heat resistance and workability were evaluated by the following tests. (1) Heat resistance test: A test piece (5
(0 × 70, mm) is held in a heating furnace maintained at a temperature of 400 to 600 ° C. for 100 hours, and then the presence or absence of discoloration on the plating surface is visually determined, and the thickness of the Fe—Al alloy layer at the interface is also determined. Measure. In the table, the symbols in the "plated layer surface appearance" column are as follows. ○… Silver-white gloss retention ×… Discolored to gray black

(2) Tube expansion test: A tube (tube diameter 28.6 mm, thickness 1.0 mm) is formed from a test plated steel sheet by roll forming and high frequency welding, and then tube expansion is performed by a mandrel (expansion rate: 1.3). After pipe expansion, the presence of cracks is determined by visual observation. The symbols in the "Pipe expansion workability" column in the table are as follows. ○: No cracks occurred ×: Cracks occurred

In the table, Nos. 1 to 6 are invention examples (Tables 1 and 2), and Nos. 101 to 114 are comparative examples (Tables 3 and 4). Of the comparative examples, No. 101 to 113 are similar to the plated steel sheet of the invention example in which the aluminum killed steel sheet is the base material steel sheet, but the base material steel sheet composition, hot rolling coiling temperature, or heat treatment conditions after plating One of the requirements (underlined in Table 3) deviates from the definition of the present invention, and No. 114 is an example in which a rimmed steel sheet is used as the base steel sheet. The No. 114 hot-dip aluminized steel sheet, which uses the rimmed steel sheet as the base steel sheet, has a temperature of 550
Up to around ℃, it keeps a silver-white glossy surface, but when it exceeds that temperature range, it changes to grayish black due to alloying of the plating layer. On the other hand, invention examples No. 1 to 6 are 600
Even in a high temperature range around ℃, the surface of the plating layer does not change to black and the silver-white surface is kept beautiful. Also,
It has good strength and ductility, and has sufficient workability to withstand pipe expansion.

In Comparative Examples Nos. 101 to 113 (all carbon steel sheets are low carbon aluminum killed steel sheets), No. 1
The heat resistance (blackening resistance) of 01 to 105 is low because the amount of Al and N in the base steel sheet is excessive or insufficient, so the amount of solute N in the steel is small, and as a result, the plating layer and the base steel sheet. This is because the barrier layer (AlN layer) at the interface with and was not sufficiently formed. In No. 106, since the base steel sheet contains an excessive amount of N, the ductility is greatly reduced due to aging of the steel sheet, and cracking occurs in the pipe expanding process. No. 107 has good elongation, but cracks have occurred during pipe expansion. This is because the metal structure of the weld heat affected zone became coarse due to the lack of the C content of the base steel sheet. In No. 108, since the C content of the base steel sheet was excessive, the ductility of the steel sheet was low and cracking occurred during pipe expansion processing.
The heat resistance of No. 109 is low because the hot rolling temperature of the base steel sheet was too high, and the amount of dissolved N in the steel required for forming the barrier layer (AlN layer) was insufficient. And No.1
The reason why cracking occurred during pipe expansion in No. 10 was that the hot rolling temperature of the base steel sheet was too low and the crystal structure of the steel sheet was excessively refined and ductility was reduced. No.111 and No.1
The heat resistance of No. 12 was low because the former had a short treatment time in the heat treatment after plating, and the latter had a too low heat treatment temperature, so that the AlN layer (barrier layer) at the plating layer interface was not sufficiently formed. It depends. In addition, No. 113 is
Since the heating temperature of the heat treatment after plating is too high, the ductility of the base steel sheet deteriorates and cracks occur during pipe expansion.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

[Table 3]

[0027]

[Table 4]

[0028]

According to the method of the present invention, an aluminum-killed steel plate, which is cheaper than a rimmed steel plate and has a good workability, is used as a base steel plate, and it has excellent heat resistance (blackening resistance) and has a workability and a weldability. Aluminized steel sheet can be manufactured.
The effect of improving the heat resistance is extremely high, and even in a high temperature range of about 600 ° C., the surface of the plating layer does not change to black, and a beautiful silver-white glossy surface is stably maintained. Further, in the production of hot-dip aluminized steel sheet using an aluminum-killed steel sheet as a base material steel sheet by the conventional method, the heat treatment after plating for imparting heat resistance must be carried out in a tight coil annealing furnace, whereas In the manufacturing method of the invention, there are few restrictions on heat treatment conditions, and it is possible to carry out by a continuous annealing furnace type heat treatment furnace.When applying a batch method, not only a tight coil annealing furnace but also an open coil annealing furnace are used. Is extremely advantageous in terms of production efficiency, such as
The industrial significance is extremely large, and it is possible to expand and diversify the heat resistant use of hot-dip aluminized steel sheet.

[Brief description of drawings]

FIG. 1 is a graph showing the influence of the Al and N contents of a base steel sheet on the heat resistance of a plated steel sheet.

FIG. 2 is a graph showing the relationship between the amount of reduction in elongation of a coated steel sheet due to artificial aging and the N content of the base steel sheet.

FIG. 3 is a graph showing the influence of the C content of the base steel sheet on the elongation and workability of the plated steel sheet.

FIG. 4 is a graph showing the effect of heat treatment temperature on the heat resistance (blackening resistance) of the plated steel sheet.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI Technical indication C23C 2/28

Claims (1)

[Claims] 1. Winding of steel strip in hot rolling is performed in a range of 500 to 60.
C: 0.03 to 0.08%, M performed in a temperature range of 0 ° C.
n: 0.1 to 0.4%, Al: 0.004 to 0.014
%, N: 0.0025 to 0.0050%, a cold rolled steel plate of low carbon aluminum killed steel consisting of balance Fe and unavoidable impurities was used as a plating base metal steel plate, and after hot dip aluminum plating, the temperature was 250 to 500 ° C. A method for producing a hot-dip aluminized steel sheet having excellent heat resistance, weldability, workability, etc., characterized by performing heat treatment for a holding time of 1 minute or more.