JPH0627315B2 - Method for producing high-strength alloyed hot-dip galvanized steel sheet - Google Patents

Description

[Detailed Description of the Invention] (Industrial field of application) High-strength galvannealed steel sheet (hereinafter referred to as HS.GA steel sheet) with a tensile strength of 35 kg / mm ² or more and excellent workability, especially total elongation. With respect to the advantageous production of the present invention, this specification intends to disclose the results of the developmental research on the advantageous improvement of the plating adhesion without the disadvantages of strain aging, natural aging and other powdering.

In recent years, high-strength steel sheets have been widely used for the purposes of safety of automobiles, reduction of vehicle body weight, and reduction of material usage. These high-strength steel sheets are often used in a thinner plate thickness than the case of using the ordinary steel due to the purpose of use, and thus are in a situation much more serious than the ordinary steel with respect to corrosion.

Therefore, it is required that high-strength steel sheets having excellent corrosion resistance and workability be manufactured by a mass production method.

Examples of methods for imparting corrosion resistance to steel sheets include Cu and Cr
There is a method of adding an element that enhances the corrosion resistance of steel into the steel and a method of applying metal plating to the surface of the steel sheet, but the former is not so effective under severe corrosion conditions such as salt damage.

Therefore, against such severe corrosion, hot-dip galvanizing is extremely effective even in metal plating, and it is possible to perform thick plating. Moreover, in recent years, high post-coating corrosion resistance, coating adhesion and spot weldability have been achieved. In consideration of the above, it is absolutely necessary to perform an alloying heat treatment after hot dip galvanizing.

However, the influences of the factors such as strength, workability, and plating adhesion on the characteristic values are generally contradictory, and almost no steel sheet satisfies these characteristic values in good harmony.

That is, as the strength increases, the total elongation (hereinafter referred to as El) generally deteriorates. Furthermore, the zinc layer attached to the steel plate surface hinders the plastic deformation of the steel surface, making El even worse. Regarding the plating adhesion, it is also known that the higher the strength of the steel plate, the more the kinds and amounts of the elements added to the steel generally increase, which is harmful to the plating property.

(Prior art) Tensile strength (hereinafter referred to as TS) -El is a good high-strength steel sheet in which solid solution strengthening elements such as Mn and P are added as disclosed in JP-A-52-44720. However, when such solid solution strengthened high-strength steel sheet is subjected to general plating treatment and plating alloying treatment, the solid solution C in the ferrite is not completely precipitated as a carbide, and as a matter of course, Strain aging and natural aging deteriorate.

For the purpose of improving the aging property and increasing the strength, there is a method of adding a carbide-forming element such as Ti as disclosed in JP-A-57-43974, but these precipitation-strengthened high-strength steel sheets are generally continuous zinc. When treated in a plating line (hereinafter abbreviated as CGL), it is reduced after weak oxidation in the so-called gas cooling process for cleaning the surface of the steel sheet.
Due to the presence of an easily oxidizable alloying element such as Ti, reduction cannot be sufficiently performed, resulting in insufficient reduction, which often causes non-plating and is not suitable for practical use.

In addition, originally, in galvanizing, molten zinc reacts with base iron to form an alloy layer first, and this alloy layer serves to bond the zinc layer and the base iron layer, but this alloy layer is hard and brittle. If it grows thicker, it will cause the peeling of the plating during processing.
In addition steel and the like, the growth rate of the alloy layer is high, the alloy layer is formed relatively thick even under normal operating conditions, and it is easy to cause powder flaking (hereinafter referred to as powdering) during processing.

(Problems to be solved by the invention) Hot dip galvanized layer applied to a steel sheet containing Si and Mn as basic components so as not to cause problems such as aging and non-plating and powdering. By combining the alloying conditions with the post-alloying cooling conditions for
HS. With 35 kgf / mm ² or more, less strain aging and natural aging, and good plating adhesion. It is an object of this invention to provide a method for manufacturing GA steel sheet.

(Means for Solving Problems) The above objectives are satisfied by the following matters.

C: 0.02-0.03wt% Si: 0.50wt% or less, Mn: 0.10-2.0wt%, hot-dip galvanizing a steel sheet with a composition consisting of residual iron and unavoidable impurities, then 650-850 ℃ And keep it at that temperature for 1 second or longer, and then cool it to a temperature below 500 ℃ and above 300 ℃ at a cooling rate of 20 ℃ / s or more, then 30
A method for producing a high-strength hot-dip galvanized steel sheet characterized by the combination of holding at a temperature of more than 0 ° C and 500 ° C or less for 5 seconds or more or coiling at that temperature (first invention) ).

C: 0.02-0.03wt% Si: 0.50wt% or less, Mn: 0.10-2.0wt%, hot-dip galvanizing a steel sheet with a composition consisting of residual iron and unavoidable impurities, then 650-850 ℃ Heat it for 1 second and keep it at that temperature for more than 1 second.
Cooling rate below 20 ° C / s until reaching a temperature below 600 ° C, then cooling to below 500 ° C at a cooling rate above 20 ° C / s, and then holding at a temperature above 200 ° C for 5 seconds or longer. A method for producing a high-strength hot-dip galvanized steel sheet (second invention), which is characterized by the combination of carrying out or coiling at the temperature.

In any case, it is compatible with both hot-rolled steel sheets and cold-rolled steel sheets, and these steel sheets are immersed in a hot-dip zinc bath for hot-dip galvanizing, which is then alloyed at high temperature as described above. Next, it is essential to apply cooling control.

In the present invention, heating for recrystallization before plating, which is generally performed when galvanizing a cold-rolled steel sheet, is not required, and the cost reduction by omitting this heat treatment is also worth noting.

Generally, hot-dip galvanizing is performed by holding a steel sheet immersed in a zinc bath at a temperature in the range of 450 to 550 ° C for 1 second or longer and then pulling it out of the bath. The temperature is maintained at that temperature for 1 second or longer to carry out the alloying treatment.

On the other hand, the normal alloying treatment temperature is usually 550 ℃ ~ 600
It is only at ℃ and the plating layer formed by this is a δ ₁ phase with an iron concentration of around 12 wt%. This δ ₁ phase is excellent in terms of coating corrosion resistance, coating adhesion and spot weldability, but strong bending and pressing cause powdering, and not only the characteristics of bending angle are not utilized, but
It causes defects such as stars even during press working.

To reduce powdering without impairing coating corrosion resistance, coating adhesion, and spot weldability, Japanese Patent Application No. 58-073498 is used.
As disclosed in Japanese Patent Application No. 60-01737 and Japanese Patent Application No. 60-01737, it is effective to set the iron concentration of the plating layer to 15 to 35 wt%. For this purpose, the alloying temperature is raised to 650 to 780 ° C. There is a need.

Detailed studies by the inventors have revealed that the characteristics of the plating layer do not deteriorate even if the alloying treatment is performed at a temperature higher than 780 ° C.

Therefore, after high-temperature alloying treatment of heating and holding at a temperature in the range of 650 to 850 ℃, 500 ℃ or less and 300 ℃
At a cooling rate of 20 ° C / s or more to a temperature exceeding 300 ° C, and then holding or coiling at a temperature below 300 ° C for 5 seconds or more at a temperature below the cooling reaching temperature, or lowering 600 ° C during the cooling process. It is cooled at a cooling rate of 20 ° C / s or more to 500 ° C or less through a process of a lower cooling rate up to a certain temperature, and then held or coiled for 5 seconds or more at a temperature of 200 ° C or more below the ultimate cooling temperature. To do,
Especially important.

(Operation) HS. In order to obtain a GA steel sheet at a cooling rate that can be implemented by an actual CGL at low cost, at least C: 0.02 wt% or more and Mn: 0.10 wt% or more are required. In addition, in order to improve the workability at low cost, it is effective to add Si at 0.5 wt% or less. However, if any one of the limits of C: 0.30 wt%, Mn: 2.0 wt% and Si: 0.50 wt% is exceeded, workability, spot weldability and coating adhesion deteriorate.

The above is the reason for limiting the component range of the starting material of the present invention.

Next, the hot dip galvanizing operation is not particularly limited as already mentioned, but the lower limit of the subsequent alloying treatment temperature was set to 650 ° C because the iron concentration in the plating layer was 15 to 35 wt% as described above.
For the purpose of, and at the time of heating for alloying treatment, the solid solution C present in the steel is supersaturated in the ferrite by the subsequent cooling process, and is maintained at a temperature of 300 ° C. or less, which is below the temperature reached by cooling. Alternatively, it is necessary in order to reduce the natural aging by precipitating all the solid solution C as a carbide during coiling. The upper limit of alloying temperature is 850 ℃
The reason is that if the temperature exceeds 850 ℃, the Fe concentration in the plating phase will be 35%.
This is because the coating corrosion resistance deteriorates.

Here, the results of hot dip galvanizing and alloying treatment experiments conducted in the laboratory using the hot rolled steel sheets having the compositions shown in Table 1 will be touched upon.

The experimental heat cycle is as follows.

Heat the test steel plate to 480 ℃ at a heating rate of 10 ℃ / s,
After hot dip galvanizing while holding 5S at 10 ℃, 10 ℃
Is the temperature increased to the alloying temperature (T ₁ ℃) at a heating rate of / s, held at this temperature for 20 seconds for alloying, and then cooled to the cooling end temperature T ₃ ℃ at a cooling rate of v ℃ / s? Alternatively, cooling is performed at the cooling rate υ ₁ ℃ / s to the first stage cooling end temperature (T ₂ ℃), and further to the second stage cooling end temperature (T ₃ ℃) Cooling rate υ ₂ ℃ /
It was cooled at s, then kept at the cooling end temperature T ₃ for t seconds and then air cooled (see FIG. 1).

Here, the tensile properties were checked by variously changing T ₁ , T ₂ , T ₃ , υ, υ ₁ , υ ₂ , and t.

The results are shown in Tables 2 (1) to (3) (first-stage cooling) and Table 3 (second-stage cooling).

In the cooling process in Table 2, the alloying temperature (T ₁ ° C) is cooled at a constant rate from the above cooling end temperature (T ₃ ° C), and this cooling rate is shown in υ ° C / s. There is no temperature T ₂ .

In addition to TS and El as material test values, AI (aging, index) was adopted as a factor representing natural aging. The AI value is So (kgf / mm ² ), which is the strength when 7.5% strain is applied in the tensile test, and 30% at 100 ℃ after applying 7.5% strain.
After heating for a minute, the tensile test is conducted again, and if the descending yield point at that time is S (kgf / mm ² ), it is given by the following formula (1) AI = SS ₀ (kgf / mm ² )… (1) . Regarding the AI value, the dislocations pinned to solid solution C become free dislocations that deviate from solid solution C due to tensile stress, but when heated, they become pinned to solid solution C again, leading to an increase in the yield point. Arises from that. Generally, if AI ≦ 3 kgf / mm ² is satisfied, it is considered that defects such as stretcher strain will not occur during processing in practical use.

From Tables 2 and 3, it can be seen that the steels having TS of 35 kgf / mm ² or more are sample Nos. 2 to 4. In addition, AI ≤
For steels satisfying 3kgf / mm ² , from Table 2, T ₁ > 650 ℃, υ ≧ 20 ℃
/ s, T ₃ ≤500 ° C and t ≥5S, or from Table 3 T ₁ > 65
0 ℃, υ ₁ ≤20 ℃ / s, T ₂ ≧ 600 ℃, υ〉 20 ℃ / s, T ₃ ≦ 500
It can be seen that at tC and t≥5S. This phenomenon generally increases as the temperature increases, including the amount of solute C in austenite, but by cooling to below 500 ° C,
This means that a temperature of 650 ° C. or higher is required before cooling in order to allow the supersaturated solid solution C to sufficiently exist in the ferrite.

After that, by holding at a cooling ultimate temperature of 500 ° C. or less, precipitation of cementite occurs rapidly, and finally the amount of dissolved C is drastically reduced. When T ₁ is lower than 650 ° C, the degree of supersaturation of solid solution carbon after cooling is low, so the driving force to precipitate as cementite is small and it does not precipitate completely in the end, but a large amount of solid solution C remains and AI rises. Cause.

When comparing Table 2 and Table 3, Table 3 is a two-stage cooling process.
It can be seen that the steel of has a lower AI.

This is because solid solution C in ferrite diffuses into austenite during the cooling process of 20 ° C / s or less in the first stage of the two-stage cooling process, and almost completely precipitates as cementite at the grain boundaries of the ferrite after the second stage cooling. This is because.

On the other hand, in the case of performing cooling in one step, it is advantageous to shift the reached cooling temperature to the high temperature side in order to more strictly control the precipitation of carbides. It was decided to control cooling in the temperature range below 300 ° C.

Based on the above results, the cooling process after the alloying process should be controlled as follows.

1. After alloying, at a cooling rate of 20 ° C / s and below 500 ° C and
Cool down to a temperature over 300 ° C, below this temperature
Hold at a temperature above 300 ° C for 5 seconds or longer.

2. After alloying, cool at a cooling rate of 20 ° C / s to a temperature below 600 ° C, and then cool at a cooling rate of more than 20 ° C / s.
Cool to below 00 ° C and maintain at a temperature below this ultimate temperature of 200 ° C.

Even if coiling is performed instead of holding for 5 seconds or more after cooling to 500 ° C or less, the same behavior as described above is brought about. Therefore, this coiling is equivalent to holding as aging treatment after cooling control. is there.

When the aging temperature is less than 200 ° C, AI is high because the solid solution C in the α phase, which is to be precipitated as cementite, is insufficiently diffused, and the maximum temperature is the ultimate cooling temperature. If the temperature is higher than 500 ° C, the amount of equilibrium C dissolved in the α phase is high and the amount of dissolved C cannot be lowered sufficiently, so that AI deteriorates.

(Example) Table 4 shows the chemical composition of the steel slab used in the examples.

The steel slab was melted in a converter, continuously cast, and hot-rolled to a plate thickness of 3.0 mm.

Furthermore, a part of this hot-rolled sheet was cold-rolled to a sheet thickness of 0.95 mm.
These hot-rolled sheets and cold-rolled sheets were plated and alloyed by various CGL heat cycles, and the tensile properties and plating adhesion were investigated.

The heat cycle used in the examples is as follows.

That is, in CGL up to 480 ℃ plating bath temperature 15 ℃
After heating at / s, hot-dip galvanizing by holding for 5 seconds, then raising the temperature to various alloying temperatures, and holding at that temperature for 10 seconds
It was cooled to 350 ° C at a cooling rate of 30 ° C / s and immediately coiled.

Figure 2 shows the AI of these steels. The invention steels corresponding to the white circles and white squares in the figure satisfy AI ≦ 3.0 kgf / mm ² .

Next, Table 6 shows the plating adhesion and powdering resistance test results.

Here, the judgment of plating adhesion is half steel ball 12.7 mmφ, weight 12.2
Drop a weight of 500 kg from a height of 500 mm and die (21 mmφ).
90% or more of the peeling condition of the side surface, ○ indicates no peeling, 50%
The peeling was judged as Δ and the other cases were judged as ×.

In addition, as a method for evaluating the powdering resistance, the test surface was bent at 90 °, the cellophane tape was adhered to this, and the amount of plating release powder adhering to the tape was compared and evaluated with a limit sample prepared according to the following criteria. .

5. No release powder adhered 4. Amount of release powder adhering 3. Amount of release powder adhering 2. Amount of release powder adhering 1. Excessive amount of release powder adhering From Table 6, both plating adhesion and powdering resistance are alloys. It can be seen that it is extremely inferior when no chemical treatment is applied or when the alloying temperature is low.

(Effect of the Invention) According to the present invention, TS is 35 kgf / mm ² or more, AI ≦ 3.0 kgf / m
It is possible to stably manufacture HS and GA steel sheets that secure m ² and have good plating adhesion and powdering resistance.

[Brief description of drawings]

 FIG. 1 is a heat cycle diagram, and FIG. 2 is an AI-T correlation graph of the example.

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Shinobu Okano 1 Kawasaki-cho, Chiba-shi, Chiba Kawasaki Steel Co., Ltd. Technical Research Division (56) References JP-A-55-122820 (JP, A)

Claims (2)

[Claims] 1. A hot-dip galvanized steel sheet containing C: 0.02-0.03 wt% Si: 0.50 wt% or less and Mn: 0.10-2.0 wt% and consisting of residual iron and unavoidable impurities. Heat to a temperature of ~ 850 ℃, hold at that temperature for 1 second or longer, and continue to 20 ℃ / s below 500 ℃ and over 300 ℃.
High tension alloying melting characterized by the combination of cooling at the above cooling rate, and then holding at a temperature of more than 300 ° C and 500 ° C or less for 5 seconds or more or coiling at that temperature. Manufacturing method of galvanized steel sheet. 2. A steel sheet containing C: 0.02 to 0.03 wt% Si: 0.50 wt% or less and Mn: 0.10 to 2.0 wt% and composed of residual iron and unavoidable impurities, after hot dip galvanizing. Heat to a temperature of ~ 850 ℃ and hold at that temperature for 1 second or longer. Continue to cool at a rate of 20 ℃ / s or less until the temperature does not drop below 600 ℃, then 20 ℃ / s to 500 ℃ or less. Manufacture of a high-strength hot-dip galvanized steel sheet characterized by a combination of cooling at a cooling rate exceeding 200 ° C., then holding at a temperature of 200 ° C. or more for 5 seconds or more, or performing coiling at that temperature. Method.