JPS624860A - Manufacture of high tension alloyed hot dip galvanized steel sheet - Google Patents

Abstract

PURPOSE:To manufacture a high tension alloyed hot dip galvanized steel sheet having superior corrosion resistance and workability by hot dip galvanizing a steel sheet contg. Si and Mn as essential components and by properly combining conditions during the alloying of the resulting Zn layer with conditions during cooling after the alloying. CONSTITUTION:The surface of a steel sheet having a composition contg. 0.02-0.30wt% C, <0.50wt% Si and 0.10-2.0wt% Mn is hot dip galvanized. The hot dip galvanized steel sheet is heated to 650-850 deg.C, held for >=1sec and cooled to >600 deg.C at <=20 deg.C/sec cooling rate or to <=500 deg.C at >=20 deg.C/sec cooling rate. The steel sheet is then held at >=200 deg.C for >=5sec or coiled at the temp. A high tension alloyed hot dip galvanized steel sheet having superior adhesion, corrosion resistance and workability is obtd.

Description

Detailed Description of the Invention (Industrial Application Field) High tensile strength alloyed hot-dip galvanized steel sheet (hereinafter referred to as H
Regarding the advantageous production of steel sheets (S, GA steel sheets), this specification attempts to disclose the results of research and development to advantageously improve the plating adhesion without strain aging, natural aging, or other disadvantages of powdering. It is.

In recent years, high-strength steel sheets have come into widespread use for the purpose of improving automobile safety, reducing vehicle weight, and reducing the amount of materials used. Because steel is often used in thinner sheets than normal steel, it is exposed to corrosion much more seriously than ordinary steel.

Therefore, it is desired that high-strength steel sheets with excellent corrosion resistance and workability be manufactured by mass production.

As a method of imparting corrosion resistance to steel sheets, for example, Cu,
There are two methods: adding elements such as Cr to improve the corrosion resistance of steel, and applying metal plating to the surface of the steel sheet.
The former is not very effective under severe corrosive conditions such as salt damage.

Therefore, hot-dip galvanizing is effective against such severe corrosion, as it has excellent corrosion resistance among metal platings and can be applied thickly.Moreover, it is effective in recent years due to its advanced post-painting corrosion resistance, coating adhesion, and spot weldability. in view of,
It is absolutely necessary to perform alloying heat treatment after hot-dip galvanizing.

However, the effects of factors on the characteristic values of strength, workability, and plating adhesion are generally contradictory, and there has been almost no steel sheet that satisfies these characteristic values in a harmonious manner.

That is, as the strength increases, the total elongation (hereinafter referred to as El) generally deteriorates. Furthermore, since the zinc layer attached to the surface of the steel sheet inhibits plastic deformation of the steel surface, all is further deteriorated. Regarding plating adhesion, it is also known that the higher the tensile strength of the steel sheet, the more types and amounts of elements added to the steel, which are harmful to the plating property.

(Prior art) As a high tensile strength steel sheet with good tensile strength (hereinafter referred to as TS) and El, Mn,
There are high-strength steel sheets with solid solution strengthening elements such as P added,
When such solid solution strengthened high tensile strength steel sheets are subjected to general plating treatment and plating alloying treatment, the solid solution C in the ferrite does not completely precipitate as carbides, and naturally the strain aging properties and Natural aging deteriorates.

In order to improve aging properties and increase strength, there is a method of adding carbide-forming elements such as Ti (as described in Japanese Patent Application Laid-Open No. 57-43974), but these precipitation-strengthened high-strength steel sheets are generally Hot dip galvanizing line (hereinafter referred to as CGL)
(abbreviated as ), it is reduced after weak oxidation in a so-called gas cooling process aimed at cleaning the surface of the steel sheet, but at this time, due to the presence of alloying elements that are easily oxidized, such as Ti, the reduction is not sufficiently performed. Since this cannot be carried out, the reduction is insufficient, and as a result, unplated areas often occur, making it unsuitable for practical use.

In addition, originally in galvanizing, molten zinc and base steel react to form an alloy layer, and this alloy layer serves to bond the zinc layer and base steel layer, but this alloy layer is hard and brittle. If the Ti grows thickly, it will cause the plating to peel off during processing, so it is desired that the Ti be as thin as possible.
With additive steel, etc., the growth rate of the alloy layer is fast, and even under normal operating conditions, a relatively thick alloy layer is formed, making it easy to cause powdering and flaking (hereinafter referred to as powdering) during processing.

(Problems to be Solved by the Invention) A hot-dip galvanized layer is applied to a steel sheet containing Si and Mn as its basic components in order to avoid problems such as aging, non-coating, and even powdering. By combining the alloying conditions and post-alloying cooling conditions for HS, which has less strain aging and natural aging at TS 35Kgf/mm" or more, and has good plating adhesion,
It is an object of this invention to provide a method for manufacturing GA steel sheets.

(Means for solving problems) The above objectives are met by:

C: O, (12~Q, 3Q wt% si: 0.50-t% or less, After galvanizing, heat to a temperature of 650 to 850°C, hold at that temperature for 1 second or more, then cool at a cooling rate of 20°C/s to 500°C or less, then raise the temperature to 200°C or more. A method for producing a high tensile strength alloy hot-dip galvanized steel sheet, characterized by holding at a temperature for 5 seconds or more or coiling at said temperature (
1st invention).

c: o, o2 ~ 0.30 wt% Si: 0.50 wt% or less, Mn: 0.10 ~ 2.0 wt%, after hot-dip galvanizing a steel sheet with a composition consisting of residual iron and unavoidable impurities. , heating to a temperature of 650 to 850°C and holding at that temperature for at least 1 second, followed by a cooling rate of less than 20°C/s until the temperature reaches no less than 600°C, then 20°C/s until below 500°C. A high-strength alloyed hot-dip galvanized steel sheet characterized by a combination of cooling at a cooling rate of 200°C or more, and then holding at a temperature of 200°C or more for 5 seconds or more, or coiling at that temperature. Manufacturing method (second invention).

In either case, it is suitable for both hot-rolled steel sheets and cold-rolled steel sheets, and these steel sheets are immersed in a hot-dip zinc bath to be hot-dip galvanized, and then alloyed at high temperatures as described above. The key is then to perform cooling control.

In addition, in the present invention, there is no need for heating for recrystallization before plating, which is generally performed when galvanizing cold-rolled steel sheets, and the cost reduction due to the omission of this heat treatment is also worth noting.

Generally, to apply hot-dip galvanizing, a steel plate is immersed in a zinc bath and held at a temperature in the range of 450 to 550°C for 1 second or more, and then pulled out of the bath.
Heat to a temperature of 50°C or higher and 850°C or lower, and 1 at that temperature.
Hold for more than a second to perform alloying treatment.

On the other hand, the normal alloying treatment temperature is usually 550 to 60
The temperature is only 0° C., and the plating layer formed thereby is a δ phase with an iron concentration of about 12 wt%. This δ1 phase is excellent in terms of paint corrosion resistance, paint film adhesion, and spot weldability, but if it is subjected to strong bending or press processing, it will cause powdering, which will not only prevent its properties from being utilized, but also cause problems during press processing. It also causes defects such as star spots.

In order to reduce powdering without impairing paint corrosion resistance, paint film adhesion, and spot weldability, the patent application No. 58-07
As disclosed in Japanese Patent Application No. 3498 and Japanese Patent Application No. 60-01737, it is effective to set the iron concentration in the plating layer to 15 to 35 wt%.
It is necessary to raise the temperature to 780°C.

According to detailed research conducted by the inventors, it has become clear that the properties of the plating layer do not deteriorate even if the alloying treatment is performed at a higher temperature than 780°C.

Therefore, after going through a high-temperature alloying process that involves heating and supplying to a temperature in the range of 650°C to 850°C, it should be cooled to below 500°C at a cooling rate of 20°C/s or more, or the temperature should be lowered to 600°C during the process. Cooling is performed including a process of lower cooling rate until the temperature reaches a point where the material does not deteriorate, and then aging treatment is performed by holding or coiling at a temperature lower than the cooling control temperature but 200°C or higher for 5 seconds or more. is particularly important.

(Function) In order to obtain the HS and GA steel sheets according to the present invention at low cost at a cooling rate that can be implemented in actual CGL, at least C:
0.02wt% or more Mn: 0.10wt%
The above is necessary. In addition, in order to reduce the cost and improve workability, it is effective to add Si at 0.5wt1 or less. However, C: 0.30 wt% and Mn: 2
．． 0wt% and Si: Exceeding the limits of 0.50wtχ causes deterioration in workability, spot weldability, and coating film adhesion.

The above is the reason for limiting the range of ingredients for the starting materials of this invention.

Next, as mentioned above, there are no particular restrictions on the operation of hot-dip galvanizing, but the lower limit of the subsequent alloying treatment temperature is 650°C.
As mentioned above, the iron concentration of the plating layer is 15 to 3.
For the purpose of making it 5 wt% and during heating for alloying treatment,
Through the subsequent cooling process, the solid solution C present in the steel is made to exist in the ferrite in a supersaturated state, and the temperature reaches 2 below the cooling temperature.
This is necessary to reduce natural aging by maintaining the temperature at 00° C. or higher or by precipitating all of the solid solution C as carbide during coiling. In addition, the upper temperature limit for alloying treatment was set at 85%.
The reason for setting the temperature to 0°C is that if the temperature exceeds 850°C, the Fe concentration in the plating phase will exceed 35%, and the corrosion resistance of the coating will deteriorate.

Here, we will discuss the results of hot-dip galvanizing and alloying treatment experiments conducted in a laboratory using hot-rolled steel sheets having the compositions shown in Table 1.

Table I 11℃% The experimental heat cycle was as follows.

The temperature of the test steel plate was raised to 480°C at a heating rate of 10°C,
After hot-dip galvanizing at 480°C while holding for 5S, the temperature was raised to the alloying temperature (T,°C) at a heating rate of 10°C/s, and alloying treatment was performed by holding at this temperature for 20 seconds. , and then cooled at a cooling rate of V'C15 to the cooling end temperature of 13°C, or cooled at a cooling rate of υ,"C/s to the cooling end temperature of the first stage (7g°C), and then cooled to the cooling end temperature of the second stage (7 g°C). 73° C.) at a cooling rate of 02° C./s, and then held at the cooling end temperature T3 for t seconds, followed by air cooling (see FIG. 1).

Here, the tensile properties were checked by varying T++Tz43+υ+'l+'2+t.

The results are shown in Tables 2 (1) to (31 (1 stage cooling) 1 Table 3 (2
(stage cooling).

The cooling process in Table 2 is cooling at a constant rate from the alloying temperature (Tl'C) to the above-mentioned cooling end temperature ('ri'c), and this cooling rate is expressed as υ'C/s. Therefore, the above-mentioned first stage There is no cooling end temperature T2.

In addition to TS, E, and J as material test values, AI (aging, index) is a factor representing natural aging.
It was adopted. ^, ■The value is the strength when a strain of 7.5χ is applied in a tensile test, So (Kgf/mm"), 7
．． After applying a strain of 5χ and heating at 100°C for 30 minutes,
Perform the tensile test again, and the descending point at that time is S (kgf/
mm”), then it is given by the following (11 formula % formula % (1). ^, ■ The value is that dislocations pinned to solid solution C detach from solid solution C due to tensile stress and become free dislocations. This occurs because heating causes pinning to the solid solution C again, leading to an increase in the yield point.Generally, if A,I≦3Kgf/mmz is satisfied, stretcher strain is reduced during practical processing. It is assumed that such defects will not occur.

From Tables 2 and 3, it can be seen that steels with a TS of 35kgf/mm" or more are sample fish 2 to 4. Also, from Table 2, steels that satisfy the above A and I≦3Kgf/mmz have a TI>650 ℃, υ≧20℃/s+rz≦500℃ and t≧53, or from Table 3, T, >650℃, υ
1≦20°C/s.

It can be seen that Tt≧600°C, υ2≧20°C, 8+73≦ to 00°C, and t≧53. This phenomenon generally includes the amount of solid solute C in austenite, and the amount of solid solute C increases as the temperature increases. This means that a temperature of 650° C. or higher is required before cooling.

By subsequently maintaining the cooling temperature below 500℃,
Precipitation of cementite occurs rapidly, and finally the amount of solid solute C is drastically reduced. When T is lower than 65θ℃, the degree of supersaturation of solid solute carbon after cooling is low, so the driving force for precipitation as cementite is small and ultimately it does not precipitate completely, and a large amount of solid solute C remains A, It causes an increase in I.

When comparing Tables 2 and 3, Table 3 shows a two-stage cooling process.
It can be seen that the steel has lower A and I values.

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 ferrite after the second stage cooling process. It's for a reason.

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

1. After alloying treatment, cooling rate of 20℃/s or more is 500℃.
℃ or below, but lower than this cooling temperature, 200℃
Hold at a temperature of ℃ or higher for 5 seconds or more.

2. After alloying treatment, cooling rate of less than 20°C/s is 600°C.
It is cooled to a temperature of no less than 200° C., further cooled to 500° C. or less at a cooling rate of 20° C./s or more, and maintained at a temperature of 200° C. or more, which is lower than the reached cooling temperature.

Note that 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 will result, so this coiling is equivalent to holding as aging treatment after cooling control. be.

When the temperature of this aging treatment is lower than 200°C, A and I become high due to insufficient diffusion of solid solution C in the α phase that is intended to precipitate as cementite, and the temperature reached by cooling is the highest temperature. If the temperature is higher than 500°C, the equilibrium amount of C dissolved in solid solution in the α phase is high and the solid solute C cannot be lowered sufficiently.
I deteriorates.

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

Table 4 wt% Steel slabs were melted in a converter, and finished to a thickness of 3.0 mm through continuous casting and hot rolling.

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

The heat cycle used in the examples is as follows.

That is, after heating at 15°C/s to a plating bath temperature of 480°C in CGL, hot-dip galvanizing was performed by holding for 5 seconds, the temperature was raised to various alloying temperatures, and 1
After holding for 0 seconds, it was cooled to 350°C at a cooling rate of 30°C/s and immediately coiled.

Figure 2 shows A, 1 of these steels. The invention steel corresponding to the white circle and white angle of view in the figure satisfies A, ■≦3.0Kgf/mm2.

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

Here, the plating adhesion is judged by dropping a half-cooked weight of 12.7 mmφ and weighing 12.2 kg from a height of 500 mm, and marking 90% or more of peeling on the side of the die (21 mmφ) as ○. No peeling of 50% or more was judged as △, and other cases were judged as ×.

In addition, as a method for evaluating powdering resistance, the test surface was bent 90'' with the test surface inside, cellophane tape was applied and peeled off, and the amount of plating flaking powder adhering to the tape was evaluated by comparing it with a limit sample prepared according to the following criteria. .

5. No flaking powder attached 4. Very small amount of flaking powder attached 3. Small amount of flaking powder attached 2. Large amount of flaking powder attached 1. Extremely large amount of flaking powder attached. From Table 6, the alloy has both plating adhesion and powdering resistance. It can be seen that the alloying properties are significantly inferior when no alloying treatment is performed or when the alloying temperature is low.

(Effect of the invention) According to this invention, the TS is 35Kgf/mm” or more, A,
It is possible to stably produce HS and GA steel sheets that ensure I≦3.0 Kgf/mm'' and have good plating adhesion and powdering resistance.

[Brief explanation of drawings]

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

Claims (1)

[Claims] 1. A steel plate having a composition containing C: 0.02 to 0.30 wt%, Si: 0.50 wt% or less, Mn: 0.10 to 2.0 wt%, and consisting of residual iron and inevitable impurities. , After hot-dip galvanizing, heat to a temperature of 650 to 850°C and hold at that temperature for 1 second or more, then cool to 500°C or less at a cooling rate of 20°C/s or more, then 200°C A method for producing a high tensile strength alloyed hot-dip galvanized steel sheet, characterized by holding the above temperature for 5 seconds or more or coiling at the above temperature. 2. Hot-dip galvanizing is applied to a steel sheet with a composition containing C: 0.02 to 0.30 wt%, Si: 0.50 wt% or less, Mn: 0.10 to 2.0 wt%, residual iron and unavoidable impurities. After that, heat it to a temperature of 650 to 850℃ and hold it at that temperature for more than 1 second, then continue to heat it at 20℃/20℃ until the temperature reaches no less than 600℃.
Cooling rate below 500°C, then 20°C/s to below 500°C
A high-strength alloyed hot-dip galvanized steel sheet characterized by a combination of cooling at a cooling rate of 200°C or more, and then holding at a temperature of 200°C or more for 5 seconds or more, or coiling at that temperature. Production method.