JPS627842A - High tension alloyed hot dip galvanized steel sheet having satisfactory ductility and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a high tension alloyed hot dip galvanized steel sheet having satisfactory ductility by specifying the concn. of iron in a zinc alloy layer formed on the surface of a steel sheet contg. specified amounts of C, Mn, S and Al by hot dip galvanization and alloying. CONSTITUTION:The composition of a hot rolled or cold rolled steel sheet is composed of, by weight, 0.02-0.3% C, 0.1-2% Mn, <0.02% S, <0.1% Al and balance Fe. The steel sheet is hot dip galvanized, heated to the Ac1 transformation point -850 deg.C, held at the temp. for >=1sec and cooled to at least 450 deg.C at >=10 deg.C/sec cooling rate. Thus, the concn. of iron in a zinc alloy layer formed on the surface of the steel sheet by the hot dip galvanization and alloying is regulated to 15-35wt.%.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides a high tensile strength alloyed galvanized steel sheet having a tensile strength of 40 kgf/- or more and excellent workability, especially total elongation, and a method for producing the same. It is related to.

In recent years, high-tensile steel plates have come to be widely used as outer panels for automobiles from the viewpoint of safety, reduction in vehicle weight, and reduction in the amount of materials used.

Naturally, such high-strength steel plates are often used with thinner walls than ordinary steel due to their intended use, but because of this, they are in a more serious situation when it comes to corrosion than ordinary steel. Q (Conventional technology) As a method of imparting corrosion resistance to steel sheets, Ou
There are known methods of adding elements such as hQr to enhance the corrosion resistance of steel, and methods of applying metal plating to the surface of steel sheets, but the former cannot be expected to be effective in severe corrosive environments such as salt damage.

In this regard, hot-dip galvanizing, which has excellent corrosion resistance and can be applied thickly among other metal platings, is effective against such severe corrosion attacks. Considering adhesion and spot weldability, it is more preferable to further perform alloying treatment.

However, the strength of a steel plate and its workability and adhesion are considered to be contradictory to each other, and so far, there has been no steel plate that satisfies each of these characteristic values in a harmonious manner. That is, when the strength is increased, the total elongation (C or less, referred to as El) generally deteriorates. In addition, the zinc layer adhering to the surface of the steel plate inhibits plastic deformation of the steel surface, so that the deterioration of El further progresses. Regarding plating adhesion, generally speaking, the higher the tensile strength of the steel sheet, the higher the number and amount of elements added to the steel, but these added elements are known to be harmful to plating adhesion.

High tensile strength steel sheets with good tensile strength (hereinafter abbreviated as TS) in both l and el include ferrite-martensite two-phase,
A so-called composite structure high-strength steel sheet consisting of two phases of ferrite-bainite and three phases of ferrite-bainite-martensite is promising, but such a composite structure aW4 can be made of materials other than ferrite even by the extremely common plating process. There was a problem in that the structure was hardened, resulting in a significant decrease in TS.

As a solution to this problem, Japanese Patent Application Laid-Open No. 56-513927
In the publication, T S
A method for improving the -El balance has been proposed.

(Problems to be Solved by the Invention) However, with the above method, it is impossible to perform alloying treatment after hot-dip galvanizing from the viewpoint of maintaining material quality, and it is also difficult to improve corrosion resistance, paint film adhesion, and spot welding after painting. Problems also remained in the inevitable decline in sexuality.

This invention advantageously solves the above problems, and T
The purpose of the present invention is to propose a high tensile strength alloyed hot-dip galvanized steel sheet that has a S 40 kgf/u2 or higher, has a high El, and has excellent plating adhesion, together with an advantageous manufacturing method thereof.

Now, in order to solve the above problem, the inventors conducted a thorough study on the chemical composition of the steel material and each treatment process. Mn-added steel or c, p.

It has been found that the intended purpose can be advantageously achieved by using Mn-added steel as a basic component and by modifying the alloying conditions and subsequent cooling conditions of the steel.

This invention is based on the above knowledge.

That is, in this invention, C: 0.02~o, so w
t% (hereinafter simply indicated as a curse), Mn: o, 10-2.
0%, S: 0.020% or less and A4: 0.
1 or less than 10%, and sometimes additionally P: 0.20
% or less and 3i: Contains one or two of 0.80% or less, the remainder is substantially Fe, and the iron-a degree in the surface alloyed galvanized Id is 15
This is a high tensile strength alloyed hot-dip galvanized steel sheet with good ductility, characterized by a ductility of ~35%.

In addition, this invention provides 0:0.02 to 0.30%, Mn
: 0.10 to 2.0%, S'O-020% or less and Al: 0.10% or less, and sometimes further P
: 0.20% or less and Si: 0.30% or less, hot-rolled or cold-rolled hot rice containing either one or both of them, and the remainder substantially having a composition of Fe, is subjected to hot-dip galvanizing, Then A. , after heating to a temperature above the transformation point and below 850°C, maintaining the temperature range for 1 second or more, and then cooling to at least 450°C at a cooling rate of 10°C/8 or more. This is a method for manufacturing high-tensile alloyed hot-dip galvanized steel sheets.

This invention will be explained in detail below.

In this invention, the reason why the component composition is limited to the above range is as follows.

0: 0.02-0.80% C is a useful element for obtaining strength, but 0.02%
If it is less than 0.80%, the effect of the addition is poor;
If the content exceeds 1%, the workability and spot weldability deteriorate, so the content was limited to 0.02% to 0.30%.

Mn: 0.10 to 2.0% Mn requires at least 0.10% in order to obtain strength, but if added in a large amount exceeding 2.0%, processability and This will cause deterioration of spot weldability.
It was limited to a range of 0.10 to 2.0%.

S: 0.020% or less S forms nonmetallic inclusions in steel and deteriorates workability, so it is desirable to reduce its content as much as possible.
It was limited to a range of 0.020% or less, preferably 0.010% or less.

1: 0.10% or less A/ effectively contributes not only to deoxidizing but also to improving plating adhesion, but the effect reaches saturation even if added in excess of 0.10%, so the content is The content was limited to 0.10% or less.

Although the basic components have been described above, in this invention, P and Si can be added in the following ranges for the purpose of further improving processability.

P: 0.20% or less, Si: 0.80% or less Both P and Si contribute effectively to improving workability, but when P exceeds 0.20%, spot weldability deteriorates. , and if Sl exceeds 0.80%, the adhesion will deteriorate significantly, so p<o, go%, S
It is necessary to add it in a range of 1≦0.80%.

Now, the steel plate adjusted to the above-mentioned preferred component composition is 450~
After hot-dip galvanizing for 1 second or more in a temperature range of 550°C, it is heated to a temperature above the Ac transformation point and below 850°C, and then alloyed in this high temperature range.

Here, the conventional general alloying temperature is 550-600℃
Because the area was close to
Although the δ□ phase is excellent in paint corrosion resistance, paint film adhesion, and spot weldability, it is often broken into powder during strong bending or press processing. Not only does this lead to flaking (hereinafter referred to as powdering), which impairs the properties mentioned above, but it also has the disadvantage of causing defects such as stars during press working.

In this respect, according to the present invention, by alloying and processing at a relatively high temperature, an alloy layer with an iron concentration of 15 to 85% can be obtained, resulting in a marked improvement in powdering resistance.

The reason why the lower limit of the alloying treatment temperature is set as the AOX transformation point is closely related to the cooling after the alloying treatment.
This is to obtain a ferrite-martensite, ferrite-bainite, or ferrite-bainite-martensite composite structure by cooling. Also, the upper limit is 850
``C'' is because when this temperature is exceeded, Fe in the plating layer
This is because if the concentration exceeds 85%, the corrosion resistance of the coating deteriorates.

Hot-rolled steel sheets having the compositions shown in Table 1 were hot-dip galvanized and then alloyed in the following manner.

That is, after heating to 480℃ at a heating rate of 10''C/S and applying hot-dip galvanizing while holding at 480℃ for 5 seconds, the alloying temperature T□ (℃ ) and held at that temperature for 10 seconds to perform alloying treatment, followed by cooling rate: v”c7
's to a temperature of 72°C, and then air-cooled.

Here is T1. Table 2 shows the results of examining the tensile properties when V I T@ was changed to type 6.

Table 1 Table 2 AWIlljM T1vT2TS” o: Invention steel □2 〃780 70 250 194.8
48.0 Chemical composition A 700702
50 aa, 548.5□ 3 // 750 70 250 82
．． 9 50.1 □4 N of this invention
800 70 250 84.0 47.9
Out of range 15 # 85070250 aa, 6.
48.46B*70070250 aa, 944.87
#7aO*545088J 45.08y 7301
045040.9, 9019 〃
80 15 450 41.5
47.8 010 p 7803
045042.047.2 011 tt 78010
25041.847.9 012 # 7801040
041.747.0 013 II 73010500
a9.144.814 // 7808025042
．． 847.0 015 N 7505045045.0
45.4 015 tt 7508030045.94
4.9 017 # 8007025048.04a,
1 018 N 8505020051. ．． 942.0
01907107025050.942.2 020
// 7505025051.740.8 021
/' 8008040055.540.1 022 p
8508025059.839.2 023 D 7
80502505t1. Oa9.8 024 tp
7507020058.989.4 025 ty 8
0050 aoo 61.7 s6.5026 z 8
5030400 g3.0 a7. o O27E 7
807045080.885.1 028 tt 75
05080064.084.5 09Q #
Qlln Qn Rini/'I
IIo 0 ri0 ha to,
For steel type A whose component composition does not satisfy the appropriate range of this invention, how should the subsequent processing conditions be adjusted (AX~5
), a high TS of 401a;tf/1lII+2 or higher could not be obtained.

Moreover, even if the component composition is suitable, the alloying treatment temperature T0 is low.
&6), when the subsequent cooling rate V was slow (black 7) and when the cooling end temperature T8 was high [black 18], a good TS of 40 kgf/ws' or higher was still not obtained.

Also 4 B, 8, 9, 10, 15 and 1?
Ni.

When the structure was observed with an optical microscope, it was found that black 6 is a ferrite-pearlite structure, ya 8 is a ferrite-bainite structure, &9, IO, 15, and 17 are all ferrite-bainite-martensite structures. confirmed.

From the above experimental results, in this invention, the cooling rate after alloying treatment is set to 10" C/S or more, and the cooling end temperature is set to 450" C/S or more.
'c or below.

Note that cooling at a cooling rate of 10°C/S or more is advantageous in stabilizing the r phase with an a degree of around 20% at low temperatures, as well as suppressing the formation of the δ phase, which has poor powdering resistance. . In other words, the alloy layer at high temperature consists of F phase and Zn phase, but when the cooling rate is slow, the alloy layer consists of 630-64
0 ” δ□ phase is generated by peritectic reaction from around C,
On the other hand, at a cooling rate of 0° C./S or more, the above reaction is suppressed, and therefore the F phase is stabilized.

[Example] Molten steel having the composition shown in Table 8 was made into a slab by continuous casting, and then hot rolled into a hot rolled plate having a thickness of 3.0 mm. Further, a part of this hot-rolled sheet was cold-rolled to a thickness Q of 955 m.

These hot-rolled sheets and cold-rolled sheets were subjected to various hot-dip galvanizing and alloying treatments in the 3GL heat cycle as shown in Table 4. After heating at 15"C/s to a plating bath temperature of 480℃ at OGL, hot-dip galvanizing was performed by holding for 5 seconds, and then heated to an alloying treatment temperature T□. After alloying treatment, 50"C/s After cooling to 800°C at a cooling rate of 8, it was rolled up.
After heating to 80°C (at a heating rate of /s
After cooling at , it was wound up at 800''C.

FIG. 1 shows the results of examining the TS of the galvanized steel sheet thus obtained.

In a temperature range where the alloying temperature T□ exceeds 700° C., TS increases as T increases. Table 5 shows the test results for plating adhesion and powdering resistance.

The plating adhesion here is a semi-steel ball with a diameter of 12.7 m and a weight of 12
．． Evaluate by the peeling status of the die (-21wφ) side when a 2 kg weight is dropped from a height of 500111 m. If the degree of peeling is less than 10%, △ if it is less than 150%, and × if it is more than that. It was determined that

For powdering resistance, the test surface was bent at 90 degrees with the test surface facing inward, and cellophane tape was applied to this.

After application, the tape was peeled off and the amount of flaking powder adhering to the tape was evaluated according to the following 5-level criteria.

5...No flaking powder adhering 4...Minor amount of flaking powder adhering 8...Small amount of flaking powder adhering 2...Large amount of flaking powder adhering 1...Very large amount of flaking powder adhering* Q...・As is clear from the results shown in Compatible Example Table 5, when the alloying treatment temperature was less than the lower limit of this invention (manufacturing AN~3)
It was difficult to say that both the plating adhesion and the powdering resistance were good.

In addition, although the powder &8 manufactured according to the conventional method had good powdering resistance, it had poor plating adhesion because it was not subjected to alloying treatment.

On the other hand, samples &4 to 7 manufactured according to the present invention all had good plating adhesion and powdering resistance, and were also excellent in TS as shown in FIG. 1 above.

(Effects of the Invention) Thus, according to the present invention, it is possible to obtain an alloyed hot-dip galvanized steel sheet that maintains TS40 of 9f/2, has excellent elongation properties, and also has good plating adhesion and powdering resistance. I can do it.

Further, according to the method of the present invention, it is possible to omit recrystallization annealing before the matting treatment, which was conventionally considered indispensable, contributing to energy saving. .

[Brief explanation of drawings]

Figure 1 shows that one cycle of hot-dip galvanizing and alloying
It is a graph showing the influence on the TS of a steel plate.

Claims (1)

[Claims] 1. Contains C: 0.02 to 0.30 wt%, Mn: 0.10 to 2.0 wt%, S: 0.020 wt% or less, and Al: 0.10 wt% or less, with the remainder being substantially A high tensile strength alloyed hot-dip galvanized steel sheet with good ductility, characterized in that the composition is Fe and the iron concentration in the surface alloyed galvanized layer is 15 to 35 wt%. 2. Contains C: 0.02 to 0.30 wt% Mn: 0.10 to 2.0 wt% S: 0.020 wt% or less and Al: 0.10 wt% or less, and P: 0.20 wt% or less and Si : Contains one or two of the following: 0.30 wt% or less, the remainder is essentially Fe, and the iron concentration in the surface alloyed galvanized layer is 15 to 35 wt%. High tensile strength alloyed hot-dip galvanized steel sheet with good ductility. 3. Contains C: 0.02 to 0.80 wt%, Mn: 0.10 to 2.0 wt%, S: 0.020 wt% or less, and Al: 0.10 wt% or less, with the remainder being substantially Fe. Hot-rolled or cold-rolled steel sheet is hot-dip galvanized, then heated to a temperature above the A_C_1 transformation point and below 850°C, held in the temperature range for 1 second or more, and then cooled at 10°C/s or more. A method for producing a high-strength alloyed hot-dip galvanized steel sheet with good ductility, characterized in that it is cooled at a rate of at least 450°C. 4. Contains C: 0.02 to 0.30 wt% Mn: 0.10 to 2.0 wt% S: 0.020 wt% or less and Al: 0.10 wt% or less, and P: 0.20 wt% or less and Si : A hot-rolled or cold-rolled steel sheet containing one or two of 0.30 wt% or less, with the remainder being substantially Fe, is hot-dip galvanized and has a transformation point of A_C_1 or higher, 850 After heating to a temperature of 10°C or lower, maintaining the temperature range for 1 second or more, cooling at a cooling rate of 10°C/s or higher at least 45°C
A method for producing a high tensile strength alloyed hot-dip galvanized steel sheet with good ductility, the method comprising cooling to 0°C.