JPS5937333B2 - Manufacturing method of alloyed hot-dip galvanized steel sheet - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION This invention has good formability and spot weldability,
Furthermore, the present invention relates to a method for producing an alloyed hot-dip galvanized steel sheet that has slow aging properties at room temperature and bake hardenability.

In general, steel sheets for automobiles, for example, not only have good press formability that does not cause wrinkles or cracks during press working, but also conform well to the press die and prevent springback from occurring when the molded product is removed from the press die. In other words, excellent formability is also required.

Regarding press formability and shapeability (hereinafter referred to as formability), it is considered that a material having a high Lanford value (hereinafter referred to as r value) and a low yield strength is good. Therefore, automotive steel sheets have the characteristics of high r-value and low yield strength before press forming, and on the other hand, after press forming and baking painting, they have the property of having high yield strength (baking). curability) is required. On the other hand, in recent years, a large amount of galvanized steel sheets has been used in the manufacture of automobile bodies in order to improve the rust resistance of automobiles.

Generally, galvanized steel sheets are manufactured by electrogalvanizing or hot-dip galvanizing, but the productivity is higher with hot-dip galvanizing, which allows annealing and plating to be carried out on the same line. Hot-dip galvanized steel sheets tend to be used in larger quantities because they are extremely good, and also from the viewpoint of manufacturing costs and corrosion resistance.

However, the above-mentioned hot-dip galvanizing method has a problem in that the forming processability and spot weldability of the steel sheet deteriorate because the steel sheet is rapidly heated and cooled.

Therefore, an Fe-Zn alloying furnace is installed on the galvanizing bath outlet side of the continuous hot-dip galvanizing line, and the galvanized steel sheet is reheated to a temperature within the temperature range of 520 to 620 °C.
Spot weldability has been improved by changing the Zn layer on the surface to a Fe-Zn alloy layer, and it is true that the alloyed hot-dip galvanized steel sheet obtained as a result of this has significantly improved spot weldability. However, due to the rapid cooling after alloying treatment, the formability and room temperature aging properties are equal to or inferior to normal hot-dip galvanized steel sheets. This was remarkable compared to steel sheets, and its properties deteriorated significantly within one month after temper rolling. Therefore, in order to change the normal temperature aging property of the above-mentioned alloyed hot-dip galvanized steel sheet to slow aging property, it may be possible to perform post-annealing on a separate line after the alloying process, but in this case, the manufacturing cost will increase. There is a problem that not only is it undesirable because it is bulky, but also the bake hardenability is lost.

The above-mentioned bake hardenability refers to the property that the yield strength of the steel plate increases when the steel plate is press-formed and then painted and baked as described above. Even if the yield strength of the steel plate is low, it is possible to ensure the strength of the molded product after painting and baking, making it possible to press form with a low yield strength, resulting in a product with good shape. Therefore, this is a desirable property for users.

Now, the bake hardenability and room temperature aging properties are mainly affected by the carbon content dissolved in the steel, and in the case of hot-dip galvanized steel sheets, the content of solid solution carbon is large, so
It has high bake hardenability and room temperature aging properties. On the other hand, hot-dip galvanized steel sheets that have been post-annealed after alloying treatment have a significantly lower solute carbon content, so they have slow aging properties at room temperature, but at the same time they have also lost their bake hardenability. It becomes like that.

From the above-mentioned viewpoints, the present inventors have developed alloyed hot-dip galvanized steel sheets with good spot weldability, without the need for post-annealing or other annealing on a separate line. As a result of conducting research to impart good formability as well as steel composition, the composition of the steel was changed to C: 0.002.
~0.15%, Si: 0.6% or less, Mn: 0.5-1
．． 6 (fl), P: 0.15% or less, SOt. At:0
．． 01 to 0.10%, and further contains Cr as necessary.
:0.04~0.80%, B:0.0005~0.00
30%, and: 0.02 to 0.40%, and the remainder is Fe and unavoidable impurities (all percentages by weight mean weight percent) ), that is, in particular, the Mn content is 0.5 to 1.
6 (F6ltC control provides both room temperature slow aging and bake hardenability, and furthermore, before hot-dip galvanizing, the temperature within the relatively low temperature range of 400°C to Al transformation point is applied to the cold-rolled steel sheet. They found that when box annealing is carried out, the r value increases significantly.This invention was made based on the above knowledge, and
C: 0.002-0.15%, Si: 0.601) or less, Mn: 0.5-1.691), P: 0.15 (fl)
The following contains SOtOAt: 0.01 to 0.10%,
Furthermore, if necessary, Cr: 0.04 to 0.80 (F6,
B: 0.0005 to 0.003 (L, and: 0.02
~0.40 (contains one or more of F6, and the remainder is Fe and unavoidable impurities. Finishing temperature: 700 ~ 950 ° C., Coiling temperature: 200 ~ 6
After hot rolling at 20°C, cold rolling at a reduction rate of 40% or more, and then box annealing at a temperature within the temperature range of 400°C to Al transformation point, the product was transferred to a continuous hot-dip galvanizing line. and heated to a temperature within the temperature range of 480 to 900 °C, and in the cooling process after the heating, immersed in a galvanizing bath with a bath temperature of 420 to 520 °C, and subsequently within the temperature range of 520 to 640 °C. Alloyed molten zinc has good formability and spot weldability, as well as room temperature slow aging and bake hardenability, by alloying it at a temperature of 3 seconds or more. This method is characterized by the method of manufacturing plated steel sheets.

Next, the reason why the composition range of the steel and the manufacturing conditions are limited as described above in the method of the present invention will be explained.
A. Component composition range of steel (a) C As mentioned above, the C component is a component that affects the room temperature aging property and bake hardenability of steel sheets, but if its content is 0.00
If the content is less than 2%, it will not be possible to obtain the desired bake hardenability (on the other hand, if the content exceeds 0.15%, the spot weldability of the steel plate will deteriorate, so the content should be reduced to 0.
．． It was set at 0.002% to 0.15%.

(b) In order to form a good Fe-Zn alloy layer through Si alloying treatment and to impart good slow aging properties to the steel sheet, it is better to keep the Si content as low as possible, and these good properties The upper limit of rejection is 0.6 (f) that can ensure the following.

On the other hand, since the Si component has the property of improving the strength of the steel sheet, it is possible to improve the strength without impairing the above properties by including Si in a range of 0.6% or less. (c) Mn In order to make the steel plate have slow aging properties and appropriate bake hardenability only on a continuous line, the Mn content is increased from 0.5 to 1.6%.
If the content is less than 0.5%, the desired slow aging properties and bake hardenability cannot be secured, while if the content exceeds 1.601), the melt This makes manufacturing difficult and causes an increase in cost, which further leads to a decrease in the r value.

(d) P The P component is normally contained to the extent that it is included as an unavoidable impurity, and it is particularly necessary to reduce the amount of P component when strength is required for the steel plate.
．． It can be contained within a range not exceeding 15%.

This is because if the P content exceeds 0.15%, spot weldability will deteriorate. (e) SOtOAt The At component precipitates as AtN during box annealing and has the effect of developing grains with a preferable crystal orientation for the r value. If At is less than 0.01%, a predetermined amount of AtN cannot be precipitated, so the desired improvement effect on the r value cannot be obtained. 0.101 at At
) Even if the content exceeds 0.01 to 0.01, no further effect of improving the r value will be obtained.
It was set at 1%.

(f) Cr, B, and these components have a uniform effect of improving the strength of steel sheets, so they are included as necessary when strength is required for steel sheets, but the content of each Cr: less than 0.04 (:fl), B: 0.0
0.005% and V: less than 0.02%, the desired strength improvement effect cannot be obtained, while Cr: 0.8% and B: 0.02%.
Even if the content exceeds Cr:0.03% and Cr:0.4%, no further strength improvement effect can be obtained and it will cause an increase in cost. .04~0.8%, B:0.0005~0
．． 003%, : 0.02 to 0.4 (fl).

B Hot rolling conditions (a) Finishing temperature When the finishing temperature is less than 700°C, the r value of the steel plate decreases because rolling occurs in the presence of a large amount of α phase, whereas when the finishing temperature exceeds 950°C, the r value of the steel plate decreases. , as the grain size of the hot-rolled sheet increases, the r value similarly decreases.
The finishing temperature was set at 700-950°C.

In addition, in practice, it is preferable to set the finishing temperature just above the Ar3 transformation point. (b) Coiling temperature The coiling temperature finely distributes Fe3c in the hot rolled steel sheet,
In addition, the amount of solid solute carbon in the steel sheet after alloying treatment is controlled to be small and to control the precipitation of AtN. However, if the coiling temperature is less than 200°C, the precipitation force of Fe3c is sufficient, and one winding When the temperature exceeds 620℃, Fe3
As c becomes coarser, AtN precipitation occurs and the r value decreases, so the coiling temperature is set to 200 to 620.
It was set as ℃.

C cold rolling rolling reduction If the rolling reduction in cold rolling is less than 40%, the desired r value cannot be secured, so the rolling reduction must be 40911 or more.

D-box annealing Box annealing is performed to increase the r-value, and by slowing down the temperature increase rate, D-box annealing has an effective K effect on the development of crystal orientations favorable to the r-value in the cold-rolled structure. AtN
This is a treatment to finely precipitate
Below 00°C, precipitation of AtN is insufficient and the desired r-value improvement effect cannot be secured, while even if box annealing is performed at a high temperature exceeding the Al transformation point, the r-value improvement effect is small. The temperature should be lowered to 4 to avoid the loss of thermal energy.
00 to Al transformation point.

E Heating in the hot-dip galvanizing line This heating is performed to impart ductility to the steel sheet, but if the box annealing in the previous process is performed at a relatively high temperature of 620°C or higher, the cold-rolled structure will change. Since recrystallization has already been completed, high-temperature heating is not necessary, and in this case Z
Simply heating to improve the adhesion of n is sufficient, so heating to a minimum of 480°C is sufficient.

On the other hand, if box annealing is performed at a temperature below 620°C, recrystallization of the cold-rolled structure has not yet been completed, so it is necessary to complete the recrystallization by heating to a relatively high temperature; Heating at a temperature of
The temperature was set at 0°C. F Zinc plating bath temperature If the temperature is lower than 420°C, Zn will solidify, but if the temperature exceeds 520°C, the plating properties will be poor, so the temperature should be set at 420 to 520°C.
It was determined that

G Alloying Treatment Conditions If the alloying treatment temperature is less than 520°C and the holding time is less than 3 seconds, the degree of alloying will be insufficient and the desired good spot weldability cannot be secured. On the other hand, if the alloying temperature exceeds 640°C, alloying will progress too much and a brittle layer will be formed on the surface of the steel sheet, so the alloying temperature should be kept at 520 to 640°C. The time was set at 3 seconds or more.

Next, the method of the present invention will be specifically explained using examples.

Example 1 C, Si, P and SOt. At
MO has a content within the range of this invention, but MO contains various steels that include the range of this invention and exceed this range, that is, C: 0.04 to 0.06%,
Si: 0.01-0.03%, P: 0.01-0.01
5%, SOt. At: 0.02-0.04%, Mn: 0
．． Various steels having compositions within the range of 1 to 1.8% are melted and cast into slabs, and then these slabs are processed at a finishing temperature of 820 to 860°C and a winding temperature of 400 to 400°C.
Hot rolled at 80℃ and plate thickness: 3.2Twn,
Then, after pickling, rolling reduction: 75%, plate thickness: 0.8wr
A cold rolled sheet was manufactured by performing cold rolling under the conditions of m.

Subsequently, the cold-rolled plate was heated at a heating rate of 40°C/H, a heating temperature of 560°C, a holding time of 1 hour, and a cooling rate of 80°C.
After box annealing under the conditions of /h, in a continuous hot-dip galvanizing line, it was first heated to a temperature of 0°C for about 30 seconds, and during the cooling process it was held at a temperature of 510°C for about 30 seconds, and then heated to a temperature of 4°C.
Immersed in a galvanizing bath at 60℃ and then heated to 560℃ again.
Alloying treatment is carried out by heating to
．． 3%, U-flat elongation rate: 0.3S, and then outside the same line, adjustment rolling is performed under the conditions of adjustment elongation rate: 0.5% to produce alloyed hot-dip galvanized steel sheets. did.

Next, a JIS No. 5 test piece was cut out in the L direction from the resulting alloyed hot-dip galvanized steel sheet, and this test piece was tested in that state and at a temperature of 30'C.
A tensile test was conducted under the condition of lfC being maintained for 2 months (after room temperature aging treatment), and changes in yield stress, elongation at yield point, and elongation at break were determined.

In addition, after applying a 2% tension equivalent to press forming to the above-mentioned as-cut test piece, heat treatment was performed at a temperature equivalent to paint baking treatment: 170°C for 20 minutes, and the increase in yield stress was (Amount of bake hardening) was determined.

These results are shown in FIG. As shown in Figure 1, when the Mn content deviates from the range of the present invention, bake hardenability increases significantly, but aging at room temperature is also significant, resulting in a decrease in elongation at break and an increase in yield stress. Furthermore, the elongation at yield point is large, resulting in poor moldability. On the other hand, in an alloyed hot-dip galvanized steel sheet whose Mn content is within the range of the present invention, the decrease in elongation at break: 296 or less, and the increase in yield stress: 1
k9/1171 or less, yield point elongation increase: 0.501)
It shows the following, and it is clear that it has substantially slow aging properties, and also shows a bake hardening amount of 4k9/Md or more, which shows that it is particularly suitable as a steel plate for automobiles.
Example 2 Using a converter, steel having the composition shown in Table 1 was melted and cast into a slab.
The slab is hot-rolled under the hot-rolling conditions shown in the table to form a hot-rolled plate with a thickness of 3.2wn, and thereafter the box annealing temperature and the line heating temperature are set to the conditions shown in Table 1. The present invention was obtained by performing cold rolling, box annealing, heat treatment in a continuous hot-dip galvanizing line, hot-dip galvanizing, alloying treatment, and temper rolling under the same conditions as in Example 1 except for the following. Methods 1 to 9 and Comparative Method 1
- 6 were carried out, respectively.

Comparative method 1 is used when box annealing is not performed, and comparative method 2 is used when the box annealing temperature is lower than the range of this invention.
Comparative method 3 results in a higher coiling temperature, comparative method 4 results in a higher Mn content, comparative methods 5 and 6 result in SO
t. This is a case where the At and Mn contents are each on the lower side.

Next, for the alloyed hot-dip galvanized steel sheet obtained as a result, the amount of decrease in elongation at break due to room temperature aging, the amount of increase in elongation at yield point due to room temperature aging, and the amount of bake hardening were determined under the same conditions as in Example 1.

The results are also shown in Table 1.

Additionally, Table 1 also shows the tensile properties and r values. From the results shown in Table 1, the steel plates manufactured in Comparative Examples 1 to 6 exhibited undesirable values in at least one of the following properties: r value, decrease in elongation at break, and increase in elongation at yield. On the other hand, the steel plates manufactured by methods 1 to 9 of the present invention all have an r value of 1.5 or more and a decrease in elongation at break of 1.
% or less, yield point elongation increase: 0.5% or less, and the bake hardening amount was about 4k9/1 or more, and it is clear that it has all the properties particularly required for automotive steel sheets. be.

As described above, according to the method of the present invention, an alloyed hot-dip galvanized steel sheet having good formability and spot weldability, as well as room-temperature slow aging and bake hardenability, is produced on a separate line. It can be manufactured using only a continuous hot-dip galvanizing line, without the need for post-annealing.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the Mn content in a steel sheet and various properties.

Claims (1)

[Claims] 1 C: 0.002 to 0.15%, Si: 0.6% or less, Mn: 0.5 to 1.6%, P: 0.15% or less, so
l. Contains Al: 0.01-0.10%, the rest is Fe
Finishing temperature: 700-950°C, coiling temperature: 200-950°C.
After hot rolling at 620°C, cold rolling at a rolling reduction of 40% or more, then box annealing at a temperature within the temperature range of 400°C to Al transformation point, followed by continuous hot-dip galvanizing. In the heating line, it is heated to a temperature within the temperature range of 480 to 900°C, and in the cooling process of the heating, the bath temperature is 420 to 520°C.
Production of an alloyed hot-dip galvanized steel sheet, characterized by immersing it in a galvanizing bath of Law. 2 C: 0.002 to 0.15%, Si: 0.6% or less, Mn: 0.5 to 1.6%, P: 0.15% or less, so
l. Contains Al: 0.01 to 0.10%, and further contains Cr
:0.04~0.80%, B:0.0005~0.00
30% and V: 0.02 to 0.40%, and the remainder is Fe and unavoidable impurities (weight%) at a finishing temperature of 7.
After hot rolling under the conditions of 00 to 950°C, coiling temperature: 200 to 620°C, cold rolling at a rolling reduction of 40% or more, and then at a temperature within the temperature range of 400°C to Al transformation point. Box annealing followed by continuous hot-dip galvanizing line, 4
Heating to a temperature within the temperature range of 80 to 900°C, immersing in a galvanizing bath with a bath temperature of 420 to 520°C during the heating and cooling process, and then reheating to a temperature within the temperature range of 520 to 640°C. 1. A method for producing an alloyed hot-dip galvanized steel sheet, characterized in that the alloying treatment is carried out under conditions of holding the temperature for 3 seconds or more.