JP2674388B2 - Method for producing hot-dip galvanized steel sheet with high formability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength (tensile strength 41 kg / mm ² or more) high formability hot-dip galvanized steel sheet used for automobile parts and the like, and more specifically, hot-rolled direct hot-dip galvanizing or The present invention relates to a method for manufacturing an alloyed hot-dip galvanized steel sheet.

[0002]

2. Description of the Related Art In recent years, there has been a significant demand for high strength steel sheets for reducing the weight of automobile steel sheets.

In order to meet this demand, JP-A-63-1493
There is a proposal such as No. 21 bulletin.

Conventionally, in the production of this kind of high strength steel sheet, when the alloy steel is used as it is in hot rolling, low temperature winding or the like is carried out by controlled rolling to obtain a bainite or fine ferrite / pearlite structure. Strength (tensile strength)
We are working to improve and improve moldability.

However, when the alloyed molten zinc-treated steel sheet having a tensile strength of 41 kg / mm ² is produced, the alloying treatment is reheated, so that the low-temperature winding control cannot be performed, and therefore the tensile strength is increased. In order to prevent deterioration and deterioration of moldability, high strength has been guaranteed and high moldability has been maintained simply by adjusting the components.

[0006]

In the above component adjustment, if the C content is increased, the weldability deteriorates, and if the Mn content is increased, the cost increases. If a large amount of Si is added, the problem of scale defects and alloying are likely to be delayed. Furthermore Nb
If a large amount of is added, the strength will increase too much, causing problems during pressing. When the strength is guaranteed only by adjusting the components as described above, various problems occur, and it is impossible to obtain a product which is completely satisfactory in terms of weldability and formability.

On the other hand, the above-mentioned publication discloses a technique similar to the present invention to be described later. However, since cooling is performed at a constant cooling rate from finishing to winding, temper brittleness due to P occurs. However, there is an extremely high risk of press cracking and further deterioration of strength.

[0008] Therefore, the object of the present invention is to exhibit high strength and excellent formability, and to prevent thermal deterioration such as strength and hole expansion performance due to alloying treatment, and to stabilize this kind of characteristics. It is to provide a method of manufacturing a steel sheet that can be obtained.

[0009]

[Means for Solving the Problems] The above problem is C: 0.08%
Below, Si: 0.03% or less, Mn: 0.50 to 0.90%, P: 0.
The steel with 03% or more, S: 0.010% or less, and the balance iron and unavoidable impurities is used for finishing stand outlet temperature.
It is hot-rolled at 810 to 900 ℃ and wound at a winding temperature of 400 to 450 ℃, and the cooling rate from finishing to winding is the first cooling step from the finish to an intermediate temperature in the range of 600 to 660 ℃. Cooling at 40-60 ° C / sec, and then as a second cooling step, cooling from less than 40 ° C / sec to 20 ° C / sec between the intermediate temperature and the coiling temperature, then pickling, hot dip galvanizing or alloying. The problem can be solved by carrying out hot dip galvanization.

[0010]

In the present invention, as will be described later, in addition to the steel composition, the finishing stand temperature, and the coiling temperature, the cooling process is divided into two stages and controlled, which is excellent as described below. It is possible to obtain a hot-dip galvanized or galvannealed steel sheet having excellent properties as compared with.

Specifically, by setting the finish outlet roll temperature within the above range, it is possible to suppress the occurrence of scale flaws and reduce variations in tensile properties. Further, within the range of the present invention, the production is carried out in a temperature range where Ar is 3 points or more and rolling is most easy, which is advantageous in terms of cost.

By setting the winding temperature within the above range, the tensile strength can be increased while preventing temper embrittlement and press cracking.

Further, the moldability can be improved by changing the cooling rate in two steps as described above.

[0014]

Next, the present invention will be specifically described with a particular focus on the reason for limiting the numerical values.

(Steel composition) In the present invention, steel is composed of
C: 0.08% or less, Si: 0.03% or less, Mn: 0.50 to 0.90
%, P: 0.03% or more, S: 0.010% or less, and the balance iron and inevitable impurities. C ≦ 0.08%: Ferrite / pearlite has good weldability and a fine ferrite / pearlite structure corresponding to the controlled rolling of hot rolling, and the effect of heat during alloying treatment is taken into consideration. This is for making the grain growth (coarsening) of the structure insensitive, and by doing so, high strength and good formability due to low temperature winding can be maintained. P ≧ 0.03%: This is for guaranteeing high strength. Strength can be increased by increasing the amount of Mn added, but in this case, M
Since n is expensive, not only the cost is increased, but also the hole expansion ratio (an index of formability as shown in FIG. 2 is shown. In the case of increasing the Mn addition amount to increase the strength, it decreases. Si ≦ 0.03% is preferable. This is to prevent so-called island scale and promote alloying of plating. Mn = 0.50-0.90%: To ensure high strength, 0.90
%, The strength becomes excessive, and if less than 0.50%, the strength becomes insufficient. S: 0.01% or less: This is to prevent the formation of sulfide-based inclusions and prevent the deterioration of cold workability. On the other hand, the tensile strength and the hole expansion ratio (index of formability) and the composition examples shown in Table 1 have the relationship shown in FIG. 5, and the steel composition of the present invention is also effective in this respect. That is, it is found that in a low C system, when Mn, Nb, Ti, V, etc. are added, the tensile strength increases but the hole expansion ratio decreases.

[0016]

[Table 1]

(Finishing outlet temperature and winding temperature) In the present invention, the finishing stand outlet temperature is hot-rolled at 810 to 900 ° C and wound at a winding temperature of 400 to 450 ° C. Here, the lower limit of the finishing stand outlet temperature is set to 810 ° C. because Ar is
It is necessary to secure ₃ points, and if the upper limit of 900 ℃ is exceeded, scale flaws are likely to occur. That is, if the temperature is less than 810 ° C, a rolling work structure is generated, resulting in large variations in tensile properties. Further, 810 ° C is the temperature range in which rolling is most easy, which is advantageous in terms of manufacturing cost. Conversely, if the temperature exceeds 900 ° C, scale defects will occur. Furthermore, the reason why the winding temperature is 400 to 450 ° C. is that if the temperature is less than 400 ° C., the variation in strength in the width direction becomes remarkable and the temperature control becomes difficult. If it exceeds 450 ° C, tempering brittleness due to P may occur, and there is a risk of causing press cracking, especially cracking in the cross section, and further, lowering of strength occurs. FIG. 6 shows the correlation with the fracture length of cracks when the coiling temperature is changed (when the tensile test is conducted, the crack length generated at the fracture surface. Due to temper embrittlement) when P = 0.02%. In the case of the above-mentioned conventional publication example,
It can be seen that tempering embrittlement becomes noticeable when wound at 470 ° C or higher.

(Two-stage cooling) In the present invention, in particular, the cooling rate from finishing to winding is as the first cooling step, cooling from finishing to an intermediate temperature in the range of 600 to 660 ° C at 40 to 60 ° C / sec. Then, as a second cooling step, cooling is performed at less than 40 ° C to 20 ° C / sec between the intermediate temperature and the coiling temperature. FIG.
Is an investigation of changes in tensile strength and hole expansion ratio by changing the intermediate temperature. When the intermediate temperature of 600 to 660 ° C. is deviated, the hole expanding property is remarkably deteriorated. Furthermore, this first
The cooling rate in the cooling stage is set to 40 to 60 ° C./second, but as shown in FIG. 4, if the temperature is less than 40 ° C./second, the hole expansion rate decreases largely. Also, if the cooling rate becomes too fast,
The ferrite / pearlite structure becomes unstable, resulting in large fluctuations in characteristics inside the coil. 3 and 4, the steel composition is C: 0.07% or less, Si: 0.01% or less, Mn: 0.70.
%, P: 0.035% or more, S: 0.005%. On the other hand, the cooling rate in the second cooling stage is set to less than 40 ° C./sec to 20 ° C./sec, but as shown in FIG. 8, tempering brittleness due to P occurs at less than 20 ° C./sec, It causes press cracking and lowers strength. Further, when the temperature is 40 ° C / sec or more, the characteristic variation in the coil becomes large.

(Example of typical temperature control) As an example of the temperature control or cooling pattern of the present invention, as shown in FIG. 1, when the finishing stand outlet temperature is, for example, 830 to 850 ° C., the intermediate temperature is 600 to 660 ° C. First cooling rate up to ≧ 40 ℃
/ Second, cooling is performed at the second cooling rate ≧ 28 ° C / second after the intermediate temperature of 600 to 660 ° C, and the winding temperature is 400 to 45 ° C.
Set to 0 ° C.

(Plating treatment) The steel sheet thus obtained is subjected to pickling and hot dip galvanizing or alloying hot dip galvanizing. The plating bath temperature can be 460 ° C. and the alloying temperature can be 500 to 550 ° C., but the conditions are not limited. Further, the obtained steel sheet can be directly conveyed to a plating step and subjected to hot dip galvanizing and alloying treatment.

[Experimental Results] Next, the present invention will be described more specifically based on the experimental results. That is, when the tensile strength, the hole expansion ratio, the weldability, the alloying property, and the surface flaw state were investigated while variously changing the steel composition, the finish stand outlet temperature, the intermediate temperature, the winding temperature, and the cooling rate, Table 2 And the results of Table 3 were obtained. In the table below, the alloying processability was evaluated as ◯: good, Δ: partially untreated, and x: poor. In addition, the welding performance and surface flaw (island scale) were evaluated by ○:
Good, Δ: Slightly generated, ×: Bad. Further, as for the hole expansion rate, as shown in FIG. 7, a hole having a diameter D (12 mmφ) is punched in the steel plate, and the cone punch is pushed up from below and the cone punch is stopped when a crack is formed. D ′ is defined by the hole expansion ratio = (DD ′) × 100 / D and is obtained by calculation.

[0022]

[Table 2]

[0023]

[Table 3]

[0024]

As described above, according to the present invention, high strength and excellent formability are exhibited, and further, thermal deterioration such as strength and hole expansion performance due to alloying treatment after hot dip galvanization can be prevented. There is an advantage that this kind of characteristic can be stably obtained.

[Brief description of the drawings]

FIG. 1 is a diagram showing a cooling pattern of the present invention.

FIG. 2 is a correlation diagram between a hole expansion ratio and a press molding cracking ratio.

FIG. 3 is a correlation diagram of intermediate temperature, tensile strength and hole expansion rate.

FIG. 4 is a relationship diagram between a first cooling rate and a hole expansion rate.

FIG. 5 is a relational diagram of steel components with respect to tensile strength and hole expansion ratio.

FIG. 6 is a relationship diagram between a winding temperature and a fracture length.

FIG. 7 is a schematic diagram for explaining a hole expansion test method according to the present invention.

FIG. 8 is a diagram showing the relationship between the second cooling rate and the length of fracture surface cracking.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical display location C22C 38/04 C22C 38/04

Claims (1)

(57) [Claims] 1. C: 0.08% or less, Si: 0.03% or less, M
n: 0.50 to 0.90%, P: 0.03% or more, S: 0.010% or less, and steel consisting of balance iron and inevitable impurities are hot-rolled at a finishing stand outlet temperature of 810 to 900 ° C,
At the winding temperature of 400 to 450 ° C, the cooling rate from the finishing to the winding is 40 to 6 as the first cooling step from the finishing to the intermediate temperature in the range of 600 to 660 ° C.
Cooling at 0 ° C./sec, then as a second cooling step, cooling from less than 40 ° C./sec to 20 ° C./sec between the intermediate temperature and the coiling temperature, then pickling, hot dip galvanizing or alloying melting A method for producing a hot-dip galvanized steel sheet with high formability, which comprises performing galvanization.