JPH0784620B2 - Method for producing hot-dip galvanized cold-rolled steel sheet for deep drawing excellent in secondary processing brittleness resistance - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a hot-dip galvanized cold-rolled steel sheet for deep drawing, which is excellent in secondary work embrittlement resistance.

(Prior Art and Problems to be Solved) In recent years, cold-rolled steel sheets used for automobile members and outer panels of electric devices are required to have high press formability and corrosion resistance.

Conventionally, as a method for producing a cold-rolled steel sheet satisfying such requirements, by adding carbonitride forming elements such as Ti and Nb to the ultra-low carbon steel individually or in combination, C and N in the steel are fixed,
Develops a (111) plane orientation texture advantageous for deep drawability,
Further, a method of applying galvanization has been proposed.

However, on the other hand, in the case of ultra-low carbon steel in which C and N in steel are sufficiently fixed by adding carbonitride forming elements such as Ti and Nb,
There is a problem that cracking due to brittle fracture (secondary working brittleness) occurs in the second working after press molding. This is because the solid solution C in the steel is fixed and does not exist, so that the segregation of C in the ferrite grain boundaries disappears and the grain boundaries become brittle. In particular, in a hot-dip galvanized steel sheet, there is a problem in that hot-dip galvanizing treatment makes it easier for hot-dip zinc to enter the weakened grain boundaries and further promotes embrittlement.

As a means for solving this grain boundary embrittlement, it has been attempted in the past to control the amount of Ti or Nb added so that C in the steel remains in advance. However, in this method, even if the component steel in which the solid solution C remains can be produced, since the solid solution C essentially deteriorates the r value and the ductility of the steel, the press formability is greatly reduced. I had no choice but to come down. That is, the press formability and the secondary work embrittlement resistance were essentially incompatible. On the other hand, remaining such a small amount of C has not been established in steelmaking technology.

In this respect, conventionally, the following proposals have been made, but it is difficult to provide both excellent press formability and secondary work brittleness resistance. For example, in order to improve the resistance to secondary work cracking of deep drawing steel sheet, Ti and Nb are added to fix C in the steel, and carburizing is performed during open coil annealing after cold rolling to carburize the steel sheet surface. There is a method of forming a layer
63-38556). However, in the case of this method, since carburizing is performed during batch annealing for a long time, a steel sheet having a different composition and structure in the sheet thickness direction, such as a difference in ferrite grain size between the surface layer portion and the central portion of the steel sheet, is obtained. In addition, such a batch annealing type has a drawback that the productivity is naturally low and that the material in the plate length direction and the plate width direction tends to be uneven.

The present invention is a method for solving the above-mentioned problems of the prior art and capable of producing a hot-dip galvanized cold-rolled steel sheet for deep drawing excellent in secondary work embrittlement resistance without impairing the characteristics as a cold-rolled steel sheet for deep drawing. It is intended to provide.

(Means for Solving the Problem) As described above, in order to obtain the secondary processing brittleness resistance, it is necessary to make a small amount of C of several ppm exist in the grain boundaries to strengthen the grain boundaries. In order to obtain press moldability, it is necessary to sufficiently fix C. Meeting these conflicting requirements has not been possible with the prior art.

Therefore, the inventors of the present invention have earnestly conducted research on measures that can be realized as a result by shifting in time instead of simultaneously realizing such conflicting requirements. as a result,
Until the completion of recrystallization during annealing when the recrystallization texture is determined, the solid solution C is kept in a state of zero and then carburized, and if C is left at the grain boundaries or within the grain in the final product stage, press It was found that both formability and resistance to secondary work brittleness can be ensured. Furthermore, when the carburization is very high, this carburizing is carried out by intergranular diffusion, which has a diffusion rate about 10 times faster than intragranular diffusion. I got the knowledge. The present invention has been made here by further studying component adjustment and manufacturing conditions for achieving this.

That is, the present invention, C: 0.01% or less, Si: 0.2% or less, M
n: 0.05-0.40%, P: 0.10% or less, S: 0.02% or less, sol.A
l: 0.01-0.08%, N: 0.005% or less, Ti and N
With the addition of b alone or in combination, the effective Ti amount according to the following formula (1) (hereinafter referred to as Ti *) and the relationship between the Nb amount and the C amount are shown in the following formula (2) Ti * (%) = totalTi (%) -{(48/32) x S (%) + (48/14) x N (%)} ... (1) 1 ≤ (Ti * / 48 + Nb / 93) / (C / 12) ≤ 4.5 ... (2) Content in the range that satisfies the above requirement, and further B: 0.003% if necessary
A steel containing the following, the balance of which is Fe and unavoidable impurities, is heated in the range of 1000 to 1250 ° C, and then hot-rolled to roll in the range of (Ar ₃ −50) to (Ar ₃ +100) ° C. Exit
After that, it was wound in the range of 500 to 800 ° C, pickled and cold-rolled, and then heated to a temperature above the recrystallization temperature in a carburizing atmosphere gas to make the solid solution C amount 3 to 30 ppm. The present invention is directed to a method for producing a hot-dip galvanized cold-rolled steel sheet for deep drawing, which is excellent in secondary work embrittlement resistance, which is characterized by controlling and subsequently performing hot dip galvanizing.

The present invention will be described in more detail below.

(Operation) First, the reason for limiting the chemical composition of steel in the present invention will be described.

C: C fixes T as C increases as its content increases
The amount of i and Nb added increases, leading to an increase in manufacturing cost.
Furthermore, since the amount of TiC and NbC precipitation increases and the grain growth is hindered and the r value deteriorates, the lower the C content, the better.
01% From the viewpoint of steelmaking technology, it is desirable to set the lower limit of the C content to 0.001%.

Si: Si is added mainly for deoxidation of molten steel, but if the addition amount is too large, the surface properties, zinc adhesion, chemical conversion treatment or paintability will deteriorate, so the content should be kept to 0.2% or less. .

Mn: Mn is added mainly for the purpose of preventing hot embrittlement, but if it is less than 0.05%, its effect is not obtained, while if the addition amount is too large, ductility deteriorates, so its content is 0.05- 0.40%
The range is.

P: P has the effect of increasing the steel strength without lowering the r value, but segregates at the grain boundaries and easily causes secondary work embrittlement, so its content is suppressed to 0.10% or less.

S: S combines with Ti to form TiS, so if the content thereof increases, the amount of Ti required to fix C and N increases. Further, the MnS-based elongated inclusions increase and deteriorate the local ductility, so the content is suppressed to 0.02% or less.

Al: Al is added for the purpose of deoxidizing molten steel, but its content is
If less than 0.01% by l.Al, the purpose is not achieved, while if over 0.08%, the deoxidizing effect is saturated and Al ₂ O ₃
Inclusions increase and workability deteriorates. Therefore, the content is sol.Al in the range of 0.01 to 0.08%.

N: N combines with Ti to form TiN, so an increase in the content increases the amount of Ti required to fix C. Further, the TiN precipitation amount increases, grain growth is hindered, and the r value deteriorates. Therefore, the smaller the content, the better.
Control to below 005%.

Ti, Nb: Ti, Nb has the effect of increasing the r value by fixing C and N. In this case, Ti is combined with S and N as described above.
Since TiS and TiN are formed, it is necessary to consider the Ti amount in the product by the amount converted as the effective Ti amount (Ti *) calculated by the following equation (1).

Ti * (%) = totalTi (%) − {(48/32) × S (%) + (48/14) × N (%)} (1) Therefore, for the purpose of the present invention, Ti * Amount, Nb amount and C
It must be contained within the range of the relationship with the amount in the formula (2) 1 ≦ (Ti * / 48 + Nb / 93) / (C / 12) ≦ 4.5 (2). If the value of the equation (2) is smaller than 1, C and N cannot be fixed sufficiently and the r value is deteriorated. On the other hand, if it exceeds 4.5, the action of increasing the r-value is saturated, and the solid solution Ti and Nb immediately fix the C that has invaded during the subsequent annealing in the atmosphere, so that the amount of C required at the grain boundaries and grains is increased. However, the desired secondary work embrittlement resistance cannot be obtained by preventing grain boundary segregation of C.

B: B is an element effective for the secondary work embrittlement resistance, so that it can be added if necessary. However, if it exceeds 0.003%, the effect is saturated and the r value is manipulated. Therefore, considering the economic efficiency as well, the content is made 0.003% or less.

Next, the manufacturing conditions of the present invention will be described.

Steel having the above chemical composition is melted and cast by a conventional method, but in the subsequent hot rolling, after heating to 1000 to 1250 ° C.,
It is necessary to perform hot rolling under conditions that the finishing temperature and _{(Ar 3 -50) ~ (Ar} 3 +100) ℃ range. This is because from the viewpoint of improving the r value, it is preferable to make the finishing temperature Ar ₃ or higher because it is necessary to reduce the grain size of the hot rolled sheet and randomize the texture. However, even in the ferrite / austenite two-phase region, when the austenite fine grains are many, the number of Ar points does not necessarily have to be ₃ or more, so the heating temperature should be in the range of 1000 to 1250 ° C and the finish of hot rolling should be the temperature is _{(Ar 3 -50) ~ (Ar} 3 +100) ℃ range. In addition,
Other hot rolling conditions are not particularly limited.

Next, after hot rolling, the coiling temperature needs to be in the range of 500 to 800 ° C. in order to fix the solid solution C and N in the steel as carbonitrides.

After winding, pickling and cold rolling are performed, but the conditions are not particularly limited, but a total rolling rate of 60 to 90% is desirable in order to develop a (111) orientation texture that is advantageous for r value. .

After this cold rolling, in the hot dip galvanizing line, continuous annealing is performed in a carburizing atmosphere gas at a temperature not lower than the recrystallization temperature to form a (111) plane orientation texture advantageous to the r value. That is, the r value mainly depends on the (111) plane orientation texture of the steel, and the solid solution C and the solid solution N are fixed as carbonitrides and completely removed by the coiling treatment before recrystallization annealing. This is for obtaining the (111) plane orientation texture.
However, once the recrystallization is completed and the texture is formed, C invading thereafter does not adversely affect the r value.
Of the C that has entered from the carburizing atmosphere, C that is not fixed as TiC or NbC segregates at the grain boundaries, improving the secondary work embrittlement resistance. Therefore, the amount of solute C must be 3 to 30 ppm. That is, if it is less than 3 ppm, the required amount of solid solution C is insufficient, and sufficient secondary processing brittleness resistance cannot be obtained.
If it exceeds ppm, workability such as elongation is deteriorated, and the stripping speed of the hot dip galvanizing line must be reduced, resulting in a decrease in productivity.

Continuous annealing in a hot dip galvanizing line is performed so that this required amount of solid solution C can be obtained. Of course, the temperature is above the recrystallization temperature. The continuous annealing atmosphere is a reducing atmosphere.
A carburizing gas with a controlled carbon potential is obtained by mixing CO or lower hydrocarbons. The residence time in the continuous annealing furnace is short, preferably 2 seconds to 2 minutes.

After performing continuous annealing in the hot dip galvanizing line,
Subsequently, hot dip galvanizing is continuously performed. Although the present invention does not require the overaging treatment, the overaging treatment may be performed at a temperature in the vicinity of the plating bath. In this case, the immersion galvanizing bath is subsequently immersed for plating. Further, alloying treatment may be performed if necessary.

Next, examples of the present invention will be described.

(Example) An ultra low carbon steel having the chemical composition shown in Table 1 was used at 1150 ° C. for 30 days.
After heat treatment for 1 minute and solution treatment, finish temperature is 890 ℃
After that, hot rolling is completed, and then winding treatment is performed at 720 ° C.
After pickling, cold rolling was performed at a reduction rate of 75%. Then, in the hot-dip galvanizing line, recrystallization annealing was performed at 780 ° C. for 40 seconds in a carburizing atmosphere or an inert gas, then hot-dip galvanizing treatment was performed at 450 ° C., and then 0.8% skin pass was applied.

Mechanical properties of the obtained hot dip galvanized cold rolled steel sheet,
Table 2 shows the results of examining the r value and the secondary working brittleness limit temperature.

In the brittleness test, after the cup obtained by forming a cup with a total drawing ratio of 2.7 was trimmed to a height of 35 mm, it was pushed into a conical punch with an apex angle of 40 ° in a refrigerant at each test temperature to prevent brittle fracture. The critical temperature was measured and used as the secondary working brittleness critical temperature.

As is clear from Table 2, the examples of the present invention have excellent secondary work embrittlement resistance while maintaining the press formability (r value) as the hot-dip galvanized cold-rolled steel sheet for deep drawing, as compared with the conventional examples. have.

In steels with a P content of 0.025% or less (Ti * / 48 +
Fig. 1 shows the results organized by the relationship between the Nb / 93) / (C / 12) value, the r value, and the secondary working brittleness limit temperature. From the figure, it is understood that the invention examples in which the value of (Ti * / 48 + Nb / 93) / (C / 12) is within the range of the invention have a high r value and a low secondary working embrittlement limit temperature.

Further, FIG. 2 shows the relationship between the P content and the secondary working embrittlement limit temperature. P segregates at the grain boundaries and easily causes secondary working brittleness. It is understood that when it is present, the secondary work embrittlement resistance is improved, and the addition of B improves the secondary work embrittlement resistance.

(Effects of the Invention) As described in detail above, according to the present invention, it is possible to improve the resistance to deep-drawing without impairing the requirements as a hot-dip galvanized cold-rolled steel sheet.
It is possible to obtain a steel sheet having excellent subsequent working brittleness with high productivity.

[Brief Description of Drawings] Fig. 1 shows (Ti * / 48) for steel with P content of 0.025% or less.
+ Nb / 93) / (C / 12) value and r value, and the relationship between secondary working brittleness limit temperature, Fig. 2 is a diagram showing the relationship between P content and secondary working brittleness limit temperature is there.

Claims (2)

[Claims] 1. By weight% (hereinafter the same), C: 0.01% or less,
Si: 0.2% or less, Mn: 0.05 to 0.40%, P: 0.10% or less, S: 0.0
2% or less, sol.Al: 0.01 to 0.08%, N: 0.005% or less, and by adding Ti and Nb alone or in combination, the effective Ti amount according to the following formula (1) (hereinafter referred to as Ti *) And the relationship between the amount of Nb and the amount of C is expressed by the following formula (2) Ti * (%) = totalTi (%)-{(48/32) × S (%) + (48/14) × N (%)} ... (1) Steel containing 1 ≦ (Ti * / 48 + Nb / 93) / (C / 12) ≦ 4.5 (2) with the balance Fe and unavoidable impurities in the range of 1000 to 1250 ° C. After that, hot rolling is performed to finish the rolling in the range of (Ar ₃ −50) to (Ar ₃ +100) ° C, and then it is wound in the range of 500 to 800 ° C, pickled and cooled. After hot rolling, it is heated to a temperature not lower than the recrystallization temperature in a carburizing atmosphere gas to control the amount of solid solution C to 3 to 30 ppm, and subsequently to continuously perform galvanizing. Hot-dip galvanizing cold for deep drawing with excellent secondary processing brittleness resistance Manufacturing method of rolled steel sheet. 2. The method according to claim 1, wherein the steel further contains B: 0.003% or less.