JPH0784619B2 - Method for producing cold-rolled steel sheet excellent in deep drawability and resistance to secondary work brittleness - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a cold rolled steel sheet, and more particularly to a method for manufacturing a cold rolled steel sheet excellent in deep drawability and secondary work embrittlement resistance.

(Prior Art) In recent years, high press formability is required for cold-rolled steel sheets used for automobile members and outer panels of electric devices.

As a method for manufacturing a cold rolled steel sheet that satisfies such requirements,
Using a steel that can fix C and N in steel by adding carbonitride forming elements such as Ti and Nb to ultra-low carbon steel, hot-rolling in the unrecrystallized ferrite region, and further cold-rolling There is proposed a method of forming a rolling texture by applying a heat treatment, and then performing recrystallization annealing by various methods to develop a (111) texture advantageous for deep drawability.

(Problems to be Solved by the Invention) However, on the other hand, in an ultra-low carbon steel in which C and N in steel are sufficiently fixed by carbonitride forming elements such as Ti and Nb, brittleness occurs in secondary working after press forming. There is a problem that cracking occurs due to breakage. This is because, since there is no solid solution C in the steel, segregation of C at the ferrite grain boundaries disappears and the grain boundaries become weaker.

Further, in the P-added steel, there is a problem that P segregates at the grain boundaries, which promotes embrittlement of the grain boundaries.

Therefore, conventionally, in order to improve the secondary processing brittleness resistance,
It has been attempted in advance to control the amount of addition of Ti and Nb so that C and N in the steel remain, and perform melting. However, in this method, even if the component steel in which the solid solution C and N remain can be produced, since the solid solution C and N essentially deteriorate the r value and ductility of the steel, Inevitably, the formability was significantly reduced. That is, the press formability and the secondary work embrittlement resistance were essentially incompatible.
On the other hand, leaving such a small amount of C and N was not 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. A method of forming a layer (Japanese Patent Laid-Open No. 63-38556) has been proposed. However, in the case of this method, since carburization is performed during batch annealing for a long time, a high-concentration carburized layer (average C amount: 0.02 to 0.10%) is formed only on the surface layer of the steel sheet, and There is a problem that the steel sheet has a different composition and structure in the plate thickness direction, such as a difference in the ferrite grain size between the center and the center.In addition, the batch annealing type naturally has low productivity and the plate length and plate width. There is a disadvantage that the material in the direction tends to be non-uniform.

Similarly, as a method for producing a deep-drawing steel sheet by adding Ti and Nb, there is also a method of performing recarburization annealing after cold rolling and then performing a carburizing treatment (JP-A-1-96330). However, it is mainly aimed at improving the strength by precipitation of a large amount of carbides and nitrides, there is no consideration for secondary work embrittlement resistance, and since carburizing is performed for a long time in batch after annealing, the carburizing amount Are prone to become excessive and non-uniform, the productivity is low, and the process is complicated.

The present invention has been made in order to solve the problems of the above-mentioned conventional techniques, and uses an ultra-low carbon steel to produce a cold-rolled steel sheet excellent in deep drawability and secondary work embrittlement resistance. The object is to provide a method of manufacturing well.

(Means for Solving the Problem) In order to achieve such an object, the present inventors examined the cause of deterioration of press formability in conventional ultra low carbon steel.

As a result, the causes of the solid C and N lowering the press formability are that the local slip system and the rearrangement of dislocations are affected at the stage of forming the rolling texture and the stage of forming the recrystallization texture, resulting in deep drawing. It was found that this is because it inhibits the development of (111) texture, which is favorable for sex.

Therefore, the inventors of the present invention have conducted intensive studies on a method for eliminating such a cause and improving the secondary work embrittlement resistance, and as a result, adjusted specific components and rolled the ultra low carbon steel. By defining the conditions, the recrystallization texture is determined and the solid C and N are kept in a state of zero until the completion of recrystallization during annealing. After that, continuous annealing is performed in a carburizing atmosphere to obtain the final Several ppm at grain boundaries at the product stage
A method of preventing embrittlement by finding a certain amount of C and strengthening the grain boundaries has been found, and the present invention has been made here.

That is, the present invention, C: 0.007% or less, Si: 0.1% or less, M
n: 0.05 to 0.50%, P: 0.12% or less, S: 0.015% or less, sol.A
l: 0.005 to 0.05%, N: 0.006% or less, Ti and
With the addition of Nb alone or in combination, the effective Ti amount (expressed as Ti *) according to the following formula (1) and the relationship between the Nb amount and the C amount are expressed by the following formula (2) Ti * = totalTi-{(48/32) × S + (48/14) × N} ...
(1) 1 ≦ (Ti * / 48 + Nb / 93) / (C / 12) ≦ 4.5 (2) The content is in the range that satisfies the above condition, and if necessary, further B: 0.0001-
Steel containing 0.0030% and the balance Fe and unavoidable impurities is roughly rolled in the temperature range of 900 to 1200 ° C, and then rolled in the unrecrystallized ferrite region in the range of 600 to 850 ° C. R ₁ hot rolling, then winding at 600 ℃ or less,
After pickling, further reduction rate R ₂ ≧ 50% (provided that the total reduction rate R = the R ₁ and _{R 2 {1- (1-R} 1/100) · (1-R 2/100)} ×
100% is 95%>R> 65%), and further continuous annealing is performed in the carburizing atmosphere gas in the temperature range above the recrystallization temperature for deep drawability and secondary work embrittlement resistance. The gist is a method of manufacturing an excellent cold-rolled steel sheet.

The present invention will be described in more detail below.

(Operation) The present invention is, in short, required to use the ultra-low carbon steel for the technology that was theoretically impossible as described above, and to fill the grain boundary defects due to the secondary work embrittlement resistance. C amount 2 to 15
It has been found that continuous annealing can be performed if ppm is secured. The reason for this is that the penetration of C is not by intragranular diffusion, but by grain boundary diffusion, which is about 10 times faster, and in the case of ultra-low carbon steel with extremely high grain boundary purity, its diffusion rate is This is because, in continuous annealing, the amount of solid solution C was zero before annealing in the continuous annealing, but it is possible to secure a predetermined amount of C in the grain boundaries first and then in the grains.

First, the reasons for limiting the chemical composition of steel in the present invention will be described.

C: C is Ti that fixes C as its content increases,
The amount of Nb added increases, leading to an increase in manufacturing cost. Further
Since the precipitation amount of TiC and NbC increases, the grain growth is hindered and the r value deteriorates, the smaller the C content is, the better the upper limit is 0.007.
%. From the viewpoint of steelmaking technology, the lower limit of C content is preferably 0.0005%.

Si: Si is added mainly for the purpose of deoxidizing molten steel, but if the addition amount is too large, the surface properties, the chemical conversion treatment property or the coating property are deteriorated, so the content is made 0.1% 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- 0.50%
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 the content is limited to 0.12% 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 thereof is suppressed to 0.015% or less.

Al: Al is added for the purpose of deoxidizing molten steel, but its content is
If less than 0.005% in l.Al, the purpose will not be achieved,
On the other hand, if it exceeds 0.05%, 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 0.005 to 0.05%.

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 006% or less.

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 C that has entered during the annealing in the carburizing atmosphere in the subsequent step is immediately combined with the solid solution Ti or Nb, and the grain boundary segregation of C occurs. Since it prevents, it is not possible to prevent the secondary processing brittleness, and the workability is deteriorated due to hardening by excessive Ti and Nb, which leads to an increase in cost.

B: B is an element effective for the secondary work embrittlement resistance, so that it can be added if necessary. If added, at least 0.0001% or more is necessary to obtain the effect, but if it exceeds 0.0030%, the effect is saturated and the r value is lowered, so the addition amount is in the range of 0.0001 to 0.0030%. And

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

In the past, it was known that the cold rolling rate should be about 90% in order to improve the r value of the cold rolled steel sheet. However, due to the capacity of the cold rolling machine, the cold rolling rate under such high pressure Was difficult to employ. As a result of intensive studies to solve this problem, the present inventors have made a part of the cold-rolled texture at the hot-rolling stage, and subsequently made it complete by cold-rolling. It has been found that the required shape, precision, and surface quality can be created in the cold rolling stage.

Specifically, the steel having the above chemical composition is melted and cast by a conventional method, and the subsequent hot rolling is performed under specific conditions.

That is, after performing rough rolling in the temperature range of 900 to 1200 ° C, hot rolling with a reduction amount R _{1 in} the unrecrystallized ferrite region in the range of 600 to 850 ° C, and then winding at 600 ° C or less. .

Then, after pickling, a reduction amount R ₂ ≧ 50% (however, the total reduction amount R of R ₁ and R ₂ is 95%>R> 65%) is cold-rolled. here,
The total rolling reduction of R ₁ and R ₂ R is defined as R = {1- (1-R 1/100) · (1-R 2/100)} × 100.

Further, after this cold rolling, continuous annealing is performed in a range of the recrystallization temperature or higher in a carburizing atmosphere to form a texture in the (111) plane orientation advantageous for the r value.

As is already known, the r value is mainly for steel (111).
It depends on the plane orientation texture, and it is known that solid solution C and N existing before recrystallization annealing have an adverse effect on the formation thereof. By the treatment, solid solution C and solid solution N are completely removed, and the above-mentioned texture is obtained. Moreover, 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, TiC and C that are not fixed as Nb segregate at the grain boundaries and improve the secondary work embrittlement resistance.

Carburizing gas with controlled carbon potential is used in the atmosphere of continuous annealing, and the target amount of carburization is controlled by selecting the combination of carbon potential, annealing temperature, and annealing time. The continuous annealing may be carried out under the condition that the amount of C required to fill the defects of 2 to 15 ppm. If it is less than 2 ppm, the amount of C necessary to fill the defects at the grain boundaries in order to obtain resistance to secondary working brittleness is insufficient, while if it exceeds 15 ppm, the workability such as elongation deteriorates, and continuous annealing It is not desirable because the plate speed must be reduced and the productivity is reduced. The residence time in the continuous annealing furnace is preferably in the range of 2 sec to 2 min.

Next, examples of the present invention will be described.

(Example) A test steel having a chemical composition shown in Table 1 and having a thickness of 50 mm was heated to 1150 ° C.
After heat treatment for 30 minutes at 1100 ~ 950 ℃
Rough rolling was performed in the temperature range of 1, and the hot rolling steel sheet was manufactured by variously changing the subsequent finish rolling temperature and the total reduction amount in the ferrite region. The winding process was simulated by holding at 400 ° C for 1 hr and cooling the furnace.

Then, after pickling, the cold rolling rate was variously changed to produce 0.8 mm thick cold rolled steel sheet, which was continuously annealed in a carburizing atmosphere.
Recrystallization annealing was performed at 850 ° C. for 1 minute.

The r value of the obtained cold rolled steel sheet and the secondary working brittleness limit temperature
In addition to being shown in the table, some of them are arranged and shown in FIGS. 1 and 2.

In the brittleness test, the cup obtained by forming a cup with a total drawing ratio of 2.7 was trimmed to a height of 35 mm, then placed in a refrigerant at each test temperature and pushed into a conical punch with an apex angle of 40 ° to make it brittle. The critical temperature at which breakage did not occur was measured. This 2
The next processing brittleness limit temperature was set.

As is clear from Table 2, in all the examples of the present invention, the r value is high, the deep drawability is excellent, and the secondary work embrittlement resistance is improved.

On the other hand, the comparative example subjected to continuous annealing in an inert gas is inferior in deep drawability or secondary work embrittlement resistance, and the comparative example subjected to continuous annealing in a carburizing atmosphere gas is outside the scope of the present invention. Since it has a chemical component, it is inferior in either deep drawability or secondary work embrittlement resistance.

Note that FIG. 1 shows the relationship between the total reduction R and the r value at 800 ° C. or less using steel No. 1 in Table 1, and the total reduction R is 65% or more and the high r It turns out that it shows a value.

Fig. 2 shows the values of (Ti * / 45 + Nb / 93) / (C / 12) and r values for steels with a total reduction R of 800 ° C or less of 65% or more and 2
It is a summary of the relationship with the next working brittleness limit temperature, by performing continuous annealing in the carburizing atmosphere according to the present invention for the steel of which the value of this formula is within the range of the present invention (1 to 4.5),
It can be seen that an excellent high r value is obtained and the secondary working brittleness limit temperature is lowered.

(Effects of the Invention) As described in detail above, according to the present invention, by using ultra-low carbon steel, and controlling the chemical composition thereof and the rolling conditions, the solid solution C, N before continuous annealing can be obtained. Is zero,
Next, continuous annealing is performed in a carburizing atmosphere gas, so cold-rolled steel sheets with excellent deep drawability and resistance to secondary work embrittlement can be obtained. Moreover, productivity is high, and the capacity of cold-rolling machines is excessively burdened. I will not force you.

[Brief description of drawings]

FIG. 1 and FIG. 2 are diagrams showing the characteristics of the cold-rolled steel sheets obtained in the examples, and FIG. 1 shows the relationship between the total reduction R and the r value at 800 ° C. or less. Figure 2 shows (Ti * / 45 + Nb / 9
The relationship between the value of 3) / (C / 12), the r value, and the secondary working brittleness limit temperature is shown.

Claims (2)

[Claims] 1. In% by weight (hereinafter the same), C: 0.007% or less, Si: 0.1% or less, Mn: 0.05 to 0.50%, P: 0.12% or less,
S: 0.015% or less, sol.Al: 0.005 to 0.05%, N: 0.006% or less, and by adding Ti and Nb alone or in combination, the effective Ti amount according to the following formula (1) (expressed as Ti *) And the relationship between the amount of Nb and the amount of C is expressed by the following equation (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 being Fe and inevitable impurities at 900-1200 ° C After performing rough rolling in the temperature range, hot rolling with a reduction amount R _{1 in} the unrecrystallized ferrite region in the range of 600 to 850 ° C., then winding at 600 ° C. or less, pickling, and further reduction ratio R ₂ ≧ 50% (provided that the total reduction rate R = {1- (1-R 1/100 of the R ₁ and _{R 2) · (1-R} 2/10
0)} × 100 is 95%>R> 65%), and is further annealed in a temperature range above the recrystallization temperature in a carburizing atmosphere gas. Secondary process Cold-rolled steel sheet manufacturing method with excellent brittleness. 2. The method according to claim 1, wherein the steel contains B: 0.0001 to 0.0030%.