JPH0784622B2 - Method for producing hot-rolled steel sheet for deep drawing excellent in secondary processing brittleness resistance - Google Patents

Description

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

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

As a method for manufacturing a hot rolled steel sheet that satisfies such requirements,
Rolling by applying hot rolling in the unrecrystallized ferrite region using steel that can fix C and N in steel by adding carbonitride forming elements such as Ti and Nb to ultra-low carbon steel alone or in combination Forming a texture, and then performing recrystallization annealing by various methods,
A method for developing a (111) texture, which is advantageous for deep drawing, has been proposed.

(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.

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 hot-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 solute C and N lowering the press formability affect the local slip system and rearrangement of dislocations in the rolling texture formation stage and the recrystallization texture formation stage, 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 solution C and N are kept in a state of zero until the completion of recrystallization at the time of 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% with the balance Fe and unavoidable impurities is roughly rolled in the usual process, then hot-rolled at a total reduction of 60% or more in the range of 400 to 800 ° C, wound, and acid After the washing, the method for producing a deep-drawing hot-rolled steel sheet excellent in secondary work embrittlement resistance is characterized by further performing continuous annealing in a carburizing atmosphere gas in a temperature range above the recrystallization temperature. is there.

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 of 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.

Steel having the above chemical composition is melted and cast by a conventional method, and then rough rolling is performed, but hot rolling needs to be performed under the following conditions.

That is, after rough rolling, hot rolling is performed at a total rolling reduction of 60% or more in a temperature range of 400 to 800 ° C., and then wound. This is high r
This is because it is necessary to obtain a rolling texture during hot rolling in order to obtain a hot-rolled steel sheet having a value, and therefore an unrecrystallized ferrite region, that is, in a temperature range of 400 to 800 ° C., and 60% or more, preferably Performs hot rolling at a reduction ratio of 60% to 90%. In this case, the better the lubrication conditions, the more homogeneous the texture can be obtained in the plate thickness direction, and the higher r value can be obtained. Therefore, it is desirable to select the lubricant according to the target r value.

The winding temperature is not particularly limited.

Further, after pickling, continuous annealing is performed in a carburizing atmosphere at a recrystallization temperature or higher, preferably at a recrystallization temperature or higher and Ac ₃ point or lower to form a texture in the (111) plane orientation that is 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 the recrystallization annealing have a bad influence on the formation, but in the present invention, the winding treatment before the recrystallization annealing. By solute C and solute N are completely removed, 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 entered from the carburizing atmosphere
C, which is not fixed as TiC and Nb, segregates at the grain boundaries and improves 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 hot rolled steel sheet, recrystallization annealing was performed at 850 ° C. for 1 minute as continuous annealing in a carburizing atmosphere and in an inert gas.

The r-value and the secondary working brittleness limit temperature of the obtained hot rolled steel sheet are
Along with the table, some of them are arranged and shown in FIG.

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 fracture did not occur was measured, and this was taken as the secondary working brittleness critical temperature.

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.

In Fig. 1, the value of (Ti * / 45 + Nb / 93) / (C / 12) and r
This is a summary of the relationship between the value and the secondary working brittleness limit temperature. Hot rolling according to the present invention and continuous annealing in a carburizing atmosphere are performed on steels whose values in this formula are within the range of the present invention (1 to 4.5). As a result, it is found 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 by controlling the chemical composition and hot rolling conditions, the solid solution C before continuous annealing, N is zero,
Then, since continuous annealing is performed in a carburizing atmosphere gas, a hot-rolled steel sheet having excellent deep drawability and secondary work embrittlement resistance can be obtained, and the productivity is high.

[Brief description of drawings]

FIG. 1 shows (Ti * / 45) in the hot rolled steel sheets obtained in the examples.
It is a figure which shows the relationship of the value of + Nb / 93) / (C / 12), r value, and secondary work embrittlement limit temperature.

Claims (2)

[Claims] 1. By weight% (hereinafter, the same), C: 0.007% or less, Si: 0.1% or less, M: n0.05 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 Fe and inevitable impurities After rolling, hot rolling with a total reduction of 60% or more in the range of 400 to 800 ℃, winding,
A method for producing a hot-rolled steel sheet for deep drawing excellent in secondary work embrittlement resistance, which comprises performing continuous annealing in a temperature range of a recrystallization temperature or higher in a carburizing atmosphere gas after pickling. 2. The method according to claim 1, wherein the steel contains B: 0.0001 to 0.0030%.