JPH0776381B2 - Manufacturing method of cold-rolled steel sheet for deep drawing - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a cold-rolled steel sheet having excellent deep drawability, which is used for automobile steel sheets and the like.

[Conventional technology]

Cold-rolled steel sheets used for automobile panels and the like are required to have excellent deep drawability. In order to improve the deep drawability, high Rankford value (r value) and high ductility (El) are required as mechanical properties of the steel sheet.

BACKGROUND ART Conventionally, assembly of an automobile body is performed by spot welding a number of press parts. Recently, there has been an increasing demand for increasing the number of parts and welding by increasing the size and integration of some of these parts. For example,
Due to the plurality of shapes, the oil pan of an automobile is currently welded and completed. There is a strong demand from automobile manufacturers to integrally mold this. Also,
In response to diversifying needs, automobile designs are becoming more complex, which increases the number of parts that are difficult to form using conventional steel sheets. In order to meet these demands, a cold rolled steel sheet having deep drawability far superior to the conventional one has been required.

Conventionally, various methods have been proposed for improving deep drawability. It is known that the deep drawability of a steel sheet is closely related to its texture, and that the more the {222} orientation is and the less the {200} orientation is, the higher r value is obtained. As a conventional method for obtaining this high r value, for example, cold-rolled low carbon steel sheets as disclosed in JP-B-44-17268, JP-B-44-17269, and JP-B-44-17270 can be used. A so-called two-stage cold rolling method has been proposed in which rolling is performed twice.

According to such a two-stage cold rolling method, the final product has many {222} oriented grains and few {200} oriented grains. This is because the primary cold rolling-annealing treatment increases {222} oriented grains and decreases {200} oriented grains as compared with the hot rolled steel sheet before cold rolling, and then cold rolling-annealing is performed again. And {222} oriented grains are further increased, while {200} oriented grains are further reduced. Therefore, a steel sheet having a high r value can be manufactured. However, although the two-stage cold rolling method is excellent in that it improves deep drawability, it requires one more cold rolling-annealing process than the conventional process, which requires enormous energy and cost. There was a drawback that it became something like.

On the other hand, in JP-A-56-62926, C / 0.008%, Si
/0.57%, Mn / 0.35%, Al / 0.43%, Nb / 0.061% steel having a composition of components is subjected to ordinary hot rolling-cold rolling and then 950 ° C-1 hr box annealing to give r = 4.73. We are proposing a technology to obtain Since this technique utilizes the formation mechanism of the transformation texture, the recrystallization annealing temperature must be raised above the Ar ₃ transformation point, so the energy cost is lower than that of recrystallization annealing below the Ar ₃ transformation point. And the difficulty in equipment and technology due to the increase in temperature and high temperature annealing. Further, there has been a problem that a large amount of Si or Al has to be added, which deteriorates the surface properties of the steel sheet.

[Problems to be Solved by the Invention]

An object of the present invention is to provide an advantageous method for producing a cold-rolled steel sheet having markedly excellent deep drawability.

Another object of the present invention is to increase the energy and cost mainly by adopting a novel method by a combination of hot rolling conditions and composition of components, namely addition of an extra cold rolling process and adjustment of only steel composition. To overcome.

[Means for Solving the Problems]

As a result of intensive studies, the present inventors have found that it is possible to manufacture a cold-rolled steel sheet excellent in ultra-deep drawability by controlling the manufacturing conditions as follows.

A first invention of the present invention is a method for manufacturing a cold-rolled steel sheet for deep drawability, which comprises the following steps.

C: 0.008 wt% or less, Si: 0.5 wt% or less, Mn: 1.0 wt% or less, P: 0.15 wt% or less, S: 0.02 wt% or less, Al: 0.010 to 0.10 wt%, N: 0.008 wt% or less In addition, the addition amount of one or two of Ti and Nb is 1.2 (C / 12 + N / 14) ≤ (Ti / 48 + Nb / 93), and steel consisting of the balance unavoidable impurities and iron is used, and less than Ar ₃ transformation point. Roll radius: R over 600 ℃
(Mm) and the plate thickness before rolling by the roll: t (mm) and the friction coefficient: μ satisfy the relation of μ ≦ −0.2log (R / t) +0.55, and the Ar ₃ transformation point of less than Total reduction rate is 60%
The above rolling is performed, and the rolling is performed under the conditions that the hot rolling finish temperature (FDT) and the coiling temperature (CT) satisfy (FDT)-(CT) ≤ 100 ° C and (CT) ≥ 600 ° C. Cold rolling at a reduction rate of 50 to 95% and recrystallization annealing.

Moreover, the 2nd invention of this invention is a manufacturing method of the cold-rolled steel sheet for deep drawing which consists of the following processes.

C: 0.008 wt% or less, Si: 0.5 wt% or less, Mn: 1.0 wt% or less, P: 0.15 wt% or less, S: 0.02 wt% or less, Al: 0.010 to 0.10 wt%, N: 0.008 wt% or less and Ti, with one or two added amount of Nb is 1.2 (C / 12 + N / 14) ≦ (Ti / 48 + Nb / 93), using a steel consisting of the balance incidental impurities, Ar less than ₃ transformation point 500 ° C. Roll radius: R in the above temperature range
(Mm) and sheet thickness before rolling by the roll: t (mm) and the coefficient of friction: mu and the μ ≦ -0.2log (R / t) of less than Ar ₃ transformation point under conditions satisfying the Tasu0.55 the relationship Total reduction rate is 6
Roll at 0% or more, perform recrystallization annealing, then cold roll at a reduction rate of 50 to 95%, and perform recrystallization annealing.

In addition, in the first invention and the second invention, in addition to the components of the steel, B: 0.0001 to 0.0020 wt% and / or Sb: 0.001 to 0.020 wt% are included, which is a deep drawing cold steel. It is a method of manufacturing a rolled steel sheet.

[Action]

Hereinafter, the research results which are the basis of the numerical limitation of the present invention will be described.

C: 0.002 wt%, Si: 0.02 wt%, Mn: 0.12 wt%, P: 0.010 wt%, S: 0.011 wt%, N: 0.002 wt%, Ti: 0.04 wt%, Nb: 0.013 wt% The hot-rolled steel sheet thus obtained was rolled at 700 ° C. for 60% in one pass, and subsequently subjected to a winding self-annealing treatment at 700 ° C. for 1 hour. At this time, roll radius: 300 mm, plate thickness: 5 mm,
The friction coefficient can be changed from 0.1 to 0.25 by changing various lubricating conditions.
Was changed in the range. The rolled steel sheet was pickled, cold rolled at 75%, and then recrystallized at 850 ° C. for 20 seconds. The effect of friction coefficient on the value of annealed steel sheet
Shown in the figure. The value strongly depends on the friction coefficient, and the value was remarkably improved by setting μ ≦ 0.19.

Also, using the same hot-rolled steel sheet, keeping the friction coefficient constant at μ = 0.19 and changing the roll radius and sheet thickness, log (R /
t) was variously changed. The effect of log (R / t) on the value of the cold rolled sheet after cold rolling-annealing is shown in FIG. The value is log (R
It strongly depends on / t) and the value is remarkably improved by setting log (R / t) ≤ 1.8. Based on the above experimental results, the scope of the present invention was limited as follows.

(1) Steel composition In the present invention, steel composition is by weight, C: 0.008% by weight or less, Si: 0.5% by weight or less, Mn: 1.0% by weight or less, P: 0.15% by weight or less, S: 0.02% by weight or less , Al: 0.010 to 0.10% by weight, N: 0.008% by weight or less, and the addition amount of one or two of Ti or Nb is 1.2 (C / 12 + N / 14) ≤ (Ti / 48 + Nb / 93). I won't.

Furthermore, in order to improve the secondary processing brittleness resistance, B: 0.0001 to 0.00
20% by weight and Sb: 0.0 to prevent nitrification during batch annealing
It is preferable to add 01 to 0.02% by weight.

If the steel components do not satisfy the above relationship, excellent deep drawability cannot be obtained.

The reasons for limitation of each component are shown below.

(A) C: 0.008% by weight or less It is preferable that the smaller the C content, the better the deep drawability. When the content is 0.008% by weight or less, it does not exert a bad influence so much, so the content is limited to 0.008% by weight or less.

(B) Si: 0.5 wt% or less Si has a function of strengthening steel, and is added in a required amount according to desired strength. If the content exceeds 0.5% by weight, the deep drawability is adversely affected, so the content was limited to 0.5% by weight or less.

(C) Mn: 1.0 wt% or less Mn has a function of strengthening steel, and is added in a required amount according to desired strength. If the content exceeds 1.0% by weight, the deep drawability is adversely affected, so the content was limited to 1.0% by weight or less.

(D) P: 0.15 wt% or less P has the effect of strengthening steel, and is added in the required amount according to the desired strength. If the content exceeds 0.15% by weight, the deep drawability is adversely affected, so the content was limited to 0.15% by weight or less.

(E) S: 0.02 wt% or less It is preferable that the S content is as small as possible because the deep drawability is improved. When the content is 0.02% by weight or less, it does not exert a bad influence so much, so the content is limited to 0.02% by weight or less.

(F) Al: 0.010 to 0.10 wt% Al is deoxidized and is added as necessary for improving the yield of carbonitride forming elements. If the addition amount is 0.010% by weight or less, there is no addition effect, while if it exceeds 0.10% by weight, a further deoxidizing effect cannot be obtained, so the Al content is 0.010 to
It was limited to 0.10% by weight.

(G) N: 0.008% by weight or less It is preferable that the smaller the amount of N, the better the deep drawability. When the content is 0.008% by weight or less, it does not exert a bad influence so much, so the content is limited to 0.008% by weight or less.

(H) Ti: 0.01 to 0.20 wt% Ti is a carbonitride forming element, which reduces the amount of C and N dissolved in steel in a solid solution and forms grains of {111} orientation, which is advantageous for deep drawability. Form preferentially. If the addition amount is less than 0.01% by weight, the above effect is not obtained, while if it is added in excess of 0.20% by weight, no further effect is obtained, and on the contrary, the surface property of the steel sheet is deteriorated. Therefore, the Ti content is limited to 0.01 to 0.20% by weight.

(I) Nb: 0.001 to 0.40 wt% Nb is a carbide-forming element and has an effect of reducing solid solution C in steel. Further, it is effective for making the structure before finish rolling fine. That is, even if there is no solid solution of C and N in the steel, if the pre-finish rolling structure is coarse, the strain introduced during rolling is not accumulated, so that the {111} orientation is difficult to form. On the other hand, if the pre-finish rolling structure is fine, strain tends to accumulate, resulting in {111}
The orientation is preferentially formed, and the deep drawability is improved. It was also clarified that solid solution Nb has the effect of accumulating strain during rolling. If the content is less than 0.001% by weight, there is no effect, while if it exceeds 0.040% by weight, the recrystallization temperature rises, so the content was limited to 0.001 to 0.040% by weight.

(J) 1.2 (C / 12 + N / 14) ≦ (Ti / 48 + Nb / 93) If solid solution C and solid solution N do not exist before finish rolling, {111} orientation is preferentially formed after rolling-annealing. The deep drawability is improved. Ti or more equivalent to the content of C and N or
It was found that by adding Nb, solid solution C and N disappeared before finish rolling. Further, it was clarified that the r value was improved at that time. Therefore, 1.2 (C / 12 + N
/ 14) ≤ (Ti / 48 + Nb / 93).

(K) B: 0.0001 to 0.0020 wt% B is effective for improving the secondary work embrittlement resistance. The amount added
Less than 0.0001% by weight has no effect, while 0.0020% by weight
If the content exceeds the range, the deep drawability deteriorates, so the amount was limited to 0.0001 to 0.0020% by weight.

(L) Sb: 0.001 to 0.020 wt% Sb is added to prevent nitrification during batch annealing. If its content is less than 0.001% by weight, it has no effect, while 0.02%
If added in excess of weight%, the surface properties of the steel sheet deteriorate, so the content was limited to 0.001 to 0.020 weight%.

(2) Hot rolling step The hot rolling step is important in the present invention. The rolling process is one of the following two.

Roll radius: R in the temperature range below Ar ₃ 600 ° C and above 600 ℃
(Mm), plate thickness before rolling by the roll: t (mm), and friction coefficient: μ are the sum less than Ar ₃ transformation point under the condition that μ ≦ −2log (R / t) +0.55 Rolling with a rolling reduction of 60% or more, and then hot rolling finish temperature (FD
Hot rolling is performed so that T) and coiling temperature (CT) satisfy the relations of (FDT)-(CT) ≤ 100 ° C and (CT) ≥ 600 ° C.

Alternatively, in the temperature range below the Ar ₃ transformation point and above 500 ° C, roll radius: R
(Mm), plate thickness before rolling by the roll: t (mm), and friction coefficient: μ satisfy the condition of μ ≤ −0.2 log (R / t) + 0.55, and if the Ar ₃ transformation point is less than Rolling with a total reduction of 60% or more is performed, and then recrystallization annealing is performed.

Furthermore, in order to further improve the deep drawability, it is preferable to finish the rough rolling at 950 ° C. or lower at the Ar ₃ transformation point or higher and set the hot rolling start temperature (FET) to 800 ° C. or lower. . That is, when rough rolling is completed in a temperature range of 950 ° C. or lower and Ar ₃ transformation point or higher, the structure before finish rolling becomes finer, so that strain introduced during finish rolling tends to be accumulated, resulting in { The 111} orientation is preferentially formed, and the deep drawability is improved. The rolling reduction during rough rolling is preferably 50% or more in order to refine the structure. Also, when the FET temperature is below 800 ° C,
Since the rolling reduction in the low temperature region is high, the amount of strain of {111} oriented grains introduced during rolling increases and {1} after recrystallization annealing.
11} orientation is preferentially formed.

Further, when the finish rolling is finished in the temperature range of the Ar ₃ transformation point or higher, the texture becomes random due to the γ → α transformation, and excellent deep drawability cannot be obtained. On the other hand, even if the finishing temperature is lowered to 500 ° C. or lower, further improvement in deep drawability cannot be expected, and the rolling load only increases. Therefore, the rolling temperature was limited to 500 ° C or higher below the Ar ₃ transformation point.

If the total rolling reduction below the Ar ₃ transformation point is not 60% or more, the {111} orientation will not be formed during rolling, resulting in poor deep drawability.

Further, it is necessary to set the relationship between the roll radius, the plate thickness before rolling, and the coefficient of friction to be μ ≦ −0.2log (R / t) +0.55. When rolling is performed under the condition of μ> -0.2log (R / t) +0.55 below the Ar ₃ transformation point, an additional shearing force acts on the surface layer of the steel sheet due to the frictional force between the roll and the steel sheet. As a result, {110} orientation, which is unfavorable for deep drawability, is preferentially formed in the surface layer of the steel sheet. Therefore, the deep drawability deteriorates.

However, by setting μ ≤ -0.2log (R / t) + 0.55, it became clear that the {110} orientation of the steel sheet surface layer portion decreases and the {111} orientation also increases. Limited It is considered that the effects of the roll radius and the plate thickness before rolling are due to changes in the deformation mode and deformation mechanism during rolling.

In addition, in the finishing middle stage or finishing rear stage with a relatively small plate thickness, the friction coefficient μ must be extremely small when the roll radius is a normal dimension of 300 mm or more. As a result, operational troubles such as slips are likely to occur. Therefore, in the finishing middle stage or the finishing second stage stand,
The roll radius is 250 mm or less, preferably 200 mm or less.

Incidentally, the effect of the roll radius and the initial plate thickness in the present invention is effective only in the normal rolling type,
For example, a planetary mill, which has a different deformation mode from normal rolling, has no effect.

In the case of the self-annealing coil material that was not subjected to recrystallization annealing after rolling, the recrystallization was not completed unless the coiling temperature was 600 ° C or higher, so the coiling temperature (CT) was set to 600 ° C or higher. Further, in order to improve the deep drawability, it is advantageous that the rolling temperature is low and the winding temperature is high. Therefore, the rolling finish temperature (FD
It is necessary to perform rolling under the condition that (T) and coiling temperature (CT) satisfy the condition of (FDT)-(CT) ≤ 100 ° C. In addition, in the case of performing recrystallization annealing after hot rolling, it is not necessary to perform winding self-annealing, so the hot rolling end temperature may be 500 ° C. or higher, and the winding temperature may be low.

Recrystallization annealing after cold rolling may be either continuous annealing or box annealing. The annealing temperature is preferably in the range of 550 ° C to 950 ° C. The heating rate may also be in the range of 10 ° C / hr to 50 ° C / s.

The steel of the present invention can be applied as various surface-treated thick plates such as hot dip galvanized.

(3) Cold rolling-annealing step The cold rolling-annealing step is important in the present invention. The hot rolled steel sheet is subsequently subjected to cold rolling and recrystallization annealing. In order to obtain super deep drawability, the reduction ratio during cold rolling must be 50 to 95%. Cold rolling rate is 50%
If it is less than the range, excellent deep drawability cannot be obtained. Meanwhile 95
If cold rolling is performed at a rolling reduction of more than%, a texture that adversely affects deep drawability develops. Recrystallization annealing is 700
~ 950 ° C is preferred. The recrystallization annealing may be either continuous annealing or box-type annealing.

The steel sheet of the present invention can be subjected to temper rolling of 0.1 to 5%. Further, the steel sheet according to the present invention may be subjected to various surface treatments such as hot dip galvanizing and can be used as a plated steel sheet.

〔Example〕

Steel slabs (A) to (K) having the compositions shown in Table 1 were heated to 1150 ° C.
After heating to 0 ° C. and soaking, rough rolling was performed and then finish rolling was performed. X (× 10 ^-4 ) in Table 1 is (Ti / 48 + Nb / 93)-
1.2 (C / 12 + N / 14) is shown, and steels C, E, and K are comparative examples.

Rough rolling finish temperature (RDT) and finish rolling start temperature (F
ET), finish rolling end temperature (FDT), winding temperature (CT), difference between finish rolling end temperature and winding temperature (FDT-CT), roll diameter (R), sheet thickness before rolling (t), Z = -0.2log (R / t) +
Table 2 shows 0.55, coefficient of friction (μ) and presence / absence of annealing of hot rolled steel sheet. 720 ° C. × 5 hours in the column of hot rolled sheet annealing indicates batch annealing conditions.

The above steel sheet was continuously hot-rolled, pickled, cold-rolled at a rolling reduction of 75%, and then annealed at 830 ° C. for 40 seconds.

Table 2 shows the material properties and El (%) after cold rolling-annealing. The tensile properties (elongation El) were measured using JIS No. 5 tensile test pieces. In addition, r value is 3 after applying 15% tensile strain.
Measured at point method, the average value of L direction (rolling direction) as r _1, D direction (45 ° direction to the rolling direction) r ₂ and C-direction (rolling direction in the 90 ° direction) r ₃ = (R ₁ + 2r ₂ + r ₃ ) / 4 Sought as. It is clear that the cold-rolled steel sheet produced by the method of the present invention has excellent deep drawability as compared with the comparative example.

〔The invention's effect〕

INDUSTRIAL APPLICABILITY According to the present invention, it is possible to manufacture a cold-rolled steel sheet having deep drawability that is far superior to the conventional one.

[Brief description of drawings]

FIG. 1 is a graph showing the effect of friction coefficient (μ) on the value after cold rolling-annealing, and FIG. 2 is a graph showing the effect of log (R / t) on the value after cold rolling-annealing. .

Claims (4)

[Claims] 1. C: 0.008% by weight or less, Si: 0.5% by weight or less, Mn: 1.0% by weight or less, P: 0.15% by weight or less, S: 0.02% by weight or less, Al: 0.010 to 0.10% by weight, N: A steel containing 0.008% by weight or less and the amount of addition of one or two of Ti and Nb being 1.2 (C / 12 + N / 14) ≦ (Ti / 48 + Nb / 93) and the balance iron and inevitable impurities Rolling radius: R (mm), plate thickness before rolling by the roll: t (mm) and friction coefficient: μ in the temperature range of less than ₃ transformation points and 600 ° C or higher is μ ≦ −0.2log (R / t) Satisfies the relationship of +0.55 and Ar ₃
After rolling at a total reduction of 60% or more below the transformation point, the hot rolling finish temperature (FDT) and coiling temperature (CT) are (FDT)-(CT) ≤ 100 ° C and (CT) ≥ 600 A method for producing a cold-rolled steel sheet for deep drawing, which comprises performing recrystallization annealing after winding under a condition satisfying a relation of ° C, cold rolling at a reduction rate of 50 to 95%. 2. The steel of the composition according to claim 1, in a temperature range of less than 500 ° C. below the Ar ₃ transformation point, roll radius: R (mm) and plate thickness before rolling by the roll: t (mm) and Friction coefficient: μ satisfies μ ≦ −0.2log (R / t) +0.55, and Ar ₃
After performing rolling with a total reduction of 60% or more below the transformation point, recrystallization annealing is performed, followed by cold rolling with a reduction of 50 to 95%, and then recrystallization annealing. Manufacturing method of cold-rolled steel sheet for drawing. 3. In addition to the above steel components, B: 0.0001 to 0.0020
The method for producing a cold-rolled steel sheet for deep drawing according to claim 1 or 2, characterized in that the method comprises the weight percentage. 4. The method for producing a cold-rolled steel sheet for deep drawing according to claim 1, characterized in that Sb: 0.001 to 0.020 wt% is contained in addition to the components of the steel.