JPH0730411B2 - Method for producing hot rolled steel sheet with excellent deep drawability - Google Patents

Description

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

[Conventional technology]

2. Description of the Related Art Conventionally, deep drawing thin steel sheets used for automobile steel sheets and the like are required to have high Rankford value (r value) and high ductility (El) as properties. Such a deep-drawing steel sheet is generally used as a cold-rolled steel sheet produced by finishing hot rolling at a transformation point of Ar ₃ or higher and then cold rolling to obtain a thin sheet having a final thickness, and then performing recrystallization annealing. It had been. However, in recent years, for the purpose of cost reduction, there has been an increasing demand to replace a member that has conventionally used a cold-rolled steel sheet with a hot-rolled steel sheet.

However, in order to secure workability, particularly ductility, the conventional hot-rolled steel sheet for working avoids the formation of an unrecrystallized ferrite structure and finishes rolling at the Ar ₃ transformation point or higher. for that reason,
In general, the texture is randomized during the γ → α transformation, so the deep drawability of hot-rolled steel sheets was significantly inferior to that of cold-rolled steel sheets.

Several methods for producing hot-rolled steel sheets having excellent deep drawability have been disclosed. For example, in JP-A-59-226149, C / 0.00
2%, Si / 0.02%, Mn / 0.23%, P / 0.009%, S / 0.008%, Al / 0.0
25%, N / 0.0021%, Ti / 0.10% low carbon Al killed steel 500 ~
An example of manufacturing a thin steel sheet having a property of r = 1.21 is shown by rolling a steel sheet having a thickness of 1.6 mm by rolling 76% while applying lubricating oil at 900 ° C. However, since strong lubrication rolling must be performed during hot rolling, operational difficulties such as defective biting of steel sheets and slippage are involved.

Further, in JP-A-62-192539, C / 0.008%, Si / 0.04%
%, Mn / 1.53%, P / 0.015%, S / 0.004%, Ti / 0.068%, Nb / 0.0
By applying a 92% rolling 24% of a low-carbon Al-killed steel by Ar ₃ ~Ar ₃ + 150 ℃, Preparation of thin steel sheet having the characteristics of r = 1.41 is shown.

However, in the above method, the hot rolling is completed in the γ region and the transformation texture due to the γ → α transformation thereafter is used, so the anisotropy of the r value inevitably becomes large, and Δr = −1.2
Is very large, and there is a limit to the r value that can be obtained.

[Problems to be Solved by the Invention]

In the present invention, steel components and rolling conditions, in particular, by appropriately controlling the roll diameter and initial plate thickness during finish rolling, the cold rolling step or cold rolling-annealing step is omitted, and a conventional cold rolled steel sheet is used. It is an object of the present invention to provide a method for manufacturing a thin steel sheet having comparable deep drawability.

[Means for Solving the Problems]

As a result of intensive studies, the present inventors have found that the hot-rolled steel sheet for deep drawing can be manufactured by regulating the manufacturing conditions as follows. The gist is 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, Ti: 0.01 to 0.20% by weight and / or Nb: 0.001 to 0.040% by weight, and the content of C, N, S and the content of Ti and / or Nb are 1.2 (C / 12 + N / 14 + S / 32) <(Ti / 48 + Nb / 93) For Ti-free steel, 1.2 (C / 12 + N / 14) <(Nb / 93) steels with a temperature of 600 ° C or more below Ar ₃ transformation point In the area
At least one pass has a roll radius: R (mm) and a plate thickness before rolling by the roll: t (mm) of R ≦ 200 and And the total rolling reduction below the Ar ₃ transformation point is 60.
After rolling more than 100%, coiling is performed under the conditions that the rolling finish temperature (FDT) and coiling temperature (CT) satisfy (FDT)-(CT) ≤ 100 ° C and (CT) ≥ 600 ° C. And a method for producing a hot-rolled steel sheet excellent in deep drawability.

Further, the present invention contains 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, Ti: 0.01 to 0.20% by weight and / or Nb: 0.001 to 0.040% by weight, and the content of C, N, S and the content of Ti and / or Nb are 1.2 (C / 12 + N / 14 + S / 32) <(Ti / 48 + Nb / 93) For Ti-free steel, a steel with a relationship of 1.2 (C / 12 + N / 14) <(Nb / 93) is used at a temperature of 500 ° C or more below the Ar ₃ transformation point. In the area
At least one pass has a roll radius: R (mm) and a plate thickness before rolling by the roll: t (mm) of R ≦ 200 and And the total rolling reduction below the Ar ₃ transformation point is 60.
%, And then recrystallization annealing is performed, which is a method for producing a hot-rolled steel sheet having excellent deep drawability.

Further, the present invention is preferable to use a steel in which B: 0.0001 to 0.0020 wt% is further added to this steel component, and prior to hot rolling in a temperature range below the Ar ₃ transformation point, 950 ° C. or less Ar ₃ transformation It is more preferable to carry out rolling that ends in the temperature range above the point, and then continue rolling in the temperature range below the Ar ₃ transformation point.

[Action]

Hereinafter, the research results which are the basis of the numerical limitation of the present invention will be described. C: 0.002%, Si: 0.01%, Mn: 0.1%, P: 0.012%, S: 0.012
%, Al: 0.045%, N: 0.002%, Ti: 0.04%, Nb: 0.010% of the hot-rolled sheet is heated at 700 ℃ -soaking, 60% rolling in one pass, then 700 ℃ It was subjected to a self-annealing treatment for 1 hr. The finish rolling is a non-lubricating rolling, and the initial thickness is 1.
It was set to 2 mm. At this time, rolling roll radius: R (mm) 50 to 300
I changed it. The effect of the roll radius on the r value of the hot rolled sheet is shown in FIG. The r value strongly depends on the roll radius, and R
It was remarkably improved by setting ≦ 200.

Further, using the same hot-rolled sheet, after heating and soaking at 700 ° C., 60% rolling was performed in one pass, and subsequently, a self-annealing treatment at 700 ° C.-1 hr was performed. The finish rolling was a non-lubricating rolling, and the roll radius used was constant at 180 mm,
The initial plate thickness was changed to 1 to 20 mm. Roll radius and initial thickness affecting r value of hot rolled sheet: The effect of is shown in FIG. The r value is Strongly depends on It was markedly improved.

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 important, 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 wt% N: 0.008 wt% or less, Ti: 0.01 to 0.20 wt% and / or Nb: 0.001 to 0.040 wt%, and C, N, S contents and Ti and Nb contents The amount should be 1.2 (C / 12 + N / 14 + S / 32) <(Ti / 48 + Nb / 93) for Ti-containing steel and 1.2 (C / 12 + N / 14) <(Nb / 93) for Ti-free steel. If the steel components do not satisfy the above relationship, excellent deep drawability cannot be obtained. further,
B: 0 for improving secondary work embrittlement resistance and r-value anisotropy.
It is preferable to contain 0001 to 0.0020% by weight.

The reasons for limitation of each component are shown below.

(A) C: 0.08% by weight or less It is preferable that the smaller the amount of C, the more the deep drawability is improved. However, if the content of C is 0.008% or less, it does not exert a bad influence so much, so it is limited to 0.008% by weight or less.

(B) Si: 0.5% by weight or less Si has a function of strengthening steel and is added in a required amount according to desired strength. However, if the added amount exceeds 0.5% by weight, deep drawability is adversely affected. Limited to 0.5% by weight or less.

(C) Mn: 1.0 wt% or less Mn has an action of strengthening steel and is added in a required amount according to desired strength. However, if the added amount exceeds 1.0 wt%, deep drawability is adversely affected. Limited to 1.0% by weight or less.

(D) P: 0.15 wt% or less P has the action of strengthening steel and is added in the required amount according to the desired strength, but if the added amount exceeds 0.15 wt%, it will adversely affect the deep drawability. It was limited to 0.15% by weight or less.

(E) S: 0.02% by weight or less It is preferable that a smaller amount of S improves the deep drawability, but if the S content is 0.02% by weight or less, it does not exert a bad influence so much, so it is limited to 0.02% by weight or less.

(F) Al: 0.010 to 0.10 wt% Al is deoxidized and added as necessary to improve the yield of carbonitride forming elements, but if it is less than 0.010 wt%, the addition effect is not Even if added in excess of 0.10% by weight, no further deoxidizing effect can be obtained, so the content was limited to 0.010-0.10%.

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

(H) Ti: 0.01 to 0.20% by weight Ti is a carbonitride forming element, and is used to reduce the solid solution C and N in steel and preferentially form the {111} orientation, which is advantageous for deep drawability. Is added. If the addition amount is less than 0.01% by weight, there is no effect.On the other hand, if the addition amount exceeds 0.20% by weight, no further effect is obtained, and conversely it leads to deterioration of the steel sheet surface property, so 0.01 to 0.20% by weight. Limited

(I) Nb: 0.001 to 0.040 wt% Nb is a carbide-forming element, which has the effect of reducing the solid solution C in steel and is also effective for refining the microstructure before finish rolling. That is, even if there is no solid solution C or N in the steel, if the structure before finish rolling is coarse, the strain introduced during rolling will not be accumulated, and the {111} orientation will be difficult to form. On the other hand, if the microstructure before finish rolling is fine, strain is likely to be accumulated, and as a result, the {111} orientation is preferentially formed and the deep drawability is improved. Furthermore, it was also clarified that solute 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 0.0
Limited to 01 to 0.040% by weight.

(J) 1.2 (C / 12 + N / 14 + S / 32) <(Ti / 48-Nb / 93) (T
i-Containing Steel) When solid solution C and N do not exist before finish rolling, {111} orientation is preferentially formed after rolling-annealing, and deep drawability is improved. In the present invention, 1.2 (C / 12 + N / 14 + S / 32) <(Ti / 48 + Nb / 93) and the equivalent amount of Ti and Nb with respect to C, N, S are added to form a solid solution C before finish rolling. I found that N disappeared. Further, it was clarified that the r value was improved at that time. Therefore, it was limited to 1.2 (C / 12 + N / 14 + S / 32) <(Ti / 48 + Nb / 93).

(K) 1.2 (C / 12 + N / 14) <(Nb / 93) (Ti-free steel) If solid solution C and N do not exist before finish rolling, {111} orientation is preferentially formed after rolling-annealing. Therefore, the deep drawability is improved. In the present invention, by adding 1.2 (C / 12 + N / 14) <(Nb / 93) and an equivalent amount or more of Nb to C, N, solid solution C, N does not exist before finish rolling. I found it.
Furthermore, it was clarified that the r value was improved at that time. Therefore, it was limited to 1.2 (C / 12 + N / 14) <(Nb / 93).

(L) B: 0.0001 to 0.0020 wt% B is effective in improving the secondary work embrittlement resistance and also in improving the anisotropy of the r value. That is, Ti and /
Alternatively, when Nb and B coexist, the crystal grains before finish rolling become fine, and as a result, the anisotropy (Δr) of the r value becomes small. If the addition amount is less than 0.0001% by weight, there is no effect, while if it exceeds 0.0020% by weight, the deep drawability deteriorates, so the amount was limited to 0.0001 to 0.0020% by weight.

(2) Rolling process The rolling process is important in the present invention and is less than the Ar ₃ transformation point 6
In a temperature range of 00 ° C or higher, the radius of the roll: R (mm) and the plate thickness before rolling by the roll: t (mm) are R ≤ 200 for at least one pass and And the total rolling reduction below the Ar ₃ transformation point is 6
After rolling 0% or more, hot rolling that satisfies the relationship of hot rolling finish temperature (FDT) and coiling temperature (CT) of (FDT)-(CT) ≤ 100 ° C and (CT) ≥ 600 ° C Or at least one pass in the temperature range of 500 ° C or higher below the Ar ₃ transformation point and the roll radius: R (mm) and the plate thickness before rolling by the roll: t
(Mm) is R ≦ 200 and And the total rolling reduction below the Ar ₃ transformation point is 6
It is necessary to perform recrystallization annealing after rolling 0% or more.

Furthermore, in order to further improve the deep drawability, rough rolling is performed at 950 ° C.
It is desirable to finish at the Ar ₃ transformation point or higher.

When the rough rolling is finished in a temperature range of 950 ° C or higher, the structure after rough rolling, that is, before finish rolling becomes coarse, so that the strain introduced during finish rolling becomes difficult to accumulate, and as a result, recrystallization occurs. It becomes difficult to form the {111} orientation later. In addition, when finished in a temperature range below the Ar ₃ transformation point, the {100} orientation is formed during rough rolling, and the deep drawability deteriorates.

On the other hand, 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. Note that the rolling reduction during rough rolling is due to the refinement of the structure.
50% or more is desirable.

Further, when the finish rolling is completed in the temperature range of the Ar ₃ transformation point or higher, the texture by the γ → α transformation becomes random, 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 only the rolling load is increased. Therefore, the rolling temperature was set 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.

Furthermore, if the roll radius is R ≦ 200 and the roll radius and the plate thickness before rolling are And need to. That is, under normal rolling conditions (R> 200 in hot rolling), when rolling is performed below the Ar ₃ transformation point, an additional shearing force acts on the steel sheet surface layer due to the frictional force between the roll and the steel sheet, As a result, the {110} orientation, which is unfavorable for deep drawability, is preferentially formed in the surface layer of the steel sheet, so that the deep drawability deteriorates. However, if R ≦ 200 and As a result, it was revealed that the {110} orientation of the surface layer of the steel sheet decreases and the {111} orientation also increases, so that R ≦ 200 and Limited. It is considered that the effects of the roll diameter and the plate thickness before rolling are due to changes in the deformation mode and deformation mechanism during rolling, but details are not clear. In addition,
Lubricant rolling does not have to be performed in the present invention, but performing lubrication rolling in order to further secure deep drawability, improve roll surface properties, and reduce rolling load does not defeat the purpose of the present invention. Absent.

The effect of the roll diameter and the initial plate thickness in the present invention is effective only in the ordinary rolling type, and is not effective for those having a different deformation mode from the ordinary rolling such as a planetary mill. Absent.

In the case of the wound self-annealed material that is not subjected to recrystallization annealing after rolling, recrystallization is not completed unless the winding temperature is 600 ° C or higher, so CT ≧ 600 ° C was set. Further, in order to improve the deep drawability, it is advantageous that the rolling temperature is low and the winding temperature is high. Therefore, hot rolling finish temperature (FDT) and coiling temperature (CT)
It is necessary to carry out rolling under the conditions that and (FDT)-(CT) ≤ 100 ° C. Note that, for those that are subjected to recrystallization annealing after hot rolling, winding self-annealing is not necessary, so the hot rolling end temperature may be 500 ° C. or higher, and the winding temperature may be low.

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

〔Example〕

Since the effect of the roll radius and the initial plate thickness in the present invention is effective if at least one pass is carried out at the time of finish rolling, in the present invention, it is carried out in the latter three stands out of seven stands, and the former four stands are ordinary rolls. Rolled in diameter.

A steel slab having the composition shown in Table 1 was heated to 1150 ° C.-after soaking,
After rough rolling, finish rolling was performed. Rough rolling finish temperature (RDT), finish rolling finish temperature (FDT), reduction ratio in the temperature range below Ar ₃ transformation point of 600 ℃ or more, coiling temperature (C
Table 2 shows T), the presence or absence of lubrication, the roll diameter of the latter three stands, and the plate thickness before rolling. The finished plate thickness is 1.2 mm.

Table 2 shows the material properties of the hot rolled sheet after pickling. Tensile properties are
It was measured using a JIS No. 5 tensile test piece. Also, the r value is 15%
After applying tensile pre-strain, it is measured by 3 point method, L direction (rolling direction), D direction (45 degree direction to rolling direction) and C
The average value of the direction (the rolling direction in the direction of 90 degrees) and anisotropic _{= (r L + 2r D +} r C) / 4, △ r = calculated as _{(r L + 2r D + r} C) / 2. Further, as the evaluation of the secondary processing brittleness resistance,
A cylindrical sample processed at a limiting drawing ratio of 3.8 was cooled to −50 ° C., and then a crush test was performed to evaluate whether or not brittle cracking occurred.

It can be seen that the hot-rolled steel sheet produced within the scope of the present invention has excellent deep drawability and secondary work embrittlement resistance as compared with the comparative example.

Further, a steel slab having the composition shown in Table 1 was heated and soaked at 1150 ° C., rough rolling was performed, and then finish rolling was performed. Rough rolling finish temperature (RDT), finish rolling finish temperature (FDT), Ar
Table 3 shows the rolling reduction (CT), the presence or absence of lubrication, the roll radius of the latter three stands and the sheet thickness before rolling in the temperature range of less than _three transformation points and 500 ° C or more. The finish plate thickness is 1.
6 mm. After pickling the rolled plate, 830 for No. 16-23
Rapid heating annealing of ℃ -60s, 75 for No.24 ~ 30
Box annealing was performed at 0 ° C for 5 hours.

Table 3 shows the material properties of the hot rolled sheet after annealing. It can be seen that the hot-rolled steel sheet produced within the scope of the present invention has excellent deep drawability and secondary work embrittlement resistance as compared with the comparative example.

[Advantages of the Invention] In the present invention, it is possible to manufacture a hot-rolled steel sheet having excellent deep drawability equivalent to that of a cold-rolled steel sheet, and it is possible to realize a significant cost reduction as compared with the production of a conventional cold-rolled steel sheet.

[Brief description of drawings]

Fig. 1 is a graph showing the influence of roll radius on the values, and Fig. 2 is the roll radius on the values and the sheet thickness before rolling: It is a graph which shows the influence of.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Hideo Abe, 1 Kawasaki-cho, Chiba-shi, Chiba, Kawasaki Steel Co., Ltd. Technical Research Headquarters (72) Inventor, Masao Kawata 1 Kawasaki-machi, Chiba, Chiba Prefecture Kawasaki Steel Co., Ltd. Research headquarters

Claims (4)

[Claims] 1. 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, Ti: 0.01 to 0.20% by weight and / or Nb: 0.001 to 0.040% by weight, and the content of C, N, S and the content of Ti and / or Nb are 1.2 ( C / 12 + N / 14 + S / 32) <(Ti / 48 + a Nb / 93) Ti the free steel 1.2 (C / 12 + N / 14) <(Nb / 93) the relationship of the steel, Ar 600 ° C. or more than ₃ transformation point In the temperature range,
At least one pass has a roll radius: R (mm) and a plate thickness before rolling by the roll: t (mm) of R ≦ 200 and And the total rolling reduction below the Ar ₃ transformation point is 60.
% After rolling, the rolling finish temperature (FDT) and coiling temperature (CT) satisfy the relationship of (FDT)-(CT) ≤ 100 ° C and (CT) ≥ 600 ° C. A method for manufacturing a hot rolled steel sheet having excellent deep drawability. 2. 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, Ti: 0.01 to 0.20% by weight and / or Nb: 0.001 to 0.040% by weight, and the content of C, N, S and the content of Ti and / or Nb are 1.2 ( C / 12 + N / 14 + S / 32) <(Ti / 48 + Nb + 93) In the Ti-free steel, 1.2 (C / 12 + N / 14) <(Nb / 93) steels with a relationship of 500 ° C or more below Ar ₃ transformation point so,
At least one pass has a roll radius: R (mm) and a plate thickness before rolling by the roll: t (mm) of R ≦ 200 and And the total rolling reduction below the Ar ₃ transformation point is 60.
% Rolling or more, and then recrystallization annealing is performed, which is a method for producing a hot-rolled steel sheet having excellent deep drawability. 3. The method for producing a hot-rolled steel sheet having excellent deep drawability according to claim 1 or 2, wherein a steel containing B: 0.0001 to 0.0020% by weight is further added to the steel composition. 4. Prior to the rolling temperature region of less than Ar ₃ transformation point, performs rolling to end at 950 ° C. or less Ar ₃ transformation point or more temperature range, subsequently rolled in a temperature range of Ar less than ₃ transformation point, The method for manufacturing a hot-rolled steel sheet having excellent deep drawability according to claim 1, 2 or 3.