JPH09176742A - Hot rolled steel sheet for working small in plane anisotropy in formability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method improving the anisotropy and the balance of strength-ductility of a hot rolled steel sheet by regulating its texture at the time of hot rolling. SOLUTION: At the time of subjecting a slab of a steel contg. <=0.1% C, <=1% Mn, <=0.5% Si, <=0.2% P, <=0.01% N and 0.005 to 1.0% Al and furthermore contg., at need, >=0.0005% B so as to satisfy 11N/l4+0.005% or below to hot rolling, if required, after rough rolling, it is joined to the preceding rough rolling stock, and rolling in which the temp. is regulated to the Ar3 transformation point+100 deg.C or below to the Ar3 transformation point or above and the total draft is regulated to >=50% is executed under the condition of <=0.2 coefficient of friction with lubrication.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a hot-rolled steel sheet for working which has a small in-plane anisotropy of formability. The hot-rolled steel sheet for processing here can also be used as a surface-treated original sheet.

[0002]

2. Description of the Related Art A standard manufacturing process for hot-rolled steel sheets for working is described below.

The pig iron obtained from the blast furnace contains about 4% of C, but by blowing in pure oxygen, C
The content is reduced to 0.1% or less. Currently in Japan,
Most slabs are manufactured by continuous casting. The slab manufactured by continuous casting is subjected to hot rolling through three routes. One is CC-DR (Continuous Ca
sting and Direct Rolling)
Is the most efficient route in terms of thermal energy when it is directly hot-rolled without reheating. In this case, equipment measures must be taken to prevent the temperature of the slab from dropping significantly, and because the slab cannot be maintained,
There are also drawbacks such as possible deterioration of the surface quality.
The second route is a route in which the slab is made into a cold block and then reheated in a heating furnace and subjected to hot rolling. Third,
HCR (Hot Charge) is a method that puts the slab in the heating furnace before completely cooling it between the first and the second.
It is called. The case where the slab temperature is reheated before the γ → α transformation is named A route, and the case where it is once below the γ / α transformation point is named B route. The reheating temperature is generally 1150 ° C to 1250 ° C.

Hot rolling is generally carried out after rough rolling several times and then at a finishing temperature not lower than the Ar ₃ transformation point in a continuous hot rolling mill having 5 to 7 stands, and a hot rolling with a plate thickness of about 1 to 6 mm. Manufacture rolled sheets.

With respect to the above standard manufacturing process, recently,
A technique has been developed in which hot rolling of IF steel is positively carried out at a temperature below the Ar ₃ transformation point (ferrite region). that time,
It was revealed that the lubrication rolling improves the deep drawability (Japanese Patent Laid-Open No. 59-59827), and from the viewpoint of rolling stability, the rough rolled material is connected to the preceding rough rolled material to finish continuously. A rolling technique (Japanese Patent Laid-Open No. 4-224635) is disclosed. This technique positively utilizes texture control in hot rolling, which has not received much attention in the past, and in order to form a texture advantageous for deep drawability, it is necessary to recrystallize the hot rolled sheet. Will be needed.

[0006]

However, the above-mentioned Ar ₃
Generally, a hot-rolled steel sheet manufactured at a finishing temperature equal to or higher than the transformation point generally exhibits a large in-plane anisotropy of inverted V type. This in-plane anisotropy is a cause of fluctuations in the plate thickness during molding and, in some cases, also causes breakage easily. This problem is particularly apparent in thin materials.

Further, in the case of the technique in which the above IF steel is finish-rolled below the Ar ₃ transformation point and then recrystallized, if it is wound by the existing hot strip equipment, the run-out table is long and the plate temperature drops. Only by the winding process, recrystallization does not sufficiently occur, and it is difficult to obtain excellent characteristics.
As a measure to solve these problems, it is conceivable to subject the hot-rolled sheet to recrystallization treatment by continuous annealing, but in this case, there is an economical disadvantage that the manufacturing cost becomes high. The texture control by hot rolling in the ferrite region contributes to the improvement of the r value, but is not necessarily effective in reducing the anisotropy.
In addition, improvement of r-value cannot be expected with aluminum-killed steel.

Therefore, the present invention advantageously solves the above-mentioned problems, and can reduce the in-plane anisotropy of the hot-rolled steel sheet to the utmost, and the hot-rolled steel sheet for working has a small in-plane anisotropy of formability. It is an object of the present invention to provide a manufacturing method of.

[0009]

Means for Solving the Problems The present inventor has made a detailed study on the in-plane anisotropy of hot-rolled steel sheet from the viewpoint of texture, and the texture of the surface layer causes a large in-plane anisotropy. I found that. That is, in hot rolling, since the friction coefficient between the roll and the rolled piece is large, near the surface of the plate thickness, shear strain that is not present in the plate thickness central portion is large, whereby crystal rotation is remarkable with the plate thickness central portion. , A heterogeneous texture is formed. This difference in texture exists even though the quality changes after the transformation. The texture of the surface layer portion after this transformation causes a large in-plane anisotropy. It is considered that the in-plane anisotropy of the material becomes apparent in the thin material because the number of layers having a texture affected by shear strain relatively increases as the plate thickness decreases.

Therefore, the present inventor energetically conducts research on texture control in the γ-region hot rolling, and makes the texture formation of the surface layer portion closer to that of the center portion to reduce the in-plane anisotropy. It was found to be a policy.

The gist of the present invention is (1) by weight, C: 0.1% or less, Mn: 1% or less,
When hot rolling a slab of steel containing Si: 1% or less, P: 0.2% or less, N: 0.01% or less, Al: 0.005% or more, 1.0% or less, Ar ₃ transformation points +100
At temperatures below ℃ and above Ar ₃ transformation point, the total rolling reduction is 50
% Or more rolling under the condition that the friction coefficient is 0.2 or less with lubrication, the method for producing a hot-rolled steel sheet for forming having a small in-plane anisotropy of formability (2) As a steel component , And by weight ratio, B: 0.00
05% or more and 11N / 14 + 0.005% or less is included, The manufacturing method of the hot-rolled steel sheet for processing with small in-plane anisotropy of the formability of the said 1 (3) After rough rolling, and a preceding material A method for producing a hot-rolled steel sheet for working having a small in-plane anisotropy of formability according to 1 or 2 above, which comprises joining with a trailing material and finish rolling (4) the preceding material after rough rolling And the succeeding material are joined and finish-rolled, and the tension between the finish-rolling machine and the winding machine is 5 MPa.
The method for manufacturing a hot-rolled steel sheet for working according to the above 1 or 2, wherein the in-plane anisotropy of formability is small.

Hereinafter, the present invention will be described in detail.

First, the reasons for limiting the components in the present invention will be described.

The upper limit of C is set to 0.1% because if it is added in an amount exceeding 0.1%, the workability is deteriorated. It is not necessary to limit the lower limit, but it is difficult to set it to 10 ppm or less in the current steelmaking capacity.

The upper limit of Mn is set to 1% because if it is added in excess of 1%, the workability is deteriorated. It is not necessary to limit the lower limit, but it is difficult to set it to 0.03 or less with the current steelmaking capacity.

The upper limit of Si is set to 1% because if added in excess of 1%, the workability is deteriorated.

The upper limit of P is set to 0.2% because if it is added in an amount exceeding 0.2%, the workability is deteriorated.

The upper limit of N is 0.01% because it is 0.01
This is because if it is added in excess of%, the workability is deteriorated. It is not necessary to limit the lower limit, but it is difficult to set it to 10 ppm or less in the current steelmaking capacity.

The lower limit of Al is 0.005% for the purpose of sufficient deoxidation. The upper limit is 1 from the viewpoint of workability.
Limited to%.

B forms BN and reduces the solid solution N and contributes to softening. Therefore, it is preferable to add B. However, 0.0005% or more is necessary for the effect to be clearly seen. Is. Further, excessive addition deteriorates workability, so the upper limit was made 11N / 14 + 0.005%.

Next, the reasons for limiting the process conditions will be described.

As described above, bringing the texture formation of the surface layer portion closer to that of the central portion reduces the in-plane anisotropy, and therefore the hot rolling conditions for realizing it are the limiting conditions. The limiting condition is that rolling at a total rolling reduction of 50% or more at a temperature of Ar ₃ transformation point + 100 ° C. or less and a temperature of Ar ₃ transformation point or more is performed under lubrication and a friction coefficient of 0.2 or less. is there.

When the friction coefficient between the roll and the rolled plate becomes 0.2 or less by applying lubrication during rolling, the shear strain on the surface can be reduced.

However, if the total reduction ratio in lubrication rolling is small, the formation of a texture that reduces the in-plane anisotropy becomes insufficient, so at least 50% or more reduction must be applied by one pass or multiple passes. There is.

At that time, if the rolling temperature is too high, recrystallization,
Since grain growth remarkably occurs to prevent sharpening of the texture, Ar ₃ + 100 ° C. is set as the upper limit. Further, if the rolling is performed at a temperature lower than the Ar ₃ transformation point, the r value is significantly deteriorated, so the lower limit of the hot rolling temperature is set to Ar ₃ .

In the lubrication rolling, when biting into the roll bite, there is a high possibility that biting failure or slip will occur. Therefore, when rolling one slab at a time, it is a common operation to not apply lubrication until the tip of the hot strip is wound up. However, in this case, the unlubricated portion and the lubricated portion are present in the longitudinal direction, and their characteristics are different from each other, which may cause trouble in quality control. As a countermeasure, after rough rolling, the rough rolled material is joined to the preceding hot rolled sheet,
Continuous hot rolling is preferable because it can eliminate unlubricated parts. This ensures the stability of quality.

Further, the tension between the finish rolling mill and the winding machine is 5
When it is at least MPa, the r value is improved. However, if excessive tension is applied, plate breakage may occur, so the upper limit is preferably 50 MPa or less.

[0028]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described with reference to examples.

In the examples, steels having the composition shown in Table 1 were used. Steel types A to G are steels of the present invention, and H and I are comparative steels. Hot rolling conditions and Δr value of product plate = (r ₀ + r ₉₀ −2
Table 2 shows the values of r ₄₅ ) / 2, rmin, total elongation El, tensile strength TS and El × TS which is an index of strength-ductility balance. Since the Δr value corresponds to the anisotropy of mechanical properties such as El, YP, and TS, it is used as a representative index of anisotropy. Here, r ₀ is the r value of the test piece cut out parallel to the rolling direction, r ₉₀ is the r value of the test piece cut out perpendicularly to the rolling direction, and r ₄₅ is the direction 45 ° from the rolling direction. It is the r value of the test piece cut out into. rmin is r ₀ , r
It is the minimum value of ₉₀ and r ₄₅ .

Other manufacturing conditions are: slab heating temperature: 1
The temperature was 200 ° C., the plate thickness was 1.4 mm, and the coiling temperature was around 600 ° C.

Experiment Nos. 1, 2, and 3 satisfying the scope of the present invention.
The materials 3, 6, 7, 9, 11, 13, 15, 17, and 19 have low absolute values of Δr values and relatively high r min. Also, the strength-ductility balance is relatively excellent. On the other hand, γ
Experiment Nos. 5, 10, 12, 14, 16, 18, 20, 22, 2 that did not reduce the friction coefficient to 0.2 or less during hot rolling
The materials Nos. 4 and 25 have a larger absolute value of Δr value and a smaller rmin than the material whose friction coefficient is almost halved by lubrication rolling. Also, the strength-ductility balance is poor. Ar ₃ transformation point +
The Δr value of the material of Experiment No. 4, which had a low total rolling reduction of 40% in hot rolling with a friction coefficient of 0.2 or less at a temperature of 100 ° C or less and the Ar ₃ transformation point or more, was lower than that of the unlubricated rolled material. No significant improvement was observed. Further, the hot finish rolling temperature is A
The material of Experiment No. 8 which was less than the r ₃ transformation point also has a poor anisotropy and strength-ductility balance. The materials of Experiment Nos. 21 to 24 using steels outside the range of the steel of the present invention have poor strength-ductility balance.

In this embodiment, the experiment numbers 1, 6, 7, 15, 1
When hot rolling, Nos. 6, 19, 20, and 25 are obtained by joining the rough rolled material to the preceding rough rolled material after the rough rolling and continuously hot rolling. The tension between the finish rolling machine and the winding machine at that time is usually around 20 MPa, but in Experiment No. 25,
The tension was intentionally set to 3 MPa for rolling. Other conditions are almost the same as the material of Experiment No. 1, but the material of Experiment No. 25 is
The anisotropy improvement margin was slightly small.

[0033]

[Table 1]

[0034]

[Table 2]

[0035]

According to the present invention, not only the rolling load and torque during hot rolling can be reduced by lubrication rolling, but also
Also in terms of material, anisotropy and strength-ductility balance can be improved, which is an industrially valuable invention.

Claims (4)

[Claims] 1. A weight ratio of C: 0.1% or less and Mn: 1
% Or less, Si: 1% or less, P: 0.2% or less, N: 0.0
When hot rolling a slab of steel containing 1% or less and Al: 0.005% or more and 1.0% or less, Ar ₃ transformation point +1
An in-plane difference in formability, which is characterized in that rolling at a temperature of 00 ° C. or lower and a temperature of Ar ₃ transformation point or higher and a total reduction ratio of 50% or higher is performed under the condition that a friction coefficient is 0.2 or less by lubrication. A method for manufacturing a hot-rolled steel sheet for processing, which has a low degree of orientation. 2. A steel component further comprising B: in a weight ratio.
The method for producing a hot-rolled steel sheet for working according to claim 1, wherein the hot-rolled steel sheet has a formability of in-plane anisotropy of less than 0.0005% and 11N / 14 + 0.005%. 3. The method according to claim 1, wherein after the rough rolling, the preceding material and the following material are joined and finish-rolled.
A method for producing a hot-rolled steel sheet for working, which has a small in-plane anisotropy of formability as described. 4. The method according to claim 1, wherein after the rough rolling, the preceding material and the following material are joined and finish-rolled, and the tension between the finish-rolling machine and the winder is set to 5 MPa or more. Item 3. A method for producing a hot-rolled steel sheet for working, wherein the in-plane anisotropy of formability is small.