WO2013129128A1

WO2013129128A1 - Production method for steel product exhibiting excellent internal properties

Info

Publication number: WO2013129128A1
Application number: PCT/JP2013/053626
Authority: WO
Inventors: 勝村　龍郎; 井口　貴朗; 駒城　倫哉; 原　浩司
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2012-02-29
Filing date: 2013-02-15
Publication date: 2013-09-06
Also published as: JP2013180302A; CN104136140A; AR090170A1; US20150027191A1; EP2821152A1; EP2821152A4

Abstract

[Problem] When hot rolling a raw material having a circular cross section to obtain a steel product having a circular cross section, it has been, hitherto, difficult to modify the internal properties thereof by applying heavy reduction. [Solution] When performing hot rolling at least 3 times on a steel raw material having a circular cross section to obtain a finished steel product having a circular cross section: a pair of flat rollers arranged above and below are used for the first time hot rolling is performed; a pair of homogeneous perforated rollers arranged above and below, or, a pair of heterogeneous perforated rollers arranged above and below are used for the second time hot rolling is performed and for every time up to and including the penultimate time hot rolling is performed; a pair of round perforated rollers arranged above and below are used the final time hot rolling is performed; and the percentage reduction in the first time hot rolling is performed is less than the total percentage reduction from the raw material to the finished product. It is preferable that the percentage reduction in the first time hot rolling is performed be at least 50% of the total percentage reduction from the second time hot rolling is performed onwards.

Description

Manufacturing method for steel with excellent quality

The present invention relates to a method for producing a steel material having excellent internal quality.

When rolling a material having a round cross section (also called a circular cross section) made of steel, generally, the product is also a round cross section, and in that case, an oval (ellipse; abbreviation O) -round (circle; abbreviation R) system is usually used. Often, a hole series is used. Conversely, when the material shape is a square, the cross-sectional area is measured by a hole roll having a hole shape such as a square (square; abbreviation S) or a box (hexagon; abbreviation B) -diamond (diamond; abbreviation D). The cross-sectional area (the same applies hereinafter) is reduced to a final desired shape. Of course, if the material is a square cross section, a combination of square (S) -oval (O) is also used (Non-patent Document 1). In addition, the example of said various hole shape is shown in FIG. 4 with a flat roll (flat roll; abbreviation F) shape. In addition, FIG. 4 is sectional drawing which cut | disconnected the up-and-down roll by the plane where those axial center lines ride. In FIG. 4, F / F is a flat roll for both upper and lower sides, O / O is for upper and lower and oval hole type rolls, R / R is a round hole type roll for both upper and lower sides, and S / S is a square hole type for both upper and lower sides. In the case of a roll, B / B is an abbreviation that represents a case of a box-hole type roll both in the upper and lower directions, and D / D is an abbreviation that represents a case of a diamond-hole type roll in the upper and lower sides (hereinafter the same).

At this time, in particular, when the material shape or the intermediate shape is substantially circular, as described above, the circular cross-section material of the final shape is manufactured by oval (O) or round (R).
On the other hand, when the raw material is an as-cast steel slab, or when a defect remains in the center of the cross-section of the steel slab, it is not suitable as a material for the final product or another production line. This is because, since defects remain, this causes wrinkles due to further processing, and there is a concern about occurrence of breakage starting from this during subsequent rolling. Then, when manufacturing a product from steel slabs, such as a slab, the method of carrying out a strong reduction is known as a method of eliminating the defect of the cross section of a raw material by rolling (nonpatent literature 2). This is because when steel sheets are manufactured, for example, when it is necessary to reduce 30 mm, it is possible to obtain a product with better internal quality by rolling 30 mm at a time rather than rolling it at 10 mm / pass. It is said that

In Patent Document 1, as a rolling method for preventing rolling cracks (particularly, side surface rolling cracks) of continuous cast billet slabs, continuous casting with a round cross-section is performed when producing steel bars by continuous rolling from continuous cast billet slabs. It is described that a billet slab is used, a perforated roll is used in the first pass of the rough rolling forming pass, and a flat roll is used in the rough rolling after the second pass.

Japanese Patent No. 3645904

However, when the material has a round cross section and the product also has a round cross section, it is not always easy to apply the technology that can be expressed in a single word as strong pressure. This is because the area reduction rate (abbreviated area reduction ratio = 1-product cross-sectional area / material cross-sectional area) until the cross-sectional area of the steel slab or slab is reduced to the cross-sectional area of the final product, This is because the required hole type and rolling reduction are limited to some extent when building the shape. Alternatively, there is a method of increasing the cross-sectional area of the material, but changing the hole shape or optimizing the rolling reduction each time requires a lot of time and labor, and is industrially difficult. In addition, when the material is made from a mold, the restrictions are large and practically difficult. As described above, generally, when rolling a round cross section, a substantially elliptical hole shape is used. However, when such a hole shape is subjected to strong pressure, biting from the hole shape occurs, and this occurs on the outer surface of the product. There is a risk of leaving traps. In addition, if the reduction is insufficient, it is difficult to apply the technique by strong reduction, such as the part that does not fill the hole mold remains on the outer surface of the final product even if there is no biting. There were things that could not be done, and this was an issue.

In order to solve the above-mentioned problems, the inventor has obtained the center of the round cross section of the material even when the rolling reduction is not necessarily high in the hot rolling of 3 passes or more to which the hole rolling is usually applied in order to obtain a desired product shape. As a result, only the first pass of the hot rolling is subjected to vertical flat roll rolling, and the second and subsequent passes are perforated rolling. The inventor has obtained the knowledge that a product that can be easily applied and has sufficient inner quality and shape can be obtained, and the present invention having the following gist configuration has been made.

That is, according to the present invention, when a steel round cross-section material is subjected to hot rolling for 3 passes or more to form a round cross-section steel product, the first pass uses a pair of upper and lower flat rolls, and the second pass and the final pass. Up to immediately before, a pair of upper and lower same type or upper and lower types of hole type rolls are used, and a final pass uses a pair of upper and lower round hole type passes, and the area reduction rate of the first pass is less than the total area reduction rate from the material to the product. It is the manufacturing method of the steel material excellent in the internal quality characterized by rolling as an inside. In the present invention, the area reduction rate of the first pass is preferably 50% or more of the total area reduction rate after the second pass.

According to the present invention, a defect existing in the center of a round cross section can be sufficiently blocked by the strong rolling under the upper and lower flat rolls in the first pass, and the flattened cross section due to the strong rolling is a relatively lightly compressed hole after the second pass. Since it can be sufficiently rounded by die rolling, a round cross-section steel material having sufficient inner quality can be obtained without deteriorating the shape.

It is a diagram which shows the example (result of experiment 1) of the influence of the roll shape which has on the correlation of the defect obstruction | occlusion rate and area reduction rate in 1 pass rolling. It is a diagram which shows the example (result of experiment 2) of the influence of the roll shape which has on the correlation of the defect obstruction | occlusion rate and area reduction rate in 1 pass rolling. It is a diagram which shows the example (result of experiment 3) of the influence of the roll shape which has on the correlation of the defect obstruction | occlusion rate and area reduction rate in 1 pass rolling. It is the schematic which shows various hole shape and flat roll shape.

As a clue to search for a solution to the above-mentioned problem, the inventors of the present invention have a defect blockage ratio (= 1−defect cross-sectional area after rolling / An experimental investigation was carried out on how the correlation between the material defect cross-sectional area) and the area reduction ratio (= 1-rolled product cross-sectional area / material cross-sectional area (including defect cross-sectional area)) could vary depending on the roll shape used. In the experiment, a lead material was used, and the rolling was performed cold. This is because the cold deformation behavior of lead is close to the hot (1000-1200 ° C) deformation behavior of steel, and the normal temperature deformation resistance of lead tends to be almost the same as the hot deformation resistance of steel. This is because it can be adopted as a good approximation.
(Experiment 1)
In Experiment 1, the outer diameter = φ50 mm, the defect diameter = φ5 mm, the upper / lower roll shapes are F / F, D / D, O / O, B / B (see FIG. 4), and the roll diameter is The material diameter was 5 times (however, the roll diameter of the perforated roll was represented by the diameter of the roll flange portion), and the area reduction rate was variously changed within a range of about 25% or less. The post-rolling defect cross-sectional area was determined from a defect photographed image in the rolled product cross section. In addition, when a defect was not recognized by the picked-up image, the color check test of the to-be-photographed cross section was conducted, and it was confirmed that there was no leaching of the penetrating liquid (the same applies hereinafter).

The results are shown in FIG. As can be seen from FIG. 1, in any roll shape, the defect occlusion rate increases with the area reduction rate, but the increase tendency is particularly steep in the case of F / F compared to other cases, and the area reduction rate = 21%. It has reached a complete blockage (defect blockage rate = 1) at a certain level, indicating that the defect blockage rate can be significantly increased by high pressure using upper and lower flat rolls.
(Experiment 2)
Experiment 2 has the same specifications as Experiment 1 except that the defect diameter in Experiment 1 is 2.5 mm. The results are shown in FIG. From FIG. 2, in any roll shape, the defect blockage rate increases with the area reduction rate, but the increase tendency is particularly steep in the case of F / F as compared to other cases, and the area reduction rate = 21%. It has reached a complete blockage (defect blockage rate = 1) at a certain level, indicating that the defect blockage rate can be significantly increased by high pressure using upper and lower flat rolls.
(Experiment 3)
Experiment 3 had the same specifications as Experiment 1 except that the outer diameter of the material was 30 mm and the defect diameter was 3 mm in Experiment 1. The results are shown in FIG. From FIG. 3, the defect blockage rate increases with the area reduction rate in any roll shape, but the increase tendency is particularly steep in the case of F / F compared to other cases, and the area reduction rate = 9%. It has reached a complete blockage (defect blockage rate = 1) at a certain level, indicating that the defect blockage rate can be significantly increased by high pressure using upper and lower flat rolls.

Next, among the rolling passes of 3 or more passes, the pass to which F / F (upper and lower flat roll) rolling is applied was examined, and the following conclusion was reached. In other words, in F / F rolling, strong rolling is performed, and this is performed in the second and subsequent passes because if the number of passes is limited, the number of hole rolling passes from after the strong rolling pass to the final pass decreases. It is difficult to round the cross section. If there is no limit on the number of passes, rounding may be possible by adding a perforated rolling stand, but the number of stands increases and there are significant disadvantages in terms of rolling efficiency and economy. Therefore, F / F rolling should be performed only in the first pass.

The area reduction rate of F / F rolling (first pass) must be less than a predetermined total area reduction rate from the material to the product. Here, in general, the total area reduction (represented by the symbol Z _{m / n} ) from the m-pass entry side to the n-pass entry side (m <n) is the m-pass entry side cross-sectional area S _m-1 . and a n-pass Medellin side cross-sectional area S _n, defined by equation (1).
Z _{m / n} = 1−S _n / S _m−1 (1)
In the case of from the material (first pass entry side) to the product (final N pass exit side), m = 1 and n = N in equation (1), and equation (2) is obtained.

Z _{1 / N} = 1−S _N / S ₀ (2)
When Expression (2) is transformed using the exit-side cross-sectional area S _i of the i-th pass and the area reduction ratio z _i (= 1−S _i / S _i−1 ), Expression (3) is obtained.

Since the material cross-sectional area S ₀ and the product target cross-sectional area S _N are both predetermined values, the total area reduction ratio Z _{1 / N} from the material to the product is a predetermined value. If z ₁ ≧ Z _{1 / N} , from formula (3), 1− _i Π _{2 / N} (1-z _i ) = Z _{2 / N} ≦ 0, and the second and subsequent passes cannot be rolled. The target round cross-sectional shape cannot be obtained. Therefore, z ₁ <Z _{1 / N} must be satisfied.
On the other hand, if z ₁ is less than 50% of Z _{2 / N} , there is a possibility that it will not be under strong pressure and the defect blocking effect will be poor. If the defect closure is performed in the first pass, it is considered that it is sufficient to adjust the shape of the remainder. Therefore, the area reduction rate z ₁ of the F / F rolling (first pass) is the total area reduction of the remaining hole rolling. It is preferable to be 50% or more of the rate Z _{2 / N.}

A test material with a through hole (circular cross section) drilled as an artificial defect at the center of the cross section of a steel round cross section material is heated and hot rolled under various rolling conditions to produce a steel material with a round cross section target. The defect blockage rate and the quality of the obtained steel were investigated. Dimensions of materials used (outer diameter, defect diameter), target dimensions of steel (outer diameter), total area reduction ratio Z _{1 / N} from the first pass entry side to the final N pass exit side, and rolling conditions (all passes) Table 1 shows the number N, roll shape used (F / F → O / O... R / R, etc.), the area reduction rate z _{1 in} the first pass, and the total area reduction rate Z _{2 / N in the} second and subsequent passes). Shown in The heating temperature was 1100 ° C. The roll diameter of the flat roll was 200 mm, and the roll diameter of the perforated roll (flange end roll diameter) was 200 mm. The final pass outlet temperature was about 50-100 ° C. lower than the heating temperature.

The defect blockage rate of the obtained steel was investigated in the same manner as in the above experiment. The quality of the shape is determined by measuring the ratio of the minimum diameter / maximum diameter in the circumferential direction as a roundness index, which is good (◯) if the roundness index is 0.975 or more, and otherwise poor (× ). The results are shown in Table 1.
From Table 1, in the present invention example (F / F in the first pass only and z ₁ <Z _{1 / N} ), the defects were completely closed and the shape was good.

Claims

When a steel round cross-section material is hot rolled for 3 passes or more to make a round cross-section steel product, a pair of upper and lower flat rolls are used for the first pass and the same type is used for the second pass and immediately before the final pass. A pair of upper and lower types of perforated rolls are used, and the final pass is a pair of upper and lower round perforated rolls, and the area reduction rate in the first pass is rolled within a range less than the total area reduction rate from the material to the product. The manufacturing method of the steel material excellent in the internal quality characterized by this.
2. The method for producing a steel material with excellent internal quality according to claim 1, wherein the area reduction rate of the first pass is 50% or more of the total area reduction rate after the second pass.